Hybrid journal (It can contain Open Access articles) ISSN (Print) 0956-540X - ISSN (Online) 1365-246X Published by Oxford University Press[370 journals]

Authors:Shcherbakov R; Zhuang J, Ogata Y. Pages: 1 - 13 Abstract: Extreme value statistics and Bayesian methods are used to constrain the magnitudes of the largest expected earthquakes in a sequence governed by the parametric time-dependent occurrence rate and frequency–magnitude statistics. The Bayesian predictive distribution for the magnitude of the largest event in a sequence is derived. Two types of sequences are considered, that is, the classical aftershock sequences generated by large main shocks and the aftershocks generated by large foreshocks preceding a main shock. For the former sequences, the early aftershocks during a training time interval are used to constrain the magnitude of the future extreme event during the forecasting time interval. For the latter sequences, the earthquakes preceding the main shock are used to constrain the magnitudes of the subsequent extreme events including the main shock. The analysis is applied retrospectively to past prominent earthquake sequences. PubDate: 2017-09-22 DOI: 10.1093/gji/ggx407 Issue No:Vol. 212, No. 1 (2017)

Authors:Retailleau L; Landès M, Gualtieri L, et al. Pages: 14 - 24 Abstract: Seismological detection methods are traditionally based on picking techniques. These methods cannot be used to analyse emergent signals where the arrivals cannot be picked. Here, we detect and locate seismic events by applying a beamforming method that combines multiple body-wave phases to USArray data. This method explores the consistency and characteristic behaviour of teleseismic body waves that are recorded by a large-scale, still dense, seismic network. We perform time-slowness analysis of the signals and correlate this with the time-slowness equivalent of the different body-wave phases predicted by a global traveltime calculator, to determine the occurrence of an event with no a priori information about it. We apply this method continuously to one year of data to analyse the different events that generate signals reaching the USArray network. In particular, we analyse in detail a low-frequency secondary microseismic event that occurred on 2010 February 1. This event, that lasted 1 d, has a narrow frequency band around 0.1 Hz, and it occurred at a distance of 150° to the USArray network, South of Australia. We show that the most energetic phase observed is the PKPab phase. Direct amplitude analysis of regional seismograms confirms the occurrence of this event. We compare the seismic observations with models of the spectral density of the pressure field generated by the interferences between oceanic waves. We attribute the observed signals to a storm-generated microseismic event that occurred along the South East Indian Ridge. PubDate: 2017-09-25 DOI: 10.1093/gji/ggx410 Issue No:Vol. 212, No. 1 (2017)

Authors:Vorobiev O; Ezzedine S, Hurley R. Pages: 25 - 41 Abstract: This paper describes analysis of non-radial ground motion generated by chemical explosions in a jointed rock formation during the Source Physics Experiment (SPE). Such motion makes it difficult to discriminate between various subsurface events such as explosions, implosions (i.e. mine collapse) and earthquakes. We apply 3-D numerical simulations to understand experimental data collected during the SPEs. The joints are modelled explicitly as compliant thin inclusions embedded into the rock mass. Mechanical properties of the rock and the joints as well as the joint spacing and orientation are inferred from experimental test data, and geophysical and geological characterization of the SPE site which is dominantly Climax Stock granitic outcrop. The role of various factors characterizing the joints such as joint spacing, frictional properties, orientation and persistence in generation of non-radial motion is addressed. The joints in granite at the SPE site are oriented in nearly orthogonal directions with two vertical sets dipping at 70–80 degrees with the same strike angle, one vertical set almost orthogonal to the first two and one shallow angle joint set dipping 15 degrees. In this study we establish the relationship between the joint orientation and azimuthal variations in the polarity of the observed shear motion. The majority of the shear motion is generated due to the effects of non-elastic sliding on the joints near the source, where the wave can create significant shear stress to overcome the cohesive forces at the joints. Near the surface the joints are less confined and are subject to sliding when the pressure waves are reflected. In the far field, where the cohesive forces on the joints cannot be overcome, additional shear motion can be generated due to elastic anisotropy of the rock mass given by preferred spatial orientations of compliant joints. PubDate: 2017-09-22 DOI: 10.1093/gji/ggx403 Issue No:Vol. 212, No. 1 (2017)

Authors:Kariche J; Meghraoui M, Timoulali Y, et al. Pages: 42 - 53 Abstract: The 2016 January 25 earthquake (Mw 6.3) follows in sequence from the1994 May 26 earthquake (Mw 6.0) and the 2004 February 24 earthquake (Mw 6.4) in the Rif Mountains and Alboran Sea. The earlier two seismic events which were destructive took place on inland conjugate faults, and the third event occurred on an offshore fault. These earthquake sequences occurred within a period of 22 yr at ∼25 km distance and 11–16-km depth. The three events have similar strike-slip focal mechanism solutions with NNE-SSW trending left-lateral faulting for the 1994 and 2016 events and NW-SE trending right-lateral faulting for the 2004 event. This shallow seismic sequence offers the possibility (i) to model the change in Coulomb Failure Function (ΔCFF with low μ΄ including the pore pressure change) and understand fault-rupture interaction, and (ii) to analyse the effect of pore fluid on the rupture mechanism, and infer the clock-time advance. The variation of static stress change has a direct impact on the main shock, aftershocks and related positive lobes of the 2004 earthquake rupture with a stress change increase of 0.7–1.1 bar. Similarly, the 2004 main shock and aftershocks indicate loading zones with a stress change (>0.25 bar) that includes the 2016 earthquake rupture. The tectonic loading of 19–24 nanostrain yr−1 obtained from the seismicity catalogue of Morocco is comparable to the 5.0 × 1017 N·m yr−1 seismic strain release in the Rif Mountains. The seismic sequence is apparently controlled by the poroelastic properties of the seismogenic layer that depend on the undrained and drained fluid conditions. The short interseismic period between main shocks and higher rate of aftershocks with relatively large magnitudes (4 < Mw < 5.5) implies the pore-fluid physical effect in undrained and drained conditions. The stress-rate ranges between 461 and 582 Pa yr−1 with a ΔCFF of 0.2–1.1 bar. The computed clock-time advance reaches 239 ± 22 yr in agreement with the ∼10 yr delay between main shocks. The calculated static stress change of 0.9–1.3 bar, under pore-fluid stimulus added with well-constrained geodetic and seismic strain rates are critical for any seismic hazard assessment. PubDate: 2017-09-13 DOI: 10.1093/gji/ggx385 Issue No:Vol. 212, No. 1 (2017)

Authors:Jing X; Yang Z, Tong Y, et al. Pages: 54 - 75 Abstract: A suspected Silurian remagnetization of the Ediacaran strata of South China was proposed decades ago by many researchers, but, there has been no systematic study of its causes and mechanisms. In this study, we investigate the multiphase remagnetization processes that affected the Ediacaran strata and the possible mechanisms of these remagnetization events. We conducted detailed palaeomagnetic, rock magnetic and scanning electron microscope (SEM) studies of samples from the Ediacaran strata in the Jiulongwan (JLWE, JLWS), Qinglinkou (QLK) and Sanxiarenjia (SXRJ) sections in the Three Gorges Area, South China. After removal of a recent viscous remanent magnetization below 150 °C, an intermediate temperature component (ITC; Dg = 27.6°, Ig = 45.3°, N = 12 sites, kg = 184.3, α95 = 3.2° for JLWE; Dg = 22°, Ig = 45.3°, N = 11 sites, kg = 789.2, α95 = 1.6° for JLWS; and Dg = 25.5°, Ig = 52.5°, N = 6 sites, kg = 533.4, α95 = 2.9° for SXRJ) was removed below 300 °C which coincides with the Jurassic results from South China, suggesting a pervasive Jurassic remagnetization. In addition, a high temperature component (HTC; Ds = 84.8°, Is = 19.2°, N = 9 sites, ks = 35.5, α95 = 8.8° for JLWE; Ds = 74.1°, Is = 49.4°, N = 7 sites, ks = 218.9, α95 = 4.1° for JLWS; and Ds = 89.5°, Is = 30.7°, N = 8 sites, ks = 129.2, α95 = 4.9° for SXRJ) was isolated between 300 and 480–540 °C. Rock magnetic and SEM studies suggest that the ITC and HTC are carried by pyrrhotite and magnetite, respectively. SEM observations also demonstrate the occurrence of massive authigenic magnetite in cavities or cracks, mineralogical changes from pyrite to Fe oxides, and the reaction between gypsum and Fe oxides. Based on similarities to the Silurian poles of South China, together with the SEM observations, we suggest that the HTC from the JLWE and SXRJ sections is a Silurian age remagnetization. The oxidation of iron sulphides and thermochemical sulphate reduction induced by the multiple generations of oil and gas in the Ediacaran and Cambrian strata are suggested as the main mechanism for remagnetization. Despite the pervasive Silurian remagnetization of the Ediacaran strata, most of the HTC from the thick-bedded dolostone of Doushantuo Formation Member 3 at the JLWS section appears to carry a primary remanence, because its pole differs from other poles of South China and the results pass both the fold and reversal tests. The relatively low-geothermic conditions and the absence of both hydrocarbon and smectite/illite explain why most results from the Doushantuo Member 3 of JLWS section were not affected by the Silurian remagnetization. This new Ediacaran pole supersedes the previous suspected remagnetized poles, which can be used to constrain the palaeoposition of South China both in Rodinia and Gondwana. PubDate: 2017-09-22 DOI: 10.1093/gji/ggx396 Issue No:Vol. 212, No. 1 (2017)

