Transtensional deformation of Montserrat revealed by shear wave
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a b s t r a c t Here we investigate seismic anisotropy of the upper crust in the vicinity of Soufriere Hills volcano using shear wave splitting (SWS) analysis from volcano-tectonic (VT) events. Soufriere Hills, which is located on the island of Montserrat in the Lesser Antilles, became active in 1995 and has been erupting ever since with five major phases of extrusive activity. We use data recorded on a network of seismometers between 1996 and 2007 partially spanning three extrusive phases. Shear-wave splitting in the crust is often assumed to be controlled either by structural features, or by stress aligned cracks. In such a case the polarization of the fast shear wave (φ) would align parallel to the strike of the structure, or to the maximum compressive stress direction. Previous studies analyzing SWS in the region using regional earthquakes observed temporal variations in φ which were interpreted as being caused by stress perturbations associated with pressurization of a dyke. Our analysis, which uses much shallower sources and thus only samples the anisotropy of the upper few kilometres of the crust, shows no clear temporal variation. However, temporal effects cannot be ruled out, as large fluctuations in the rate of VT events over the course of the study period as well as changes in the seismic network configuration make it difficult to assess. Average delay times of approximately 0.2 s, similar in magnitude to those reported for much deeper slab events, suggest that the bulk of the anisotropy is in the shallow crust. We observe clear spatial variations in anisotropy which we believe are consistent with structurally controlled anisotropy resulting from a left-lateral transtensional array of faults which crosses the volcanic complex.Keywords:
Extrusive
Shear wave splitting
Seismometer
Stress field
S-wave
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Seismic anisotropy
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Abstract We present new observations of crustal anisotropy in the southern Cascadia fore arc from tectonic tremor. The abundance of tremor activity in Oregon and northern California during slow‐slip events offers an enormous amount of information with which to measure and analyze anisotropy in the upper brittle continental crust. To accomplish this, we performed analyses of wave polarization and shear wave splitting of tectonic tremor signals by using three component broadband seismic stations. The splitting times range between 0.11 and 0.32 s and are consistent with typical values observed in the continental crust. Fast polarization azimuths are, in general, margin parallel and trend N‐S, which parallels the azimuths of the maximum compressive stresses observed in this region. This pattern is likely to be controlled by the stress field. Comparatively, the anisotropic structure of fast directions observed in the northern section of the Cascadia margin is oblique with respect to the southern section of Cascadia, which, in general, trends E‐W and is mainly controlled by active faulting and geological structures. Source distribution analysis using a bivariate normal distribution that expresses the distribution of tremors in a preferred direction shows that in northern California and Oregon, the population of tremors tends to distribute parallel to fast polarization azimuths and maximum compressive stresses, suggesting that both tremor propagation and anisotropy are influenced by the stress field. Results show that the anisotropy reflects an active tectonic process that involves the northward movement of the Oregon Block, which is rotating as a rigid body. In northern Cascadia, previous results of anisotropy show that the crust is undergoing a shortening process due to velocity differences between the Oregon Block and the North America plate, which is moving more slowly with respect to the Oregon Block, making it clash against Vancouver Island.
