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

Authors:Červený V; Pšenčík I. First page: 561 Abstract: Integral superposition of Gaussian beams is a useful generalization of the standard ray theory. It removes some of the deficiencies of the ray theory like its failure to describe properly behaviour of waves in caustic regions. It also leads to a more efficient computation of seismic wavefields since it does not require the time-consuming two-point ray tracing. We present the formula for a high-frequency elementary Green function expressed in terms of the integral superposition of Gaussian beams for inhomogeneous, isotropic or anisotropic, layered structures, based on the dynamic ray tracing (DRT) in Cartesian coordinates. For the evaluation of the superposition formula, it is sufficient to solve the DRT in Cartesian coordinates just for the point-source initial conditions. Moreover, instead of seeking 3 × 3 paraxial matrices in Cartesian coordinates, it is sufficient to seek just 3 × 2 parts of these matrices. The presented formulae can be used for the computation of the elementary Green function corresponding to an arbitrary direct, multiply reflected/transmitted, unconverted or converted, independently propagating elementary wave of any of the three modes, P, S1 and S2. Receivers distributed along or in a vicinity of a target surface may be situated at an arbitrary part of the medium, including ray-theory shadow regions. The elementary Green function formula can be used as a basis for the computation of wavefields generated by various types of point sources (explosive, moment tensor). PubDate: 2017-05-09 DOI: 10.1093/gji/ggx183

Authors:Li Y; Pan J, Wu Q, et al. First page: 570 Abstract: A new 3-D shear wave velocity model of the northeastern (NE) Tibetan Plateau and western Sino-Korea Craton is presented. The model is based on Rayleigh waves recorded at 650 portable stations deployed in the region. Interstation phase and group velocity dispersions for more than 18 000 paths were estimated using the two-station method and then inverted to produce phase velocity maps for 10–80 s period and group velocity maps for 10–60 s period. Local 1-D shear wave velocity profiles for each 0.5° × 0.5°grid node were obtained by inverting Rayleigh wave dispersions obtained in this study together with previously published Rayleigh wave group velocities between 60 and 145 s and then assembled into a 3-D shear wave velocity model. The images obtained reveal an obvious mid-crustal low-velocity zone (LVZ) and low-velocity anomaly in the upper mantle beneath the Songpan–Ganzi terrane. The mid-crustal LVZ can be explained by the presence of partial melting, most likely from asthenospheric upwelling. The existence of a relatively weak mid-crustal LVZ beneath the Qilian orogeny is also confirmed. This LVZ is likely caused by the presence of deep crustal fluids. The Yinchuan–Hetao graben is characterized by relatively low velocities that extend to at least 200 km below the Earth's surface, strongly contrasting with the seismically fast lithosphere of the Ordos and Alax blocks. The model presented here shows the presence of a relatively thick and high-velocity lithosphere beneath the Ordos and NE Alxa blocks, as well as evidence of thinning of the lithosphere beneath the southwestern Alxa and Qilian blocks, indicating that the Alxa block is not subducting beneath NE Tibet. PubDate: 2017-05-03 DOI: 10.1093/gji/ggx181

Authors:Hainzl SS; Christophersen AA. First page: 585 Abstract: Earthquake clustering can be well described by the Epidemic Type Aftershock Sequence model (ETAS), where each earthquake potentially triggers its own aftershocks. The temporal decay of aftershocks is most commonly modelled with a power law, the so-called Omori–Utsu law. However, new results suggest that alternative decay functions may be more appropriate. One recent study found that a version of the ETAS model fitted the data better when the Omori–Utsu law was truncated in time. A finite triggering time is consistent with the rate-state model that expects an exponential roll-off after a finite time following the initial power law decay. Another recent study compared a power law, pure exponential and stretched exponential and found that the stretched exponential described the overall decay of aftershocks best. Our aim is to find the best temporal aftershock decay function within the ETAS model framework. We investigate six decay functions; three power laws and three exponential decays. The power laws are an unlimited Omori–Utsu law, a sharply truncated Omori–Utsu law, and an exponential roll-off consistent with the rate-state friction model. The exponential decay functions are the pure exponential, stretched exponential and a modified stretched exponential. We fit model parameters for each decay function to 326 individual earthquake sequences from four regional and one global earthquake catalogue. The three models that fit most of the sequences the best are the truncated Omori–Utsu law (32 per cent of sequences), the power law based on the rate-state friction model (26 per cent) and the unlimited Omori–Utsu law (23 per cent). When the parameters are not fitted individually but the median model parameters are used for each function, the modified stretched exponential function fits most (28 per cent) sequences the best, followed by the unlimited Omori–Utsu law (22 per cent) and the stretched exponential (18 per cent). However, the majority of sequences (53 per cent) is still best fit by a power law. Out of all the tested decay functions, the one based on the rate-state friction model is the only one that performs in a majority of cases better than the Omori–Utsu law for fixed parameters. This suggests that it could be a potential candidate to replace the unlimited Omori–Utsu law in ETAS-model-based earthquake forecasts. PubDate: 2017-05-04 DOI: 10.1093/gji/ggx184