Authors:Ren Z; Qiu L, Tang J, et al. Pages: 76 - 87 Abstract: Although accurate numerical solvers for 3-D direct current (DC) isotropic resistivity models are current available even for complicated models with topography, reliable numerical solvers for the anisotropic case are still an open question. This study aims to develop a novel and optimal numerical solver for accurately calculating the DC potentials for complicated models with arbitrary anisotropic conductivity structures in the Earth. First, a secondary potential boundary value problem is derived by considering the topography and the anisotropic conductivity. Then, two a posteriori error estimators with one using the gradient-recovery technique and one measuring the discontinuity of the normal component of current density are developed for the anisotropic cases. Combing the goal-oriented and non-goal-oriented mesh refinements and these two error estimators, four different solving strategies are developed for complicated DC anisotropic forward modelling problems. A synthetic anisotropic two-layer model with analytic solutions verified the accuracy of our algorithms. A half-space model with a buried anisotropic cube and a mountain-valley model are adopted to test the convergence rates of these four solving strategies. We found that the error estimator based on the discontinuity of current density shows better performance than the gradient-recovery based a posteriori error estimator for anisotropic models with conductivity contrasts. Both error estimators working together with goal-oriented concepts can offer optimal mesh density distributions and highly accurate solutions. PubDate: 2017-06-23 DOI: 10.1093/gji/ggx256 Issue No:Vol. 212, No. 1 (2017)

Authors:Chen Y. Pages: 88 - 102 Abstract: Effective and efficient arrival picking plays an important role in microseismic and earthquake data processing and imaging. Widely used short-term-average long-term-average ratio (STA/LTA) based arrival picking algorithms suffer from the sensitivity to moderate-to-strong random ambient noise. To make the state-of-the-art arrival picking approaches effective, microseismic data need to be first pre-processed, for example, removing sufficient amount of noise, and second analysed by arrival pickers. To conquer the noise issue in arrival picking for weak microseismic or earthquake event, I leverage the machine learning techniques to help recognizing seismic waveforms in microseismic or earthquake data. Because of the dependency of supervised machine learning algorithm on large volume of well-designed training data, I utilize an unsupervised machine learning algorithm to help cluster the time samples into two groups, that is, waveform points and non-waveform points. The fuzzy clustering algorithm has been demonstrated to be effective for such purpose. A group of synthetic, real microseismic and earthquake data sets with different levels of complexity show that the proposed method is much more robust than the state-of-the-art STA/LTA method in picking microseismic events, even in the case of moderately strong background noise. PubDate: 2017-09-27 DOI: 10.1093/gji/ggx420 Issue No:Vol. 212, No. 1 (2017)

Authors:Larnier H; Sailhac P, Chambodut A. Pages: 103 - 118 Abstract: Atmospheric electromagnetic waves created by global lightning activity contain information about electrical processes of the inner and the outer Earth. Large signal-to-noise ratio events are particularly interesting because they convey information about electromagnetic properties along their path. We introduce a new methodology to automatically detect and characterize lightning-based waves using a time–frequency decomposition obtained through the application of continuous wavelet transform. We focus specifically on three types of sources, namely, atmospherics, slow tails and whistlers, that cover the frequency range 10 Hz to 10 kHz. Each wave has distinguishable characteristics in the time–frequency domain due to source shape and dispersion processes. Our methodology allows automatic detection of each type of event in the time–frequency decomposition thanks to their specific signature. Horizontal polarization attributes are also recovered in the time–frequency domain. This procedure is first applied to synthetic extremely low frequency time-series with different signal-to-noise ratios to test for robustness. We then apply it on real data: three stations of audio-magnetotelluric data acquired in Guadeloupe, oversea French territories. Most of analysed atmospherics and slow tails display linear polarization, whereas analysed whistlers are elliptically polarized. The diversity of lightning activity is finally analysed in an audio-magnetotelluric data processing framework, as used in subsurface prospecting, through estimation of the impedance response functions. We show that audio-magnetotelluric processing results depend mainly on the frequency content of electromagnetic waves observed in processed time-series, with an emphasis on the difference between morning and afternoon acquisition. Our new methodology based on the time–frequency signature of lightning-induced electromagnetic waves allows automatic detection and characterization of events in audio-magnetotelluric time-series, providing the means to assess quality of response functions obtained through processing. PubDate: 2017-09-28 DOI: 10.1093/gji/ggx418 Issue No:Vol. 212, No. 1 (2017)

Authors:Schmeling H; Marquart G, Grebe M. Pages: 119 - 138 Abstract: We develop an approach for heat exchange between a fluid and a solid phase of a porous medium where the temperatures of the fluid and matrix are not in thermal equilibrium. The formulation considers moving of the fluid within a resting or deforming porous matrix in an Eulerian coordinate system. The approach can be applied, for example, to partially molten systems or to brine transport in porous rocks. We start from an existing theory for heat exchange where the energy conservation equations for the fluid and the solid phases are separated and coupled by a heat exchange term. This term is extended to account for the full history of heat exchange. It depends on the microscopic geometry of the fluid phase. For the case of solid containing hot, fluid-filled channels, we derive an expression based on a time-dependent Fourier approach for periodic half-waves. On the macroscopic scale, the temporal evolution of the heat exchange leads to a convolution integral along the flow path of the solid, which simplifies considerably in case of a resting matrix. The evolution of the temperature in both phases with time is derived by inserting the heat exchange term into the energy equations. We explore the effects of thermal non-equilibrium between fluid and solid by considering simple cases with sudden temperature differences between fluid and solid as initial or boundary conditions, and by varying the fluid velocity with respect to the resting porous solid. Our results agree well with an analytical solution for non-moving fluid and solid. The temperature difference between solid and fluid depends on the Peclet number based on the Darcy velocity. For Peclet numbers larger than 1, the temperature difference after one diffusion time reaches 5 per cent of $\tilde{T}$ or more ($\tilde{T}$ is a scaling temperature, e.g. the initial temperature difference). Thus, our results imply that thermal non-equilibrium can play an important role for melt migration through partially molten systems where melt focuses into melt channels near the transition to melt ascent by dykes. Our method is based on solving the convolution integration for the heat exchange over the full flow history, which is numerically expensive. We tested to replace the heat exchange term by an instantaneous, approximate term. We found considerable errors on the short timescale, but a good agreement on the long timescale if appropriate parameters for the approximate terms are used. We derived these parameters which may be implemented in fully dynamical two-phase flow formulations of melt migration in the Earth. PubDate: 2017-10-09 DOI: 10.1093/gji/ggx406 Issue No:Vol. 212, No. 1 (2017)

