Hybrid journal (It can contain Open Access articles) ISSN (Print) 0956-540X - ISSN (Online) 1365-246X Published by Oxford University Press[370 journals]
Authors:Zhan W; Li F, Hao W, et al. Pages: 1295 - 1304 Abstract: SummarySeasonal Vertical Crustal Motions (SVCM) in Yunnan are assessed from 27 continuous Global Positioning System (cGPS) stations observations over the period 2010–2015. Results demonstrate significant SVCM in Yunnan with annual cyclic movements as the dominant pattern, the mean annual cyclic amplitude and initial phase is 9.7 mm and 2.9 months, respectively. These annual cyclic movements exhibit a high degree of spatial uniformity as a whole, while some spatial differences exist across regions. GPS seasonal vertical time-series show strong consistency with GRACE (Gravity Recovery and Climate Experiment) data, providing further indication that regional deformation exerts a dominant effect on the GPS vertical time-series. Over the period, the Yunnan area featured substantial precipitation that shows significant annual periodicity and a strong correlation with the GPS vertical time-series. The seasonal precipitation is therefore concluded to be the regional dominant factor that governs vertical cyclic crustal movements in this area. Differences in topography and precipitation between western and eastern Yunnan (i.e. across the Jinshajiang–Red River Fault) result in a greater magnitude of SVCM in western Yunnan compared to that in eastern Yunnan. Vertical linear movement rates of Yunnan derived from GPS also exhibit evident differences between eastern and western Yunnan. Therefore, results suggest that the Red River Fault is an important factor in the spatial variability of vertical crustal movements in Yunnan. PubDate: 2017-06-08 DOI: 10.1093/gji/ggx246 Issue No:Vol. 210, No. 3 (2017)
Authors:Sun Y; Ditmar P, Riva R. Pages: 1305 - 1322 Abstract: SummaryIn this study, we develop a methodology to estimate monthly variations in degree-1 and C20 coefficients by combing Gravity Recovery and Climate Experiment (GRACE) data with oceanic mass anomalies (combination approach). With respect to the method by Swenson et al., the proposed approach exploits noise covariance information of both input data sets and thus produces stochastically optimal solutions supplied with realistic error information. Numerical simulations show that the quality of degree-1 and -2 coefficients may be increased in this way by about 30 per cent in terms of RMS error. We also proved that the proposed approach can be reduced to the approach of Sun et al. provided that the GRACE data are noise-free and noise in oceanic data is white. Subsequently, we evaluate the quality of the resulting degree-1 and C20 coefficients by estimating mass anomaly time-series within carefully selected validation areas, where mass transport is small. Our validation shows that, compared to selected Satellite Laser Ranging (SLR) and joint inversion degree-1 solutions, the proposed combination approach better complements GRACE solutions. The annual amplitude of the SLR-based C10 is probably overestimated by about 1 mm. The performance of the C20 coefficients, on the other hand, is similar to that of traditionally used solution from the SLR technique. PubDate: 2017-06-01 DOI: 10.1093/gji/ggx241 Issue No:Vol. 210, No. 3 (2017)
Authors:Zhang J; Wu X, Yang X, et al. Pages: 1323 - 1331 Abstract: SummaryThe observation data of seven circumjacent terrestrial resistivity stations before two strong historical earthquakes in China were studied through performing the method of normalized monthly variation rate. We analysed the relationship between anisotropic variations of apparent resistivity and the corresponding maximum principal stress’s direction of the focal mechanism solution. The results may demonstrate that significant apparent resistivity variation occurs in the direction nearly perpendicular to the azimuth of the maximum compressive principal stress, while only small fluctuations were recorded in nearly parallel direction. In the first case, a decrease in 1 yr scale and a short-impending increase of apparent resistivity were clear. We also suggest that anisotropic variation of apparent resistivity is more substantial in (or near) the seismic fracture zone than at two sides of the fault. We conclude that the most effective way to estimate the epicentre of an earthquake is to increase the density of observation networks within the earthquake fault zone and to reasonably dispose electrode orientations based on this physical phenomenon. PubDate: 2017-05-29 DOI: 10.1093/gji/ggx235 Issue No:Vol. 210, No. 3 (2017)