Stress field
Shear wave splitting
Continental Margin
Passive margin
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Shear wave splitting
Seismic anisotropy
Shear waves
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The shear waves of local earthquakes were recorded during a 5‐month deployment of seven three‐component digital seismographs on the Wellington Peninsula, New Zealand. The seismographs were spaced an average of less than 5 km apart, and over 300 local earthquakes were recorded with phase arrivals within the shear wave window. A significant number (≈ 37%) of the earthquakes recorded showed clear evidence of shear wave splitting: identifiable fast and slow shear wave arrivals with similar pulse shapes. Consistent polarization directions at particular stations were also observed, even when poor signal‐to‐noise ratio or scattering meant that no slow shear wave arrival could be identified. However, there were large station‐to‐station differences in the polarization directions. Correcting for the observed shear wave splitting improved the fit between the measured shear wave polarizations and those calculated assuming a double‐couple focal mechanism. The cause of the observed shear wave splitting is therefore most likely to be seismic anisotropy. Large station‐to‐station differences in the polarization alignments, ranging from 61°±24° to 137°±18°, suggest that most anisotropy is confined to the top 2–3 km of the crust. However, there is evidence from one station for a small amount of pervasive anisotropy; if such a trend existed on the other stations, it could not be identified because of the large scatter in the data points. The measured delay times between split shear waves vary from 0.02 to 0.22 s, with a mean value of 0.1±0.06 s. This translates to a near‐surface shear wave velocity anisotropy of about 10%, with up to 2% pervasive anisotropy possible throughout the crust. This data set indicates that extensive dilatancy anisotropy cannot be the sole cause of crustal seismic anisotropy and that foliations in the rock fabric and the fracture zones of active faults may also be important. There is no evidence for temporal change in the shear wave splitting parameters during the period of the experiment.
Shear wave splitting
Seismometer
Shear waves
Seismic anisotropy
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[1] Understanding the geotectonic evolution of the southeastern Tibetan plateau requires knowledge about the structure of the lithosphere. Using data from 77 broadband stations in SW China, we invert Rayleigh wave phase velocity dispersion curves from ambient noise interferometry (T =1 0–40 s) and teleseismic surface waves (T =2 0–150 s) for 3‐D heterogeneity and azimuthal anisotropy in the lithosphere to ∼150 km depth. Our surface wave array tomography reveals (1) deep crustal zones of anomalously low shear wave speed and (2) substantial variations with depth of the pattern of azimuthal anisotropy. Upper crustal azimuthal anisotropy reveals a curvilinear pattern around the eastern Himalayan syntaxis, with fast directions generally parallel to the main strike slip faults. The mantle pattern of azimuthal anisotropy is different from that in the crust and varies from north to south. The tomographically inferred 3‐D variation in azimuthal anisotropy helps constrain the source region of shear wave splitting. South of ∼26°N (off the high plateau) most of the observed splitting can be accounted for by upper mantle anisotropy, but for stations on the plateau proper (with thick crust) crustal anisotropy cannot be ignored. On long wavelengths, the pattern of azimuthal anisotropy in the crust differs from that in the mantle. This is easiest explained if deformation varies with depth. The deep crustal zones of low shear wave speed (and, presumably, mechanical strength) may represent loci of ductile deformation. But their lateral variation suggests that in SE Tibet (localized) crustal channel flow and motion along the major strike slip faults are both important.
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Seismic anisotropy
Rayleigh Wave
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This study conducts a comprehensive investigation of crustal seismic anisotropy over varied geological regimes of Taiwan. With a large amount of earthquake data, the lateral variation of seismic shear wave splitting (SWS) is fully examined in terms of crustal deformation process. As the well-known vigorous orogeny subjected to the Eurasian—Philippine plate collision, tectonic convergence of Taiwan is presumably propagating from east to west. The acquired SWS waveform data cover areas from the slightly deformed Western Plains to the intermediate-to-high metamorphic Western Foothills and central mountain ranges. By means of waveform cross-correlation, the SWS parameters—the fast-wave polarization orientation and delay time—infer that the mechanism of lithologic deformation of Taiwan convergence can be classified into two domains: the convergence-parallel laminating west of the Deformation Front and the convergence-perpendicular striking east of the Deformation Front. The convergence-parallel SWS measurement presents the internal fabrics consisting of microfractures subject to lateral compression before the yielding of the lithologic strength, whereas the convergence-perpendicular measurements reveal the lateral accommodation of deformation as the stress/strain surpass the yielding strength of rock, where the predominant SWS polarization is in the NE—SW direction similar to the general trend of Taiwan's mountain ranges. It is remarkable that there is no correlation between metamorphic degrees with SWS parameters. The geological province which corresponds to higher metamorphism is not consistent with large SWS parameters. This may be because of anisotropic weakness caused by multiple tectonic processes at considerable metamorphic zone. Furthermore, comparison of the SWS delay times with corresponding focal depths suggests that seismic anisotropy in the upper crust may come from multiple layers, and the fabric lamination causing the anisotropy may be confined only within the shallow crust.