Authors:Cambiotti G; Sabadini R, Yuen D. Pages: 139 - 150 Abstract: We model the geoid anomalies excited during a megathrust earthquake cycle at subduction zones, including the interseismic phase and the contribution from the infinite series of previous earthquakes, within the frame of self-gravitating, spherically symmetric, compressible, viscoelastic Earth models. The fault cuts the whole 50 km lithosphere, dips 20°, and the slip amplitude, together with the length of the fault, are chosen in order to simulate an Mw = 9.0 earthquake, while the viscosity of the 170 km thick asthenosphere ranges from 1017 to 1020 Pa s. On the basis of a new analysis from the Correspondence Principle, we show that the geoid anomaly is characterized by a periodic anomaly due to the elastic and viscous contribution from past earthquakes and to the back-slip of the interseismic phase, and by a smaller static contribution from the steady-state response to the previous infinite earthquake cycles. For asthenospheric viscosities from 1017–1018 to 1019–1020 Pa s, the characteristic relaxation times of the Earth model change from shorter to longer timescales compared to the 400 yr earthquake recurrence time, which dampen the geoid anomaly for the higher asthenospheric viscosities, since the slower relaxation cannot contribute its whole strength within the interseismic cycle. The geoid anomaly pattern is characterized by a global, time-dependent positive upwarping of the geoid topography, involving the whole hanging wall and partially the footwall compared to the sharper elastic contribution, attaining, for a moment magnitude Mw = 9.0, amplitudes as high as 6.6 cm for the lowermost asthenospheric viscosities during the viscoelastic response compared to the elastic maximum of 3.8 cm. The geoid anomaly vanishes due to the back-slip of the interseismic phase, leading to its disappearance at the end of the cycle before the next earthquake. Our results are of importance for understanding the post-seismic and interseismic geoid patterns at subduction zones. PubDate: 2017-09-28 DOI: 10.1093/gji/ggx421 Issue No:Vol. 212, No. 1 (2017)

Authors:Liu J; Sarout J, Zhang M, et al. Pages: 151 - 163 Abstract: Quantifying rock physical properties is essential for the mining and petroleum industry. Microtomography provides a new way to quantify the relationship between the microstructure and the mechanical and transport properties of a rock. Studies reporting the use microtomographic images to derive permeability and elastic moduli of rocks are common; only rare studies were devoted to yield and failure parameters using this technique. In this study, we simulate the macroscale plastic properties of a synthetic sandstone sample made of calcite-cemented quartz grains using the microscale information obtained from microtomography. The computations rely on the concept of representative volume elements (RVEs). The mechanical RVE is determined using the upper and lower bounds of finite-element computations for elasticity. We present computational upscaling methods from microphysical processes to extract the plasticity parameters of the RVE and compare results to experimental data. The yield stress, cohesion and internal friction angle of the matrix (solid part) of the rock were obtained with reasonable accuracy. Computations of plasticity of a series of models of different volume-sizes showed almost overlapping stress-strain curves, suggesting that the mechanical RVE determined by elastic computations is also valid for plastic yielding. Furthermore, a series of models were created by self-similarly inflating/deflating the porous models, that is keeping a similar structure while achieving different porosity values. The analysis of these models showed that yield stress, cohesion and internal friction angle linearly decrease with increasing porosity in the porosity range between 8 and 28 per cent. The internal friction angle decreases the most significantly, while cohesion remains stable. PubDate: 2017-09-25 DOI: 10.1093/gji/ggx409 Issue No:Vol. 212, No. 1 (2017)

Authors:Deschamps F; Rogister Y, Tackley P. Pages: 164 - 188 Abstract: Mantle flow induces dynamic topography at the core–mantle boundary (CMB), with distribution and amplitude that depend on details of the flow. To assess whether observations of CMB topography can give constraints on deep mantle structure, we determine CMB dynamic topography associated with different models of mantle convection, including thermochemical and purely thermal models. We investigate the influence of key controlling parameters, specifically the thermal viscosity ratio (ΔηT) and, for thermochemical models, the density contrast (ΔρC) and viscosity ratio (ΔηC) between primordial and regular materials. In purely thermal models, plume clusters induce positive topography with an amplitude that decreases with increasing ΔηT. In thermochemical models with moderate density contrasts, around 100–200 kg m−3, reservoirs of dense material induce depressions in CMB topography, surrounded by a ridge of positive topography. The average depression depth and ridge height increase with increasing ΔρC and ΔηC, but decrease with increasing ΔηT. We find that for purely thermal models or thermochemical models with ΔρC ∼ 90 kg m−3 and less, the long-wavelength (spherical harmonic degrees up to l = 4) dynamic topography and shear wave velocity anomalies predicted by thermochemical distributions anticorrelate. By contrast, for models with ΔρC ≥ 100 kg m−3 and ΔηC > 1, long-wavelength dynamic topography and shear wave velocity anomalies correlate well. This potentially provides a test to infer the nature, that is, either purely or mostly thermal (ΔρC ≤ 100 kg m−3 m−3) or strongly thermochemical (ΔρC ≥ 100 kg m−3), of the low shear wave velocity provinces observed by global tomographic images. The presence of post-perovskite, provided that its viscosity is similar to that of bridgmanite, does not alter these conclusions. PubDate: 2017-09-25 DOI: 10.1093/gji/ggx402 Issue No:Vol. 212, No. 1 (2017)

Authors:Szewczyk D; Bauer A, Holt R. Pages: 189 - 210 Abstract: Knowledge about the stress sensitivity of elastic properties and velocities of shales is important for the interpretation of seismic time-lapse data taken as part of reservoir and caprock surveillance of both unconventional and conventional oil and gas fields (e.g. during 4-D monitoring of CO2 storage). Rock physics models are often developed based on laboratory measurements at ultrasonic frequencies. However, as shown previously, shales exhibit large seismic dispersion, and it is possible that stress sensitivities of velocities are also frequency dependent. In this work, we report on a series of seismic and ultrasonic laboratory tests in which the stress sensitivity of elastic properties of Mancos shale and Pierre shale I were investigated. The shales were tested at different water saturations. Dynamic rock engineering parameters and elastic wave velocities were examined on core plugs exposed to isotropic loading. Experiments were carried out in an apparatus allowing for static-compaction and dynamic measurements at seismic and ultrasonic frequencies within single test. For both shale types, we present and discuss experimental results that demonstrate dispersion and stress sensitivity of the rock stiffness, as well as P- and S-wave velocities, and stiffness anisotropy. Our experimental results show that the stress-sensitivity of shales is different at seismic and ultrasonic frequencies, which can be linked with simultaneously occurring changes in the dispersion with applied stress. Measured stress sensitivity of elastic properties for relatively dry samples was higher at seismic frequencies however, the increasing saturation of shales decreases the difference between seismic and ultrasonic stress-sensitivities, and for moist samples stress-sensitivity is higher at ultrasonic frequencies. Simultaneously, the increased saturation highly increases the dispersion in shales. We have also found that the stress-sensitivity is highly anisotropic in both shales and that in some of the cases higher stress-sensitivity of elastic properties can be seen in the direction parallel to the bedding plane. PubDate: 2017-09-22 DOI: 10.1093/gji/ggx392 Issue No:Vol. 212, No. 1 (2017)

Authors:Heinicke J; Woith H, Alexandrakis C, et al. Pages: 211 - 228 Abstract: Fluid driven seismicity has been observed worldwide. The occurrence of intraplate seismicity triggered by pore pressure perturbations is a widely accepted process. Past analysis of earthquake swarms in the NW-Bohemia/Vogtland region provided evidence for the diffusion of pore pressure fronts during the migration of earthquakes within each swarm. Here, we test the hypothesis whether the diffusion of hydraulically induced pore pressure perturbations from the surface to the hypocentral depth could be a valid trigger mechanism. We test this hypothesis for the earthquake swarms in the Nový Kostel focal zone based on the analysis of 121 earthquake swarms and microswarms which occurred between 1992 and 2016.During the past 30 yr, 90 per cent of the earthquake swarms clustered beneath the village Nový Kostel within a depth range between 6 and 12 km. The spatial distribution of the swarms suggests a main diffusion pathway to the hypocentres, namely conduits along a NW-SE trending fracture zone which can be correlated at the surface with a part of the Libocký creek. A secondary branch of this fault zone exits in the southern part below the Horka reservoir. The filling of this reservoir began in 1969 and might have triggered the first significant earthquake swarm during the last century within the Nový Kostel area in the year 1985/1986. Since then, both the number of potentially triggered swarms as well as the number of microswarms increased with time.According to a robust statistical evaluation, hydraulically induced pore pressure perturbations could have triggered at least 19 per cent of the analysed earthquake swarms assuming effective diffusivities of 0.5 m2 s–1. Thus, hydroseismicity seems to be a valid option to explain at least a part of the earthquake swarms. The alternate hypothesis that ascending/intruding magmatic fluids trigger the remaining part of the earthquake swarms is another potential but weak option. PubDate: 2017-10-03 DOI: 10.1093/gji/ggx412 Issue No:Vol. 212, No. 1 (2017)