Authors:Dicelis G; Assumpção M, Prado R, et al. First page: 594 Abstract: We investigate a long series of small earthquakes (magnitude < 3) with annual cycles between 2004 and 2010 that occurred in the northeastern Paraná Basin, Brazil. These events were attributed to pore pressure increase in a fractured aquifer caused by the drilling of several water wells down to depths of ∼200 m. Because of the poor depth constraints of the initial study, we relocated ∼1000 microtremors recorded by the local seismographic network using a more accurate velocity model. To better relocate the events, we based the velocity model on geophysical survey data obtained using several techniques, including seismic refraction, surface wave dispersion, vertical electrical sounding, high-frequency receiver functions and the time-domain electromagnetic method. The best 1-D model was calibrated using a simultaneous inversion of hypocentres and a velocity model including station corrections. The resulting focal depths, which are mostly between 100 and 175 m, place the events at depths consistent with the confined aquifer within the basaltic pack. This result confirms that the earthquakes are related to geological stress disturbances (pore pressures) in pre-existing fracture zones within the basalt layer caused by the perforation of water wells used for irrigation. We used a combination of cross-correlations and arrival times to analyse a cluster of 19 earthquakes. The improved hypocentre distribution of this cluster allowed the determination of a clear fault-plane solution, which indicates a normal fault striking WNW-ESE and dipping to the north, with an NNE-SSW extension (T-axis). This mechanism complements stress data collected in southeastern Brazil and is consistent with the regional stress regime. PubDate: 2017-05-01 DOI: 10.1093/gji/ggx180

Authors:Hartstra IE; Almagro Vidal CC, Wapenaar KK. First page: 609 Abstract: Our objective is to complement lithospheric seismic tomography with an interferometric method to retrieve high-resolution reflectivity images from local earthquake recordings. The disadvantage of using local earthquakes for the retrieval of reflected body-waves is their usual sparse distribution. We propose an alternative formulation of passive seismic interferometry by multidimensional deconvolution (MDD) which uses the multiples in the full recordings to compensate for missing illumination angles. This method only requires particle-velocity recordings at the surface from passive transient sources and retrieves body-wave reflection responses without free-surface multiples. We conduct an acoustic modelling experiment to compare this formulation to a previous MDD method and Green's function retrieval by crosscorrelation for different source distributions. We find that in the case of noise-contaminated recordings obtained under severely limited and irregular illumination conditions, the alternative MDD method introduced here still retrieves the complete reflection response without free-surface multiples where the other interferometric methods break down. PubDate: 2017-04-01 DOI: 10.1093/gji/ggx120

Authors:Peacock S; Douglas A, Bowers D. First page: 621 Abstract: Body-wave magnitudes (mb) of 606 seismic disturbances caused by presumed underground nuclear test explosions at specific test sites between 1964 and 1996 have been derived from station amplitudes collected by the International Seismological Centre (ISC), by a joint inversion for mb and station-specific magnitude corrections. A maximum-likelihood method was used to reduce the upward bias of network mean magnitudes caused by data censoring, where arrivals at stations that do not report arrivals are assumed to be hidden by the ambient noise at the time. Threshold noise levels at each station were derived from the ISC amplitudes using the method of Kelly and Lacoss, which fits to the observed magnitude–frequency distribution a Gutenberg–Richter exponential decay truncated at low magnitudes by an error function representing the low-magnitude threshold of the station. The joint maximum-likelihood inversion is applied to arrivals from the sites: Semipalatinsk (Kazakhstan) and Novaya Zemlya, former Soviet Union; Singer (Lop Nor), China; Mururoa and Fangataufa, French Polynesia; and Nevada, USA. At sites where eight or more arrivals could be used to derive magnitudes and station terms for 25 or more explosions (Nevada, Semipalatinsk and Mururoa), the resulting magnitudes and station terms were fixed and a second inversion carried out to derive magnitudes for additional explosions with three or more arrivals. 93 more magnitudes were thus derived. During processing for station thresholds, many stations were rejected for sparsity of data, obvious errors in reported amplitude, or great departure of the reported amplitude-frequency distribution from the expected left-truncated exponential decay. Abrupt changes in monthly mean amplitude at a station apparently coincide with changes in recording equipment and/or analysis method at the station. PubDate: 2017-05-08 DOI: 10.1093/gji/ggx130