Authors:Simon HH; Buske SS, Krauß FF, et al. Pages: 1332 - 1346 Abstract: SummaryThe Scandinavian Caledonides provide a well-preserved example of a Palaeozoic continent–continent collision, where surface geology in combination with geophysical data provides information about the geometry of parts of the Caledonian structure. The project COSC (Collisional Orogeny in the Scandinavian Caledonides) investigates the structure and physical conditions of the orogen units and the underlying basement with two approximately 2.5 km deep cored boreholes in western Jämtland, central Sweden. In 2014, the COSC-1 borehole was successfully drilled through a thick section of the Seve Nappe Complex. This tectonostratigraphic unit, mainly consisting of gneisses, belongs to the so-called Middle Allochthons and has been ductilely deformed and transported during the collisional orogeny. After the drilling, a major seismic survey was conducted in and around the COSC-1 borehole with the aim to recover findings on the structure around the borehole from core analysis and downhole logging. The survey comprised both seismic reflection and transmission experiments, and included zero-offset and multiazimuthal walkaway Vertical Seismic Profile (VSP) measurements, three long offset surface lines centred on the borehole, and a limited 3-D seismic survey. In this study, the data from the multiazimuthal walkaway VSP and the surface lines were used to derive detailed velocity models around the COSC-1 borehole by inverting the first-arrival traveltimes. The comparison of velocities from these tomography results with a velocity function calculated directly from the zero-offset VSP revealed clear differences in velocities for horizontally and vertically travelling waves. Therefore, an anisotropic VTI (transversely isotropic with vertical axis of symmetry) model was found that explains first-arrival traveltimes from both the surface and borehole seismic data. The model is described by a vertical P-wave velocity function derived from zero-offset VSP and the Thomsen parameters ε = 0.03 and δ = 0.3, estimated by laboratory studies and the analysis of the surface seismic and walkaway VSP data. This resulting anisotropic model provides the basis for further detailed geological and geophysical investigations in the direct vicinity of the borehole. PubDate: 2017-05-20 DOI: 10.1093/gji/ggx223 Issue No:Vol. 210, No. 3 (2017)
Authors:Arneitz P; Leonhardt R, Schnepp E, et al. Pages: 1347 - 1359 Abstract: SummaryRecords of the past geomagnetic field can be divided into two main categories. These are instrumental historical observations on the one hand, and field estimates based on the magnetization acquired by rocks, sediments and archaeological artefacts on the other hand. In this paper, a new database combining historical, archaeomagnetic and volcanic records is presented. HISTMAG is a relational database, implemented in MySQL, and can be accessed via a web-based interface (http://www.conrad-observatory.at/zamg/index.php/data-en/histmag-database). It combines available global historical data compilations covering the last ∼500 yr as well as archaeomagnetic and volcanic data collections from the last 50 000 yr. Furthermore, new historical and archaeomagnetic records, mainly from central Europe, have been acquired. In total, 190 427 records are currently available in the HISTMAG database, whereby the majority is related to historical declination measurements (155 525). The original database structure was complemented by new fields, which allow for a detailed description of the different data types. A user-comment function provides the possibility for a scientific discussion about individual records. Therefore, HISTMAG database supports thorough reliability and uncertainty assessments of the widely different data sets, which are an essential basis for geomagnetic field reconstructions. A database analysis revealed systematic offset for declination records derived from compass roses on historical geographical maps through comparison with other historical records, while maps created for mining activities represent a reliable source. PubDate: 2017-06-07 DOI: 10.1093/gji/ggx245 Issue No:Vol. 210, No. 3 (2017)
Authors:Loewer MM; Günther TT, Igel JJ, et al. Pages: 1360 - 1373 Abstract: SummaryWe combined two completely different methods measuring the frequency-dependent electrical properties of moist porous materials in order to receive an extraordinary large frequency spectrum. In the low-frequency (LF) range, complex electrical resistivity between 1 mHz and 45 kHz was measured for three different soils and sandstone, using the spectral induced polarization (SIP) method with a four electrode cell. In the high-frequency (HF) radio to microwave range, complex dielectric permittivity was measured between 1 MHz and 10 GHz for the same samples using dielectric spectroscopy by means of the coaxial transmission line technique. The combined data sets cover 13 orders of magnitude and were transferred into their equivalent expressions: the complex effective dielectric permittivity and the complex effective electrical conductivity. We applied the Kramers-Kronig relation in order to justify the validity of the data combination. A new phenomenological model that consists of both dielectric permittivity and electrical conductivity terms in a Debye- and Cole–Cole-type manner was fitted to the spectra. The combined permittivity and conductivity model accounts for the most common representations of the physical quantities with respect