Authors:Ebrahimi M; Tatar M, Aoudia A, et al. Pages: 229 - 243 Abstract: In order to define the precise shallow velocity structure beneath the second largest dam reservoir in Iran and to understand the loading effects on the underlying crust, the shear wave velocity of the shallow structure beneath the Gotvand-e Olya (hereinafter referred to as Gotvand) reservoir is determined through the inversion of group velocities obtained from seismic ambient noise tomography, using continuous data from 10 stations of a local network, installed to monitor the induced seismicity in the region surrounding the Gotvand and Masjed Soleyman dams for potential hazard. We obtained Rayleigh waves from cross-correlation of waveforms recorded 10 months before and the same duration after impoundment of the Gotvand reservoir and calculated the group velocity from dispersion analysis in the period range 2–8 s. The group velocity dispersion curves are used to produce 2-D group velocity tomographic maps. The resulting tomographic maps at short periods are well correlated with subsurface geological features and delineate distinct low- and high-velocity zones separated mainly by geological boundaries. The 3-D shear wave velocity structure provides detailed information about the crustal features underneath the reservoir. The results are consistent with the lithology of the region, and attest that ambient noise tomography (ANT) can be used for detailed studies of the velocity structure and lithology at shallow depths using continuous data from a dense local seismic network. An increase of shear wave velocity is observed at the deep parts (4–6 km) underneath the reservoir after impoundment of the dam, which could be caused by the changes in rocks properties after impoundment. However, at shallow depths (2–4 km), a decrease of Vs velocity is observed that can be associated to the penetration of water after the impoundment.Changes of material strength underneath the Gotvand reservoir, calculated using loading effects of the reservoir by considering the pore pressure are in good agreement with the observed shear wave velocity changes across the reservoir. PubDate: 2017-09-25 DOI: 10.1093/gji/ggx411 Issue No:Vol. 212, No. 1 (2017)

Authors:Davy R; Morgan J, Minshull T, et al. Pages: 244 - 263 Abstract: Continental hyperextension during magma-poor rifting at the Deep Galicia Margin is characterized by a complex pattern of faulting, thin continental fault blocks and the serpentinization, with local exhumation, of mantle peridotites along the S-reflector, interpreted as a detachment surface. In order to understand fully the evolution of these features, it is important to image seismically the structure and to model the velocity structure to the greatest resolution possible. Traveltime tomography models have revealed the long-wavelength velocity structure of this hyperextended domain, but are often insufficient to match accurately the short-wavelength structure observed in reflection seismic imaging. Here, we demonstrate the application of 2-D time-domain acoustic full-waveform inversion (FWI) to deep-water seismic data collected at the Deep Galicia Margin, in order to attain a high-resolution velocity model of continental hyperextension. We have used several quality assurance procedures to assess the velocity model, including comparison of the observed and modeled waveforms, checkerboard tests, testing of parameter and inversion strategy and comparison with the migrated reflection image. Our final model exhibits an increase in the resolution of subsurface velocities, with particular improvement observed in the westernmost continental fault blocks, with a clear rotation of the velocity field to match steeply dipping reflectors. Across the S-reflector, there is a sharpening in the velocity contrast, with lower velocities beneath S indicative of preferential mantle serpentinization. This study supports the hypothesis that normal faulting acts to hydrate the upper-mantle peridotite, observed as a systematic decrease in seismic velocities, consistent with increased serpentinization. Our results confirm the feasibility of applying the FWI method to sparse, deep-water crustal data sets. PubDate: 2017-10-03 DOI: 10.1093/gji/ggx415 Issue No:Vol. 212, No. 1 (2017)

Authors:Su Z; Hu J, Wang E, et al. Pages: 264 - 283 Abstract: The Ilan Plain, located in Northeast Taiwan, represents a transition zone between oblique collision (between the Luzon Arc and the Eurasian Plate) and backarc extension (the Okinawa Trough). The mechanism for this abrupt transition from arc-continent collision to backarc extension remains uncertain. We used Global Positioning System (GPS), leveling and multi-interferogram Small Baseline Persistent Scatterer Interferometry (SBAS-PSI) data to monitor the interseismic activity in the basin. A common reference site was selected for the data sets. The horizontal component of GPS and the vertical measurements of the leveling data were converted to line-of-sight (LOS) data and compared with the SBAS-PSI data. The comparison shows that the entire Ilan Plain is undergoing rapid subsidence at a maximum rate of −11 ± 2 mm yr−1 in the LOS direction. We speculate that vertical deformation and anthropogenic activity may play important roles in this deformation. We also performed a joint inversion modeling that combined both the DInSAR and strong motion data to constrain the source model of the 2005 Ilan earthquake. The best-fitting model predicts that the Sansing fault caused the 2005 Ilan earthquake. The observed transtensional deformation is dominated by the normal faulting with a minor left-lateral strike-slip motion. We compared our SBAS-PSI results with the short-term (2005–2009) groundwater level changes. The results indicate that although pumping-induced surface subsidence cannot be excluded, tectonic deformation, including rapid southward movement of the Ryukyu arc and backarc extension of the Okinawa Trough, characterizes the opening of the Ilan Plain. Furthermore, a series of normal and left-lateral strike-slip transtensional faults, including the Choshui and Sansing faults, form a bookshelf-like structure that accommodates the extension of the plain. Although situated in a region of complex structural interactions, the Ilan Plain is primarily controlled by extension rather than by shortening. As the massive, pre-existing Philippines–Ryukyu island arc was pierced by the Philippine Sea Plate, the Ilan Plain formed as a remnant backarc basin on the northeastern corner of Taiwan. PubDate: 2017-09-22 DOI: 10.1093/gji/ggx394 Issue No:Vol. 212, No. 1 (2017)

Authors:Johnson T; Thomle J. Pages: 284 - 296 Abstract: Complex conductivity imaging (also called induced polarization imaging or spectral induced polarization imaging when conducted at multiple frequencies) involves estimating the frequency-dependent complex electrical conductivity distribution of the subsurface. The superior diagnostic capabilities provided by complex conductivity spectra have driven advancements in mechanistic understanding of complex conductivity as well as modelling and inversion approaches over the past several decades. In this work, we demonstrate the theory and application for an approach to 3-D modelling and inversion of complex conductivity data in the real number domain. Beginning from first principles, we demonstrate how the equations for the real and imaginary components of the complex potential may be decoupled. This leads to a description of the real and imaginary source current terms, and a corresponding assessment of error arising from an assumption necessary to complete the decoupled modelling. We show that for most earth materials, which exhibit relatively small phases (e.g. less than 0.2 radians) in complex conductivity, these errors become insignificant. For higher phase materials, the errors may be quantified and corrected through an iterative procedure. We demonstrate the accuracy of numerical forward solutions by direct comparison to corresponding analytic solutions. We demonstrate the inversion using both synthetic and field examples with data collected over a waste infiltration trench, at frequencies ranging from 0.5 to 7.5 Hz.In addition to insight provided by the decoupled equations, we see two primary advantages of the decoupled inversion in comparison to fully coupled inversion in complex number domain. First, memory requirements are reduced by a factor of two because it is unnecessary to simultaneously store both the real and complex components and associated Jacobian matrices. Second, and most importantly, data noise is specified separately for the real and imaginary components of potential (or magnitude and phase), which removes the ambiguity in data misfit residuals that exists in the complex number domain, thereby enabling the inversion to appropriately fit the data. Additionally, the modelling and inversion algorithms are equivalent to the direct current electrical resistivity tomography counterpart. This enables direct application of advancements and approaches developed originally for electrical resistivity tomography. PubDate: 2017-10-03 DOI: 10.1093/gji/ggx416 Issue No:Vol. 212, No. 1 (2017)

Authors:Burjánek J; Gischig V, Moore J, et al. Pages: 297 - 310 Abstract: We analyse the ambient vibration response of Alpe di Roscioro (AdR), an incipient rock slope failure located above the village Preonzo in southern Switzerland. Following a major failure in May 2012 (volume ∼210 000 m3), the remaining unstable rock mass (∼140 000 m3) remains highly fractured and disrupted, and has been the subject of intensive monitoring. We deployed a small-aperture seismic array at the site shortly after the 2012 failure. The measured seismic response exhibited strong directional amplification (factors up to 35 at 3.5 Hz), higher than previously recorded on rock slopes. The dominant direction of ground motion was found to be parallel to the predominant direction of deformation and perpendicular to open fractures, reflecting subsurface structure of the slope. We then equipped the site with two semi-permanent seismic stations to monitor the seismic response with the goal of identifying changes caused by internal damage that may precede subsequent failure. Although failure has not yet occurred, our data reveal important variations in the seismic response. Amplification factors and resonant frequencies exhibit seasonal trends related (both directly and inversely) to temperature changes and are sensitive to freezing periods (resonant frequencies increase with temperature and during freezing). We attribute these effects to thermal expansion driving microcrack closure, in addition to ice formation, which increase fracture and bulk rock stiffness. We find the site response at AdR is linear over the measured range of weak input motions spanning two orders of magnitude. Our results further develop and refine ambient vibration methods used in rock slope hazard assessment. PubDate: 2017-10-03 DOI: 10.1093/gji/ggx424 Issue No:Vol. 212, No. 1 (2017)