Authors:Poggi VV; Burjanek JJ, Michel CC, et al. First page: 645 Abstract: The Swiss Seismological Service (SED) has recently finalised the installation of ten new seismological broadband stations in northern Switzerland. The project was led in cooperation with the National Cooperative for the Disposal of Radioactive Waste (Nagra) and Swissnuclear to monitor micro seismicity at potential locations of nuclear-waste repositories. To further improve the quality and usability of the seismic recordings, an extensive characterization of the sites surrounding the installation area was performed following a standardised investigation protocol. State-of-the-art geophysical techniques have been used, including advanced active and passive seismic methods. The results of all analyses converged to the definition of a set of best-representative 1-D velocity profiles for each site, which are the input for the computation of engineering soil proxies (traveltime averaged velocity and quarter-wavelength parameters) and numerical amplification models. Computed site response is then validated through comparison with empirical site amplification, which is currently available for any station connected to the Swiss seismic networks. With the goal of a high-sensitivity network, most of the NAGRA stations have been installed on stiff-soil sites of rather high seismic velocity. Seismic characterization of such sites has always been considered challenging, due to lack of relevant velocity contrast and the large wavelengths required to investigate the frequency range of engineering interest. We describe how ambient vibration techniques can successfully be applied in these particular conditions, providing practical recommendations for best practice in seismic site characterization of high-velocity sites. PubDate: 2017-05-06 DOI: 10.1093/gji/ggx192

Authors: T; Boxberger , Pilz M, et al. First page: 660 Abstract: The assessment of the shear wave velocity (vs) and shear wave quality factor (Qs) for the shallow structure below a site is necessary to characterize its site response. In the past, methods based on the analysis of seismic noise have been shown to be very efficient for providing a sufficiently accurate estimation of the vs versus depth at reasonable costs for engineering seismology purposes. In addition, a slight modification of the same method has proved to be able to provide realistic Qs versus depth estimates. In this study, data sets of seismic noise recorded by microarrays of seismic stations in different geological environments of Europe and Central Asia are used to calculate both vs and Qs versus depth profiles. Analogous to the generally adopted approach in seismic hazard assessment for mapping the average shear wave velocity in the uppermost 30 m (vs30) as a proxy of the site response, this approach was also applied to the quality factor within the uppermost 30 m (Qs30). A slightly inverse correlation between both parameters is found based on a methodological consistent determination for different sites. Consequently, a combined assessment of vs and Qs by seismic noise analysis has the potential to provide a more comprehensive description of the geological structure below a site. PubDate: 2017-04-26 DOI: 10.1093/gji/ggx161