to the individual measuring method. A maximum number of four relaxation processes was identified in the analysed frequency range. Among these are the free water and different interfacial relaxation processes, the Maxwell-Wagner effect, the counterion relaxation in the electrical double layer and the direct-current electrical conductivity. There is evidence that free water relaxation does not affect the electrical response in the SIP range. Moreover, direct current conductivity contribution (bulk and interface) dominates the losses in the HF range. Interfacial relaxation processes with relaxations in the HF range are broadly distributed down to the LF range. The slowest observed process in the LF range has a minor contribution to the HF response. PubDate: 2017-06-03 DOI: 10.1093/gji/ggx242 Issue No:Vol. 210, No. 3 (2017)
Authors:Kusakabe T; Kame N. Pages: 1374 - 1387 Abstract: SummaryWe present analytical time-domain expressions for 2-D extended boundary integral equation method (XBIEM) kernel functions, which represent the transient displacement and stress responses of an infinite homogeneous isotropic medium to a constant traction applied on a line surface element. Using kernels, we propose an implicit time-stepping scheme to simulate a dynamic rupture crossing a bimaterial interface and show the significant control that medium inhomogeneity exerts on rupture propagation. PubDate: 2017-06-03 DOI: 10.1093/gji/ggx240 Issue No:Vol. 210, No. 3 (2017)
Authors:Xu Z; Mikesell T. Pages: 1388 - 1393 Abstract: SummarySeismic interferometry is routinely used to image and characterize underground geology. The vertical component cross-correlations (CZZ) are often analysed in this process; although one can also use radial component and multicomponent cross-correlations (CRR and CZR, respectively), which have been shown to provide a more accurate Rayleigh-wave Green’s function than CZZ when sources are unevenly distributed. In this letter, we identify the relationship between the multicomponent cross-correlations (CZR and CRR) and the Rayleigh-wave Green’s functions to show why CZR and CRR are less sensitive than CZZ to non-stationary phase source energy. We demonstrate the robustness of CRR with a synthetic seismic noise data example. These results provide a compelling reason as to why CRR should be used to estimate the dispersive characteristics of the direct Rayleigh wave with seismic interferometry when the signal-to-noise ratio is high. PubDate: 2017-05-22 DOI: 10.1093/gji/ggx228 Issue No:Vol. 210, No. 3 (2017)
Authors:Reynen A; Audet P. Pages: 1394 - 1409 Abstract: SummaryA new method using a machine learning technique is applied to event classification and detection at seismic networks. This method is applicable to a variety of network sizes and settings. The algorithm makes use of a small catalogue of known observations across the entire network. Two attributes, the polarization and frequency content, are used as input to regression. These attributes are extracted at predicted arrival times for P and S waves using only an approximate velocity model, as attributes are calculated over large time spans. This method of waveform characterization is shown to be able to distinguish between blasts and earthquakes with 99 per cent accuracy using a network of 13 stations located in Southern California. The combination of machine learning with generalized waveform features is further applied to event detection in Oklahoma, United States. The event detection algorithm makes use of a pair of unique seismic phases to locate events, with a precision directly related to the sampling rate of the generalized waveform features. Over a week of data from 30 stations in Oklahoma, United States are used to automatically detect 25 times more events than the catalogue of the local geological survey, with a false detection rate of less than 2 per cent. This method provides a highly confident way of detecting and locating events. Furthermore, a large number of seismic events can be automatically detected with low false alarm, allowing for a larger automatic event catalogue with a high degree of trust. PubDate: 2017-05-31 DOI: 10.1093/gji/ggx238 Issue No:Vol. 210, No. 3 (2017)
Authors:Liu X; Zhao D. Pages: 1410 - 1431 Abstract: SummaryWe present a new 3-D anisotropic P-wave velocity (Vp) model for the crust and upper mantle of the Japan subduction zone obtained by inverting a large number of high-quality P-wave traveltime data of local earthquakes and teleseismic events. By assuming orthorhombic anisotropy with a vertical symmetry axis existing in the modeling space, isotropic Vp tomography and 3-D Vp azimuthal and radial anisotropies are determined simultaneously. According to a simple flow field and the obtained Vp anisotropic tomography, we estimate the distribution of olivine fabrics in the mantle wedge. Our results show that the forearc mantle wedge above the subducting Pacific slab beneath NE Japan exhibits an azimuthal anisotropy with trench-parallel fast velocity directions (FVDs) and Vhf > Vv > Vhs (here Vv is Vp in the vertical