Authors:Huang H; Zhang Z, Chen X. Pages: 311 - 323 Abstract: In this work, we investigate the effect of irregular topography on the dynamic rupture and resultant ground motions using the curved grid finite-difference method. The research is based on spontaneous dynamic rupture on vertical strike-slip faults by varying the shapes and relative locations of irregular topography to the critical supershear transition distance. The results show that seismic energy of a supershear earthquake can be transmitted farther with large amplitudes. However, its ground motion near the fault is weaker than that caused by a subshear (namely the sub-Rayleigh) rupture. Whether the irregular topography exhibits stronger ground motion overall depends on the irregular topography's ability to prevent the subshear-to-supershear transition. Finally, we also discuss the effects of the strength parameter S and a larger size of the irregular topography on the resultant ground motion. The modellings of San Andreas Fault with real and inverted topographical surfaces show the implications of the topographical effects from the real earthquake. PubDate: 2017-10-09 DOI: 10.1093/gji/ggx425 Issue No:Vol. 212, No. 1 (2017)

Authors:Xie J; Ni S, Chu R, et al. Pages: 324 - 332 Abstract: Accurate seismometer clock plays an important role in seismological studies including earthquake location and tomography. However, some seismic stations may have clock drift larger than 1 s (e.g. GSC in 1992), especially in early days of global seismic networks. The 26 s Persistent Localized (PL) microseism event in the Gulf of Guinea sometime excites strong and coherent signals, and can be used as repeating source for assessing stability of seismometer clocks. Taking station GSC, PAS and PFO in the TERRAscope network as an example, the 26 s PL signal can be easily observed in the ambient noise cross-correlation function between these stations and a remote station OBN with interstation distance about 9700 km. The travel-time variation of this 26 s signal in the ambient noise cross-correlation function is used to infer clock error. A drastic clock error is detected during June 1992 for station GSC, but not found for station PAS and PFO. This short-term clock error is confirmed by both teleseismic and local earthquake records with a magnitude of 25 s. Averaged over the three stations, the accuracy of the ambient noise cross-correlation function method with the 26 s source is about 0.3–0.5 s. Using this PL source, the clock can be validated for historical records of sparsely distributed stations, where the usual ambient noise cross-correlation function of short-period (<20 s) ambient noise might be less effective due to its attenuation over long interstation distances. However, this method suffers from cycling problem, and should be verified by teleseismic/local P waves. Further studies are also needed to investigate whether the 26 s source moves spatially and its effects on clock drift detection. PubDate: 2017-09-22 DOI: 10.1093/gji/ggx401 Issue No:Vol. 212, No. 1 (2017)

Authors:Li G; Li Y, Han B, et al. Pages: 333 - 344 Abstract: SUMMARYIn this study, the complex frequency-shifted perfectly matched layer (CFS-PML) in stretching Cartesian coordinates is successfully applied to 3-D frequency-domain marine controlled-source electromagnetic (CSEM) field modelling. The Dirichlet boundary, which is usually used within the traditional framework of EM modelling algorithms, assumes that the electric or magnetic field values are zero at the boundaries. This requires the boundaries to be sufficiently far away from the area of interest. To mitigate the boundary artefacts, a large modelling area may be necessary even though cell sizes are allowed to grow toward the boundaries due to the diffusion of the electromagnetic wave propagation. Compared with the conventional Dirichlet boundary, the PML boundary is preferred as the modelling area of interest could be restricted to the target region and only a few absorbing layers surrounding can effectively depress the artificial boundary effect without losing the numerical accuracy. Furthermore, for joint inversion of seismic and marine CSEM data, if we use the PML for CSEM field simulation instead of the conventional Dirichlet, the modelling area for these two different geophysical data collected from the same survey area could be the same, which is convenient for joint inversion grid matching. We apply the CFS-PML boundary to 3-D marine CSEM modelling by using the staggered finite-difference discretization. Numerical test indicates that the modelling algorithm using the CFS-PML also shows good accuracy compared to the Dirichlet. Furthermore, the modelling algorithm using the CFS-PML shows advantages in computational time and memory saving than that using the Dirichlet boundary. For the 3-D example in this study, the memory saving using the PML is nearly 42 per cent and the time saving is around 48 per cent compared to using the Dirichlet. PubDate: 2017-09-13 DOI: 10.1093/gji/ggx382 Issue No:Vol. 212, No. 1 (2017)

Authors:Oliveira B; Afonso J, Zlotnik S, et al. Pages: 345 - 388 Abstract: SUMMARYWe present a conceptual and numerical approach to model processes in the Earth’s interior that involve multiple phases that simultaneously interact thermally, mechanically and chemically. The approach is truly multiphase in the sense that each dynamic phase is explicitly modelled with an individual set of mass, momentum, energy and chemical mass balance equations coupled via interfacial interaction terms. It is also truly multicomponent in the sense that the compositions of the system and its constituent phases are expressed by a full set of fundamental chemical components (e.g. SiO2, Al2O3, MgO, etc.) rather than proxies. These chemical components evolve, react with and partition into different phases according to an internally consistent thermodynamic model. We combine concepts from Ensemble Averaging and Classical Irreversible Thermodynamics to obtain sets of macroscopic balance equations that describe the evolution of systems governed by multiphase multicomponent reactive transport (MPMCRT). Equilibrium mineral assemblages, their compositions and physical properties, and closure relations for the balance equations are obtained via a ‘dynamic’ Gibbs free-energy minimization procedure (i.e. minimizations are performed on-the-fly as needed by the simulation). Surface tension and surface energy contributions to the dynamics and energetics of the system are taken into account. We show how complex rheologies, that is, visco-elasto-plastic, and/or different interfacial models can be incorporated into our MPMCRT ensemble-averaged formulation. The resulting model provides a reliable platform to study the dynamics and nonlinear feedbacks of MPMCRT systems of different nature and scales, as well as to make realistic comparisons with both geophysical and geochemical data sets. Several numerical examples are presented to illustrate the benefits and limitations of the model. PubDate: 2017-09-22 DOI: 10.1093/gji/ggx399 Issue No:Vol. 212, No. 1 (2017)

Authors:Hu S; Zhu L. Pages: 390 - 399 Abstract: SUMMARYSolving seismic waveform inverse problem by the linearization approach requires calculating partial derivatives of seismograms (differential seismograms) with respect to velocity model parameters. Here we extend our previous work on calculation of differential teleseismic receiver functions using analytic partial derivatives to regional waveforms. We derived analytic partial derivatives of displacement kernels for both P–SV and SH waves generated by a point source in a multilayered half-space. They are expressed explicitly in terms of the analytic partial derivatives of the Haskell propagator matrix and the source vector. Differential seismograms are then computed using the wavenumber–frequency double integration method. We implemented an efficient algorithm to speed up the computation by storing intermediate matrix products. The total computation time for computing differential seismograms with respect to all the layers is only about one extra forward-problem computation time. Numerical tests show that our derivations are correct and the implementation is efficient. Synthetic and real data waveform inversions using more accurate analytic differential seismograms perform better than using finite-difference differential seismograms. PubDate: 2017-10-10 DOI: 10.1093/gji/ggx433 Issue No:Vol. 212, No. 1 (2017)

Authors:Xu X; Sandwell D, Bassett D. Pages: 400 - 411 Abstract: SUMMARYWe have developed a data-driven spectral expansion inversion method to place bounds on the downdip rupture depth of large megathrust earthquakes having good InSAR and GPS coverage. This inverse theory approach is used to establish the set of models that are consistent with the observations. In addition, the inverse theory method demonstrates that the spatial resolution of the slip models depends on two factors, the spatial coverage and accuracy of the surface deformation measurements, and the slip depth. Application of this method to the 2010 Mw 8.8 Maule Earthquake shows a slip maximum at 19 km depth tapering to zero at ∼40 km depth. In contrast, the continent–continent megathrust earthquakes of the Himalayas, for example 2015 Mw 7.8 Gorkha Earthquake, shows a slip maximum at 9 km depth tapering to zero at ∼18 km depth. The main question is why is the maximum slip depth of the continental megathrust earthquake only 50 per cent of that observed in oceanic megathrust earthquakes. To understand this difference, we have developed a simple 1-D heat conduction model that includes the effects of uplift and surface erosion. The relatively low erosion rates above the ocean megathrust results in a geotherm where the 450–600 °C transition is centred at ∼40 km depth. In contrast, the relatively high average erosion rates in the Himalayas of ∼1 mm yr–1 results in a geotherm where the 450–600 °C transition is centred at ∼20 km. Based on these new observations and models, we suggest that the effect of erosion rate on temperature explains the difference in the maximum depth of the seismogenic zone between Chile and the Himalayas. PubDate: 2017-09-22 DOI: 10.1093/gji/ggx408 Issue No:Vol. 212, No. 1 (2017)