Authors:Beall AP; Moresi L, Stern T. First page: 671 Abstract: Under some conditions, dense parts of the lower crust or mantle lithosphere can become unstable, deform internally and sink into the less dense, underlying asthenosphere. Two end-member mechanisms for this process are delamination and dripping. Numerical calculations are used to compare the time taken for each instability to grow from initiation to the point of rapid descent through the asthenosphere. This growth period is an order of magnitude shorter for delamination than dripping. For delamination, the growth rate varies proportionally to the buoyancy and viscosity of the sinking material, as with dripping. It also depends on the relative thickness ($L^{\prime }_c$) and viscosity ($\eta ^{\prime }_c$) of the weak layer which decouples the sinking material from the upper crust, varying proportionally to $L_c^{\prime 2}/\eta ^{\prime \frac{2}{3}}_c$. As instabilities commonly resemble a mix of dripping and delamination, the analysis of initial instability growth includes a range of mechanisms in-between. Dripping which begins with a large perturbation and low $\eta ^{\prime }_c$ reproduces many of the characteristic features of delamination, yet its growth timescale is still an order of magnitude slower. Previous diagnostic features of delamination may therefore be ambiguous and if rheology is to be inferred from observed timescales, it is important that delamination and this ‘triggered dripping’ are distinguished. Transitions from one mechanism or morphology to another, during the initial growth stage, are also examined. 3-D models demonstrate that when $\eta ^{\prime }_c$ is small, a dripping, planar sheet will only transition into 3-D drips if the initial triggering perturbation is less than a third of the dense material’s thickness. This transition occurs more easily at large $\eta ^{\prime }_c$, so rheological heterogeneity may be responsible for morphological transitions through time. We also calculate the rates at which delamination grows too slowly to outpace cooling of the upwelling asthenosphere, resulting in stalling and switching to dripping. Common lithospheric viscosities and observed timescales indicate that both instability transitions are feasible. Overall, the timescale and persistence of delamination depends on three parameter groups, which characterize the properties of the anomalously dense material, weak lower crustal layer and relative rate of thermal diffusion. These scalings appear to unify the varying results of previous delamination studies. PubDate: 2017-05-10 DOI: 10.1093/gji/ggx202

Authors:Vackář J; Burjánek J, Gallovič F, et al. First page: 693 Abstract: We have developed a new, fully automated tool for the centroid moment tensor (CMT) inversion in a Bayesian framework. It includes automated data retrieval, data selection where station components with various instrumental disturbances are rejected and full-waveform inversion in a space–time grid around a provided hypocentre. A data covariance matrix calculated from pre-event noise yields an automated weighting of the station recordings according to their noise levels and also serves as an automated frequency filter suppressing noisy frequency ranges. The method is tested on synthetic and observed data. It is applied on a data set from the Swiss seismic network and the results are compared with the existing high-quality MT catalogue. The software package programmed in Python is designed to be as versatile as possible in order to be applicable in various networks ranging from local to regional. The method can be applied either to the everyday network data flow, or to process large pre-existing earthquake catalogues and data sets. PubDate: 2017-04-21 DOI: 10.1093/gji/ggx158

Authors:Foster DJ; Lane FD, Zhao Z. First page: 706 Abstract: We examine wave propagation in transversely homogeneous media characterized by variations of properties with respect to depth. Our intent is to provide analytic formulae that quantify scattering effects from fine scale inhomogeneities typically observed on seismic field data. We employ a Born scattering series approach that systematically includes higher order phenomena (transmission losses, mode conversions and multiples). Although, the Born series is inexact, it provides useful approximations. Analytic examples show how various terms of the Born series include higher order scattering effects. We also document the inexact nature of the approximation. Numerical examples illustrate the magnitude of scattering induced time delays and attenuation. A generalized (complex) reflection coefficient is introduced to describe backscattered energy from a discrete interval instead of an isolated interface. The main goal of this paper is to quantify thin-bed scattering in order to provide a more complete representation of seismic reflections in realistic geological settings. It is our hope that this insight will lead to useful applications. PubDate: 2017-05-05 DOI: 10.1093/gji/ggx186

Authors:Dales P; Audet P, Olivier G, et al. First page: 731 Abstract: In active underground mining environments, monitoring of the rockmass has important implications for both safety and productivity. Monitoring can be accomplished by exploiting the many passive seismic sources (microearthquakes, drilling, ore-crushers etc.) around the mine on the condition they can be accurately detected and located. We implement a popular beamforming-like approach that uses cross-correlation functions in a maximum likelihood search to locate sources of seismic energy. We illustrate the technique with a synthetic example in which two simultaneous sources are located and discuss briefly the effects of different processing parameters. We demonstrate the effectiveness of this technique by monitoring both impulsive sources (microearthquakes) and other persistent sources (drilling and ore-crushers) in two active underground mines. We then propose how this information can be used in conjunction with ambient seismic noise interferometry to estimate seismic Green's functions under temporally variable and anisotropic wavefield conditions. Alternatively, we demonstrate how stable persistent sources, typically seen as contaminants in ambient noise applications, can be used to monitor changing rockmass conditions and potentially guide mining operations. PubDate: 2017-05-31 DOI: 10.1093/gji/ggx189