direction, Vhf and Vhs are P-wave velocities in the fast and slow directions in the horizontal plane), where B-type olivine fabric with vertical trench-parallel flow may dominate. Such an anisotropic feature is not obvious in the forearc mantle wedge above the Philippine Sea (PHS) slab under SW Japan, probably due to higher temperatures and more fluids there associated with the young and warm PHS slab subduction. Trench-normal FVDs and Vhf > Vv > Vhs are generally revealed in the mantle wedge beneath the arc and backarc in Japan, where E-type olivine fabric with FVD-parallel horizontal flow may dominate. Beneath western Honshu, however, the mantle wedge exhibits an anisotropy of Vv > Vhf > Vhs and so C-type olivine fabric may dominate, suggesting that the water content is the highest there, because both the PHS and Pacific slabs exist there and their dehydration reactions release abundant fluids to the overlying mantle wedge. PubDate: 2017-06-08 DOI: 10.1093/gji/ggx247 Issue No:Vol. 210, No. 3 (2017)
Authors:Seydoux L; de Rosny J, Shapiro NM. Pages: 1432 - 1449 Abstract: SummaryPassive imaging techniques from ambient seismic noise requires a nearly isotropic distribution of the noise sources in order to ensure reliable traveltime measurements between seismic stations. However, real ambient seismic noise often partially fulfils this condition. It is generated in preferential areas (in deep ocean or near continental shores), and some highly coherent pulse-like signals may be present in the data such as those generated by earthquakes. Several pre-processing techniques have been developed in order to attenuate the directional and deterministic behaviour of this real ambient noise. Most of them are applied to individual seismograms before cross-correlation computation. The most widely used techniques are the spectral whitening and temporal smoothing of the individual seismic traces. We here propose an additional pre-processing to be used together with the classical ones, which is based on the spatial analysis of the seismic wavefield. We compute the cross-spectra between all available stations pairs in spectral domain, leading to the data covariance matrix. We apply a one-bit normalization to the covariance matrix eigenspectrum before extracting the cross-correlations in the time domain. The efficiency of the method is shown with several numerical tests. We apply the method to the data collected by the USArray, when the M8.8 Maule earthquake occurred on 2010 February 27. The method shows a clear improvement compared with the classical equalization to attenuate the highly energetic and coherent waves incoming from the earthquake, and allows to perform reliable traveltime measurement even in the presence of the earthquake. PubDate: 2017-06-08 DOI: 10.1093/gji/ggx250 Issue No:Vol. 210, No. 3 (2017)
Authors:Kondopoulou DD; Gómez-Paccard MM, Aidona EE, et al. Pages: 1450 - 1471 Abstract: SummaryThe Balkan area provides an almost continuous record of the geomagnetic field variations during the last eight millennia but important data dispersion and chronological gaps are still observed. In order to improve this pattern, we oriented our research towards the study of Greek prehistoric ceramics and pottery collections. We present here new archaeointensity and mineralogical results from pottery and ceramics collected in two Bronze Age sites in Northern Greece, corresponding to the middle third and middle second millennium BC. Thermal demagnetization experiments, thermomagnetic curves, thermal demagnetization measurements of three axes composite isothermal magnetization and first-order reversal curves (FORC) diagrams were performed in order to define the main magnetic carriers and select the most promising samples for archaeointensity determination. The results suggest that the majority of the studied materials are dominated by a mixture of superparamagnetic and stable single domain grains, with very little magnetic interactions. In almost all samples the main magnetic carrier observed is a low coercivity mineral, most probably magnetite and/or Ti-magnetite. Classical Thellier experiments, including both the thermoremanent magnetization anisotropy and cooling rate corrections were performed on the selected samples. Two new archaeointensities were obtained and compared with previous data available for Greece and neighbouring countries and with global geomagnetic field models results. In order to investigate the causes of the high rate of failure observed in our archaeointensity experiments, we re-examined the results obtained from the study of four collections already published by better characterizing the magnetic and mineralogical properties of the studied fragments. Specific hysteresis curves performed at different layers of ceramic fragments reveal an important degree of inhomogeneity. Mineralogical analysis of selected samples from the totality of six Greek prehistoric sites studied, including new X-ray powder diffraction analysis and a detailed description of the different geological contexts, is provided. The results suggest that the sherds were all fired in medium-high temperatures (mostly between 500° and 800 °C). The nature of the baked clay paste in response to the local geology appears to account for a part of the variability in magnetic properties. We suggest that X-ray powder diffraction analyses and thorough examination of the geological environment in combination with a satisfactory knowledge of the corresponding excavations history might provide an additional tool to appropriately select pottery fragments before performing future archaeointensity experiments. PubDate: 2017-05-23 DOI: 10.1093/gji/ggx224 Issue No:Vol. 210, No. 3 (2017)