Authors:Spica Z; Perton M, Nakata N, et al. Pages: 412 - 421 Abstract: SUMMARYA new interpretation of the horizontal to vertical (H/V) spectral ratio in terms of the Diffuse Field Assumption (DFA) has fuelled a resurgence of interest in that approach. The DFA links H/V measurements to Green’s function retrieval through autocorrelation of the ambient seismic field. This naturally allows for estimation of layered velocity structure. In this contribution, we further explore the potential of H/V analysis. Our study is facilitated by a distributed array of surface and co-located borehole stations deployed at multiple depths, and by detailed prior information on velocity structure that is available due to development of the Groningen gas field. We use the vertical distribution of H/V spectra recorded at discrete depths inside boreholes to obtain shear wave velocity models of the shallow subsurface. We combine both joint H/V inversion and borehole interferometry to reduce the non-uniqueness of the problem and to allow faster convergence towards a reliable velocity model. The good agreement between our results and velocity models from an independent study validates the methodology, demonstrates the power of the method, but more importantly provides further constraints on the shallow velocity structure, which is an essential component of integrated hazard assessment in the area. PubDate: 2017-10-05 DOI: 10.1093/gji/ggx426 Issue No:Vol. 212, No. 1 (2017)

Authors:Kumar N; Narayan J. Pages: 422 - 441 Abstract: SUMMARYThis paper presents the site–city interaction (SCI) effects on the response of closely spaced structures under double resonance condition ($F_{02{\rm{D}}}^{\rm{S}} = F_{02{\rm{D}}}^{\rm{B}}$), where $F_{02{\rm{D}}}^{\rm{S}}$ and $F_{02{\rm{D}}}^{\rm{B}}$ are fundamental frequencies of 2-D structure and 2-D basin, respectively. This paper also presents the development of empirical relations to predict the $F_{02{\rm{D}}}^{\rm{B}}$ of elliptical and trapezoidal basins for both the polarizations of the S wave. Simulated results revealed that $F_{02{\rm{D}}}^{\rm{B}}$ of a 2-D basin very much depends on its geometry, shape ratio and polarization of the incident S wave. The obtained spectral amplification factor (SAF) at $F_{02{\rm{D}}}^{\rm{S}}$ of a standalone structure in a 2-D basin is greater than that in the 1-D case under double resonance condition. A considerable reduction of the fundamental resonance frequency of structures due to the SCI effects is observed for both the polarizations of the S wave. The SAFs at $F_{02{\rm{D}}}^{\rm{S}}$ of closely spaced structures due to SCI effects is larger in the case of SV than SH waves. A splitting of the fundamental-mode frequency bandwidth along with the drastic decrease of SAF due to the SCI effects is obtained. The findings of this paper raise the question concerning the validity of the predicted response of standalone structure based on soil–structure interaction for the design of structures in a 2-D small basin, in an urban environment. PubDate: 2017-09-22 DOI: 10.1093/gji/ggx397 Issue No:Vol. 212, No. 1 (2017)

Authors:Meng H; Ben-Zion Y. Pages: 442 - 457 Abstract: SUMMARYWe present a technique to detect small earthquakes not included in standard catalogues using data from a dense seismic array. The technique is illustrated with continuous waveforms recorded in a test day by 1108 vertical geophones in a tight array on the San Jacinto fault zone. Waveforms are first stacked without time-shift in nine non-overlapping subarrays to increase the signal-to-noise ratio. The nine envelope functions of the stacked records are then multiplied with each other to suppress signals associated with sources affecting only some of the nine subarrays. Running a short-term moving average/long-term moving average (STA/LTA) detection algorithm on the product leads to 723 triggers in the test day. Using a local P-wave velocity model derived for the surface layer from Betsy gunshot data, 5 s long waveforms of all sensors around each STA/LTA trigger are beamformed for various incident directions. Of the 723 triggers, 220 are found to have localized energy sources and 103 of these are confirmed as earthquakes by verifying their observation at 4 or more stations of the regional seismic network. This demonstrates the general validity of the method and allows processing further the validated events using standard techniques. The number of validated events in the test day is >5 times larger than that in the standard catalogue. Using these events as templates can lead to additional detections of many more earthquakes. PubDate: 2017-09-25 DOI: 10.1093/gji/ggx404 Issue No:Vol. 212, No. 1 (2017)

Authors:Guan W; Shi P, Hu H. Pages: 458 - 475 Abstract: SUMMARYIn this study, we theoretically analyse the contributions of the four poroelastic-wave potentials to seismoelectromagnetic (SEM) wavefields, verify the validity of the quasi-static calculation of the electric field and provide a method to calculate the magnetic field by using the curl-free electric field. Calculations show that both the fast and slow P waves and the SH and SV waves have non-negligible contributions to the SEM fields. The S waves have indirect contribution to the electric field through the EM conversion from the magnetic field, although the direct contribution due to streaming current is negligible if EM wavenumbers are much smaller than those of the S waves. The P waves have indirect contribution to the magnetic field through EM conversion from the electric field, although the direct contribution is absent. The quasi-static calculation of the electric field is practicable since it is normally satisfied in reality that the EM wavenumbers are much smaller than those of poroelastic waves. While the direct contribution of the S waves and the higher-order EM conversions are ignored, the first-order EM conversion from the S-wave-induced magnetic field is reserved through the continuity of the electric-current density. To calculate the magnetic field on this basis, we separate the quasi-static electric field into a rotational and an irrotational part. The magnetic-field solutions are derived through Hertz vectors in which the coefficients of the magnetic Hertz vector are determined from the magnetic-field continuities and those of the electric Hertz vector originate from the irrotational part of the quasi-static electric field. PubDate: 2017-10-09 DOI: 10.1093/gji/ggx417 Issue No:Vol. 212, No. 1 (2017)

Authors:Király-Proag E; Gischig V, Zechar J, et al. Pages: 476 - 490 Abstract: SUMMARYIn recent years, human-induced seismicity has become a more and more relevant topic due to its economic and social implications. Several models and approaches have been developed to explain underlying physical processes or forecast induced seismicity. They range from simple statistical models to coupled numerical models incorporating complex physics. We advocate the need for forecast testing as currently the best method for ascertaining if models are capable to reasonably accounting for key physical governing processes—or not. Moreover, operational forecast models are of great interest to help on-site decision-making in projects entailing induced earthquakes. We previously introduced a standardized framework following the guidelines of the Collaboratory for the Study of Earthquake Predictability, the Induced Seismicity Test Bench, to test, validate, and rank induced seismicity models. In this study, we describe how to construct multicomponent ensemble models based on Bayesian weightings that deliver more accurate forecasts than individual models in the case of Basel 2006 and Soultz-sous-Forêts 2004 enhanced geothermal stimulation projects. For this, we examine five calibrated variants of two significantly different model groups: (1) Shapiro and Smoothed Seismicity based on the seismogenic index, simple modified Omori-law-type seismicity decay, and temporally weighted smoothed seismicity; (2) Hydraulics and Seismicity based on numerically modelled pore pressure evolution that triggers seismicity using the Mohr–Coulomb failure criterion. We also demonstrate how the individual and ensemble models would perform as part of an operational Adaptive Traffic Light System. Investigating seismicity forecasts based on a range of potential injection scenarios, we use forecast periods of different durations to compute the occurrence probabilities of seismic events M ≥ 3. We show that in the case of the Basel 2006 geothermal stimulation the models forecast hazardous levels of seismicity days before the occurrence of felt events. PubDate: 2017-09-16 DOI: 10.1093/gji/ggx393 Issue No:Vol. 212, No. 1 (2017)

Authors:Roselli P; Marzocchi W, Mariucci M, et al. Pages: 491 - 508 Abstract: SUMMARYIn this paper, we put forward a procedure that aims to forecast focal mechanism of future earthquakes. One of the primary uses of such forecasts is in probabilistic seismic hazard analysis (PSHA); in fact, aiming at reducing the epistemic uncertainty, most of the newer ground motion prediction equations consider, besides the seismicity rates, the forecast of the focal mechanism of the next large earthquakes as input data. The data set used to this purpose is relative to focal mechanisms taken from the latest stress map release for Italy containing 392 well-constrained solutions of events, from 1908 to 2015, with Mw ≥ 4 and depths from 0 down to 40 km. The data set considers polarity focal mechanism solutions until to 1975 (23 events), whereas for 1976–2015, it takes into account only the Centroid Moment Tensor (CMT)-like earthquake focal solutions for data homogeneity. The forecasting model is rooted in the Total Weighted Moment Tensor concept that weighs information of past focal mechanisms evenly distributed in space, according to their distance from the spatial cells and magnitude. Specifically, for each cell of a regular 0.1° × 0.1° spatial grid, the model estimates the probability to observe a normal, reverse, or strike-slip fault plane solution for the next large earthquakes, the expected moment tensor and the related maximum horizontal stress orientation. These results will be available for the new PSHA model for Italy under development. Finally, to evaluate the reliability of the forecasts, we test them with an independent data set that consists of some of the strongest earthquakes with Mw ≥ 3.9 occurred during 2016 in different Italian tectonic provinces. PubDate: 2017-09-16 DOI: 10.1093/gji/ggx383 Issue No:Vol. 212, No. 1 (2017)

Authors:Katou M; Abe S, Saito H, et al. Pages: 509 - 521 Abstract: SUMMARYIn this study, experiments on seismic reciprocity utilizing various acquisition tools, including airguns, vibrators, dynamite, hydrophones and geophones, in combination are conducted on onshore–offshore transects that crossed the coastline. Because airguns, vibrators and dynamite generate different wavelets (i.e. have different source functions) and because hydrophones and geophones observe different physical field quantities, waveform matching processes must be applied to the recorded time-series to perform integrated seismic profiling. For the best integration of data acquired with onshore and offshore instruments, processes which compensate the instrumental differences with no loss of bandwidth should be employed. We present a combined onshore–offshore acquisition system and associated signal processing procedures for obtaining broad-band seismograms that support the comparison of reciprocal data. In the first of two experiments, we considered the data acquired by using two receiver–source pairs, one consisting of an onshore geophone and offshore airgun and the other consisting of an offshore hydrophone and onshore vibrator. The two waveforms were found to be in very good agreement after applying data processing including signature deconvolution. In the second experiment, we succeeded in matching time-series data from a pair consisting of an onshore geophone and offshore airgun and a pair consisting of an offshore hydrophone and an onshore dynamite source, by tracing subsurface dilatation through the use of a buried array of multicomponent geophones and the moving least squares method. Both signature deconvolution and the moving least squares method preserved the bandwidth of each recorded seismogram and were noise-independent. This study confirms that seismic reciprocity for onshore–offshore seismic profiling can be achieved with a combined survey system using all required instrument types and modern data processing preserving recorded bandwidth. PubDate: 2017-09-13 DOI: 10.1093/gji/ggx374 Issue No:Vol. 212, No. 1 (2017)

Authors:Ray A; Kaplan S, Washbourne J, et al. Pages: 522 - 542 Abstract: SUMMARYLimited illumination, insufficient offset, noisy data and poor starting models can pose challenges for seismic full waveform inversion. We present an application of a tree based Bayesian inversion scheme which attempts to mitigate these problems by accounting for data uncertainty while using a mildly informative prior about subsurface structure. We sample the resulting posterior model distribution of compressional velocity using a trans-dimensional (trans-D) or Reversible Jump Markov chain Monte Carlo method in the wavelet transform domain of velocity. This allows us to attain rapid convergence to a stationary distribution of posterior models while requiring a limited number of wavelet coefficients to define a sampled model. Two synthetic, low frequency, noisy data examples are provided. The first example is a simple reflection + transmission inverse problem, and the second uses a scaled version of the Marmousi velocity model, dominated by reflections. Both examples are initially started from a semi-infinite half-space with incorrect background velocity. We find that the trans-D tree based approach together with parallel tempering for navigating rugged likelihood (i.e. misfit) topography provides a promising, easily generalized method for solving large-scale geophysical inverse problems which are difficult to optimize, but where the true model contains a hierarchy of features at multiple scales. PubDate: 2017-10-09 DOI: 10.1093/gji/ggx428 Issue No:Vol. 212, No. 1 (2017)

Authors:Cheng Y; Renner J. Pages: 543 - 565 Abstract: SUMMARYPeriodic pumping tests were conducted using a double-packer probe placed at four different depth levels in borehole GDP-1 at Grimselpass, Central Swiss Alps, penetrating a hydrothermally active fault. The tests had the general objective to explore the potential of periodic testing for hydraulic characterization of faults, representing inherently complex heterogeneous hydraulic features that pose problems for conventional approaches. Site selection reflects the specific question regarding the value of this test type for quality control of hydraulic stimulations of potential geothermal reservoirs. The performed evaluation of amplitude ratio and phase shift between pressure and flow rate in the pumping interval employed analytical solutions for various flow regimes. In addition to the previously presented 1-D and radial-flow models, we extended the one for radial flow in a system of concentric shells with varying hydraulic properties and newly developed one for bilinear flow. In addition to these injectivity analyses, we pursued a vertical-interference analysis resting on observed amplitude ratio and phase shift between the periodic pressure signals above or below packers and in the interval by numerical modeling of the non-radial-flow situation. When relying on the same model the order of magnitude of transmissivity values derived from the analyses of periodic tests agrees with that gained from conventional hydraulic tests. The field campaign confirmed several advantages of the periodic testing, for example, reduced constraints on testing time relative to conventional tests since a periodic signal can easily be separated from changing background pressure by detrending and Fourier transformation. The discrepancies between aspects of the results from the periodic tests and the predictions of the considered simplified models indicate a hydraulically complex subsurface at the drill site that exhibits also hydromechanical features in accord with structural information gained from logging. The exploratory modeling of vertical injectivity shows its potential for analysing hydraulic anisotropy. Yet, more comprehensive modeling will be required to take full advantage of all the pressure records typically acquired when using a double-packer probe for periodic tests. PubDate: 2017-09-16 DOI: 10.1093/gji/ggx390 Issue No:Vol. 212, No. 1 (2017)

Authors:Sager K; Ermert L, Boehm C, et al. Pages: 566 - 590 Abstract: SUMMARYIn this work we investigate fundamentals of a method—referred to as full waveform ambient noise inversion—that improves the resolution of tomographic images by extracting waveform information from interstation correlation functions that cannot be used without knowing the distribution of noise sources. The fundamental idea is to drop the principle of Green function retrieval and to establish correlation functions as self-consistent observables in seismology. This involves the following steps: (1) We introduce an operator-based formulation of the forward problem of computing correlation functions. It is valid for arbitrary distributions of noise sources in both space and frequency, and for any type of medium, including 3-D elastic, heterogeneous and attenuating media. In addition, the formulation allows us to keep the derivations independent of time and frequency domain and it facilitates the application of adjoint techniques, which we use to derive efficient expressions to compute first and also second derivatives. The latter are essential for a resolution analysis that accounts for intra- and interparameter trade-offs. (2) In a forward modelling study we investigate the effect of noise sources and structure on different observables. Traveltimes are hardly affected by heterogeneous noise source distributions. On the other hand, the amplitude asymmetry of correlations is at least to first order insensitive to unmodelled Earth structure. Energy and waveform differences are sensitive to both structure and the distribution of noise sources. (3) We design and implement an appropriate inversion scheme, where the extraction of waveform information is successively increased. We demonstrate that full waveform ambient noise inversion has the potential to go beyond ambient noise tomography based on Green function retrieval and to refine noise source location, which is essential for a better understanding of noise generation. Inherent trade-offs between source and structure are quantified using Hessian-vector products. PubDate: 2017-10-09 DOI: 10.1093/gji/ggx429 Issue No:Vol. 212, No. 1 (2017)