Authors:Blackman DK; Boyce DE, Castelnau OO, et al. Pages: 1481 - 1493 Abstract: SummaryInsight into upper-mantle processes can be gained by linking flow-induced mineral alignment to regional deformation and seismic anisotropy patterns. Through a series of linked micro–macro scale numerical experiments, we explore the rheologic effects of crystal preferred orientation (CPO) and evaluate the magnitude of possible impacts on the pattern of flow and associated seismic signals for mantle that includes a cooling, thickening young oceanic lithosphere. The CPO and associated anisotropic rheology, computed by a micromechanical polycrystal model, are coupled with a large scale flow model (Eulerian Finite Element method) via a local viscosity tensor field, which quantifies the stress:strain rate response of a textured polycrystal. CPO is computed along streamlines throughout the model space and the corresponding viscosity tensor field at each element defines the local properties for the next iteration of the flow field. Stable flow and CPO distributions were obtained after several iterations for the two dislocation glide cases tested: linear and nonlinear stress:strain rate polycrystal behaviour. The textured olivine polycrystals are found to have anisotropic viscosity tensors in a significant portion of the model space. This directional dependence in strength impacts the pattern of upper-mantle flow. For background asthenosphere viscosity of ∼1020 Pa s and a rigid lithosphere, the modification of the corner flow pattern is not drastic but the change could have geologic implications. Feedback in the development of CPO occurs, particularly in the region immediately below the base of the lithosphere. Stronger fabric is predicted below the flanks of a spreading centre for fully coupled, power-law polycrystals than was determined using prior linear, intermediate coupling polycrystal models. The predicted SKS splitting is modestly different (∼0.5 s) between the intermediate and fully coupled cases for oceanic plates less than 20 Myr old. The magnitude of azimuthal anisotropy for surface waves, on the other hand, is predicted to be twice as large for fully coupled power-law flow/polycrystals than for linear, intermediate coupled flow/polycrystal models. PubDate: 2017-06-08 DOI: 10.1093/gji/ggx251 Issue No:Vol. 210, No. 3 (2017)
Authors:Arisa D; Heki K. Pages: 1494 - 1502 Abstract: SummaryThe Pacific Plate subducts beneath the Philippine Sea Plate along the Izu–Bonin Trench. We investigated crustal movements at the Bonin Islands, using Global Navigation Satellite System and geodetic Very Long Baseline Interferometry data to reveal how the two plates converge in this subduction zone. These islands are located ∼100 km from the trench, just at the middle between the volcanic arc and the trench, making these islands suitable for detecting signatures of episodic deformation such as slow slip events (SSEs). During 2007–2016, we found five SSEs repeating quasi-periodically with similar displacement patterns. In estimating their fault parameters, we assumed that the fault lies on the prescribed plate boundary, and optimized the size, shape and position of the fault and dislocation vectors. Average fault slip was ∼5 cm, and the average moment magnitude was ∼6.9. We also found one SSE occurred in 2008 updip of the repeating SSE in response to an M6 class interplate earthquake. In spite of the frequent occurrence of SSEs, there is no evidence for long-term strain accumulation in the Bonin Islands that may lead to future megathrust earthquakes. Plate convergence in Mariana-type subduction zones may occur, to a large extent, episodically as repeating SSEs. PubDate: 2017-06-15 DOI: 10.1093/gji/ggx258 Issue No:Vol. 210, No. 3 (2017)