Authors:Asgharzadeh M; Hashemi H, von Frese R. Pages: 591 - 611 Abstract: SUMMARYForward modeling is the basis of gravitational anomaly inversion that is widely applied to map subsurface mass variations. This study uses numerical least-squares Gauss–Legendre quadrature (GLQ) integration to evaluate the gravitational potential, anomaly and gradient components of the vertical cylindrical prism element. These results, in turn, may be integrated to accurately model the complete gravitational effects of fluid bearing rock formations and other vertical cylinder-like geological bodies with arbitrary variations in shape and density. Comparing the GLQ gravitational effects of uniform density, vertical circular cylinders against the effects calculated by a number of other methods illustrates the veracity of the GLQ modeling method and the accuracy limitations of the other methods. Geological examples include modeling the gravitational effects of a formation washout to help map azimuthal variations of the formation’s bulk densities around the borehole wall. As another application, the gravitational effects of a seismically and gravimetrically imaged salt dome within the Laurentian Basin are evaluated for the velocity, density and geometric properties of the Basin’s sedimentary formations. PubDate: 2017-10-05 DOI: 10.1093/gji/ggx413 Issue No:Vol. 212, No. 1 (2017)

Authors:Wang H; Fehler M. Pages: 612 - 626 Abstract: SUMMARYThe bonding quality of the seal formed by the cement or collapse material between casing and formation rock is critical for the hydraulic isolation of reservoir layers with shallow aquifers, production and environmental safety, and plug and abandonment issues. Acoustic logging is a very good tool for evaluating the condition of the bond between different interfaces. The understanding of the acoustic logging wavefields in wells with single casing is still incomplete. We use a 3-D finite difference method to simulate wireline monopole wavefields in a single cased borehole with different bonding conditions at two locations: (1) between the cement and casing and (2) between the cement and formation. Pressure snapshots and waveforms for different models are shown, which allow us to better understand the wave propagation. Modal dispersion curves and data processing methods such as velocity–time semblance and dispersion analysis facilitate the identification of propagation modes in the different models. We find that the P wave is submerged in the casing modes and the S wave has poor coherency when the cement is replaced with fluid. The casing modes are strong when cement next to the casing is partially or fully replaced with fluid. The amplitude of these casing modes can be used to determine the bonding condition of the interface between casing and cement. However, the limited variation of the amplitude with fluid thickness means that amplitude measurements may lead to an ambiguous interpretation. When the cement next to the formation is partially replaced with fluid, the modes propagate in the combination of steel casing and cement and the velocities are highly dependent on the cement thickness. However, if the cement thickness is large (more than 2/3 of the annulus between casing and rock), the arrival time of the first arrival approximates that of the formation compressional wave when cement is good. It would highly likely that an analyst could misjudge cement quality because the amplitudes of these modes are very small and their arrival times are very near to the formation P arrival time. It is possible to use the amplitude to estimate the thickness of the cement sheath because the variation of amplitude with thickness is strong. While the Stoneley mode (ST1) propagates in the borehole fluid, a slow Stoneley mode (ST2) appears when there is a fluid column in the annulus between the casing and formation rock. The velocity of ST2 is sensitive to the total thickness of the fluid column in the annulus independent of the location of the fluid in the casing annulus. We propose a full waveform method, which includes the utilization of the amplitude of the first arrival and also the velocity of the ST2 wave, to estimate the bonding condition of multiple interfaces. These two measurements provide more information than the current method that uses only the first arrival to evaluate the bonding interfa next to the casing. PubDate: 2017-10-12 DOI: 10.1093/gji/ggx437 Issue No:Vol. 212, No. 1 (2017)

Authors:Wang J; Tang H, Xiao H, et al. Pages: 646 - 658 Abstract: SUMMARYFlood hazards caused by tsunamis can cause enormous amounts of casualties and property losses. However, these hazards often occur at unexpected locations, which poses challenges for direct observations of flood source characteristics (e.g. flow speed and water depth in tsunamis-induced inundations). For this reason, inverse modelling methods based on the information contained in the deposit are indispensable for deciphering the quantitative characteristics of the flood sources. In this work, we propose an inversion scheme based on Ensemble Kalman Filtering (EnKF) to infer tsunami flow characteristics from sediment deposits. In contrast to traditional data assimilation methods using EnKF, in the current work the system state is augmented by including both the physical states (sediment fluxes) that are observable and the unknown parameters (flow speed and flow depth) to be inferred. A novelty of this work lies in formulating the tsunami inverse problem in a statistically rigorous way. Based on the Bayesian inference theory, the inversion scheme provides quantified uncertainties on the inferred quantities, which clearly distinguishes the present method with existing schemes for tsunami inversion. Two test cases with synthetic observation data are used to verify the proposed inversion scheme. Numerical results show that the tsunami flow variables inferred from the sediment deposit information agree with the synthetic truths very well, which clearly demonstrated the merits of the proposed inversion scheme. Moreover, a realistic application of the proposed inversion scheme with the field data from the 2006 South Java Tsunami is studied, and the results are validated against both previous inversion results and field data reported in the literature. The comparisons show excellent performance of the proposed approach. PubDate: 2017-10-11 DOI: 10.1093/gji/ggx435 Issue No:Vol. 212, No. 1 (2017)

Authors:Li L; Becker D, Chen H, et al. Pages: 659 - 678 Abstract: SUMMARYWaveform-based seismic location methods can reliably and automatically image weak seismic sources, such as microseismic events and microtremors. Besides the classical diffraction stacking operator which is based on the one-way traveltime, correlation-based imaging methods are another subcategory of waveform-based methods using differential traveltime. In this work, we systematically analyse the existing correlation-based methods and propose a novel hybrid correlation stacking method, which belongs to waveform-based relative location methods. The double differential traveltime from an event pair (i.e. a master event and a target event) to a receiver pair is used to stack the corresponding double correlation waveforms in this new approach. We generalize the correlation-based methods using a unified formula by describing cross-correlation stacking with beamforming. A thorough analysis of these imaging operators using synthetic and field data examples reveals their different characteristics of imaging resolution and level of redundancy, and a moderate level of redundancy can ensure both the accuracy and stability of correlation-based imaging methods, while an extremely high or low level of redundancy will hinder their performance in locating weak seismic events. The examples also demonstrate the potential disadvantage of using multiple phases with inaccurate velocity models for waveform-based location methods. PubDate: 2017-10-12 DOI: 10.1093/gji/ggx436 Issue No:Vol. 212, No. 1 (2017)

Authors:Wang L; Chen C, Thomas M, et al. Pages: 679 - 693 Abstract: SUMMARYTerrestrial water storage (TWS) changes in the Tibetan Plateau (TP) are sensitive indicators for water dynamics associated with climate variability. Joint analyses using both GRACE space mission and satellite altimetry data are increasingly being used to monitor TWS. The objective of this study is to confirm that it is possible to reliably monitor water storage changes in large lakes based on integrative analysis of GRACE data. This study focuses on data integrated and analysed for Lake Qinghai located in the northeast TP, and shows a clear continuous water-level rise since 2004. We have developed a simple framework to estimate water storage variations in individual regions using a spatial averaging kernel, while simultaneously minimizing the effects resulting from uncertainties of GRACE data using Land Surface Models (LSMs) and in situ measurements. Water storage anomalies not related to lakes, such as soil moisture, snow and reservoirs, are estimated using GLDAS/Noah and reservoir gauge station for the period 2004–2012. Our results show that the rate of rise in mass of the GRACE-derived TWS (post GLDAS/Noah anomaly removal) is calculated to be 0.27 ± 0.12 cm yr−1, or an average water-level increase rate of 0.20 ± 0.09 m yr−1 after GRACE-derived TWS (i.e. 0.27 ± 0.12 cm yr−1) multiplied by 1/1.34 to recover the ‘real’ mass variation signal using scaling factor method based on basin function from grids of Lake Qinghai from 2004 to 2012, which is equivalent to the volume change 0.86 ± 0.37 km3 yr−1 and mainly caused by the fast expansion of Lake Qinghai (0.44 ± 0.04 km3 yr−1 from altimetry/in situ measurements) and impoundment of Longyangxia reservoir (0.28 ± 0.17 km3 yr−1 based on the linear height–volume relationship and in situ water-level observations). The residual signal (GRACE−GLDAS/Noah−reservoir−Lake Qinghai) likely reflects the mass leakage from the surrounding lakes in the regions (i.e. Har, Gyaring and Ngoring) and groundwater contributions (i.e. groundwater is not included in GLDAS/Noah) due to the limited spatial resolution of GRACE. The results suggest that the combined use of LSM and GRACE measurements is a useful method for monitoring changes in the mass of large lakes. Additionally, our analysis shows that it is necessary to improve LSM results with in situ forcing parameters and groundwater level data, which will reduce the uncertainty in the application of GRACE data. There is still a need to use complementary models or in situ observations to eliminate the influence of glacial isostatic adjustments and tectonic processes. PubDate: 2017-10-14 DOI: 10.1093/gji/ggx443 Issue No:Vol. 212, No. 1 (2017)