Authors:Gillet NN; Jault DD, Canet EE. Pages: 1503 - 1516 Abstract: SummaryThe proximity between the 6 yr recurrence time of the torsional Alfvén waves that have been inferred in the Earth’s outer core over 1940–2010 and their 4 yr traveltime across the fluid core is nicely explained if these travelling waves are to be considered as normal modes. We discuss to what extent the emergence of free torsional modes from a stochastic forcing in the fluid core is compatible with some dissipation, specifically with an electromagnetic torque strong enough to account for the observed length of day variations of 6 yr period. In a spherical cavity enclosed by an insulating mantle, torsional normal modes consist of standing waves. In the presence of a conducting mantle, they transform into outward travelling waves very similar to the torsional waves that have been detected in the Earth’s outer core. With such a resonant response a periodic forcing is not required to explain the regular recurrence of torsional waves; neither is the search for a source of motions in the vicinity of the cylindrical surface tangent to the inner core, where travelling waves seem to emerge. We discuss these results in the light of the reflection properties of torsional waves at the equator. We are able to reproduce the properties found for geophysical time-series of geostrophic flows (detection of a normal mode, almost total absorption at the equator) if the conductance of the lowermost mantle is 3 × 107 to 108 S. PubDate: 2017-06-01 DOI: 10.1093/gji/ggx237 Issue No:Vol. 210, No. 3 (2017)
Authors:Tao K; Grand SP, Niu F. Pages: 1517 - 1524 Abstract: SummaryIn seismic full-waveform inversion (FWI), the choice of misfit function determines what information in data is used and ultimately affects the resolution of the inverted images of the Earth's structure. Misfit functions based on traveltime have been successfully applied in global and regional tomographic studies. However, wave propagation through the upper mantle results in multiple phases arriving at a given receiver in a narrow time interval resulting in complicated waveforms that evolve with distance. To extract waveform information as well as traveltime, we use a misfit function based on the normalized correlation coefficient (CC). This misfit function is able to capture the waveform complexities in both phase and relative amplitude within the measurement window. It is also insensitive to absolute amplitude differences between modeled and recorded data, which avoids problems due to uncertainties in source magnitude, radiation pattern, receiver site effects or even miscalibrated instruments. These features make the misfit function based on normalized CC a good candidate to achieve high-resolution images of complex geological structures when interfering phases coexist in the measurement window, such as triplication waveforms. From synthetic tests, we show the advantages of this misfit function over the cross-correlation traveltime misfit function. Preliminary inversion of data from an earthquake in Northeast China images a sharper and stronger amplitude slab stagnant in the middle of the transition zone than FWI of cross-correlation traveltime. PubDate: 2017-06-07 DOI: 10.1093/gji/ggx249 Issue No:Vol. 210, No. 3 (2017)
Authors:Andriampenomanana F; Nyblade AA, Wysession ME, et al. Pages: 1525 - 1544 Abstract: SummaryThe lithosphere of Madagascar was initially amalgamated during the Pan-African events in the Neoproterozoic. It has subsequently been reshaped by extensional processes associated with the separation from Africa and India in the Jurassic and Cretaceous, respectively, and been subjected to several magmatic events in the late Cretaceous and the Cenozoic. In this study, the crust and uppermost mantle have been investigated to gain insights into the present-day structure and tectonic evolution of Madagascar. We analysed receiver functions, computed from data recorded on 37 broad-band seismic stations, using the H–κ stacking method and a joint inversion with Rayleigh-wave phase-velocity measurements. The thickness of the Malagasy crust ranges between 18 and 46 km. It is generally thick beneath the spine of mountains in the centre part (up to 46 km thick) and decreases in thickness towards the edges of the island. The shallowest Moho is found beneath the western sedimentary basins (18 km thick), which formed during both the Permo-Triassic Karro rifting in Gondwana and the Jurassic rifting of Madagascar from eastern Africa. The crust below the sedimentary basin thickens towards the north and east, reflecting the progressive development of the basins. In contrast, in the east there was no major rifting episode. Instead, the slight thinning of the crust along the east coast (31–36 km thick) may have been caused by crustal uplift and erosion when Madagascar moved over the Marion hotspot and India broke away from it. The parameters describing the crustal structure of Archean and Proterozoic terranes, including average thickness (40 km versus 35 km), Poisson's ratio (0.25 versus 0.26), average shear-wave velocity (both 3.7 km s–1), and thickness of mafic lower crust (7 km versus 4 km), show weak evidence of secular variation. The uppermost mantle beneath Madagascar is generally characterized by shear-wave velocities typical of stable lithosphere (∼4.5 km s–1). However, markedly slow shear-wave velocities (4.2–4.3 km s–1) are observed beneath the northern tip, central part and southwestern region of the island where the major Cenozoic volcanic provinces are located, implying the lithosphere has been significantly modified in these places. PubDate: 2017-06-03 DOI: 10.1093/gji/ggx243 Issue No:Vol. 210, No. 3 (2017)
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[370 journals]