Seismic anisotropy
Orogeny
Shear wave splitting
Lithology
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Seafloor Spreading
Seismic anisotropy
Seismometer
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Here we investigate seismic anisotropy of the upper crust in the vicinity of Soufrière Hills volcano using shear wave splitting (SWS) analysis from volcano-tectonic (VT) events. Soufrière Hills, which is located on the island of Montserrat in the Lesser Antilles, became active in 1995 and has been erupting ever since with five major phases of extrusive activity. We use data recorded on a network of seismometers between 1996 and 2007 partially spanning three extrusive phases. Shear-wave splitting in the crust is often assumed to be controlled either by structural features, or by stress aligned cracks. In such a case the polarization of the fast shear wave (ϕ) would align parallel to the strike of the structure, or to the maximum compressive stress direction. Previous studies analyzing SWS in the region using regional earthquakes observed temporal variations in ϕ which were interpreted as being caused by stress perturbations associated with pressurization of a dyke. Our analysis, which uses much shallower sources and thus only samples the anisotropy of the upper few kilometres of the crust, shows no clear temporal variation. However, temporal effects cannot be ruled out, as large fluctuations in the rate of VT events over the course of the study period as well as changes in the seismic network configuration make it difficult to assess. Average delay times of approximately 0.2 s, similar in magnitude to those reported for much deeper slab events, suggest that the bulk of the anisotropy is in the shallow crust. We observe clear spatial variations in anisotropy which we believe are consistent with structurally controlled anisotropy resulting from a left-lateral transtensional array of faults which crosses the volcanic complex.
Shear wave splitting
Seismometer
Extrusive
Seismic anisotropy
S-wave
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Shear wave splitting has been detected in three‐component seismograms from a number of small earthquakes taking place either within the upper crust or the uppermost mantle beneath the Shikoku area, Japan. The faster shear waves are polarized nearly in E‐W at the stations in the central part of Shikoku, and in NEN‐SWS at the southern stations. The arrivals of slower shear waves polarized orthogonally to the faster ones are also clearly recognized in many seismograms at one of the stations. The faster shear wave polarizations at the stations in the central part of Shikoku and the pattern of travel time differences between split shear waves are successfully explained in terms of crustal anisotropy generated by vertical alignment of stress‐induced microcracks. The difference in polarization directions suggests that the tectonic stress state beneath the southern Shikoku region is significantly different from that beneath the central part of Shikoku. A quantitative analysis of travel time differences between split shear waves suggests that the presence of seismic anisotropy which causes the observed shear wave splitting is limited to the upper 10 or 15 km of the crust. Nevertheless, the possibility that anisotropy is concentrated in a much thinner surface layer cannot be ruled out. The average aspect ratio of crack might be greater than of the order of 10 −2 , or the cracks may not be purely vertical but have some fluctuations in the dip angle.
Seismogram
Shear wave splitting
Seismic anisotropy
Shear waves
Love wave
S-wave
Rayleigh Wave
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We measure crustal anisotropy parameters from several hundreds of aftershocks (ML > 2.5) of the 1997 Umbria–Marche seismic sequence which occurred in a carbonatic fold and thrust belt in the shallow crust of central Apennines (Italy). The analysis of shear wave polarization shows clear S-wave splitting with prevalent fast direction ∼140°N and average delay times of 0.06 s. The observed fast direction is parallel to the strike of the activated normal-fault system and to the maximum horizontal stress (σ2) active in the region. This is explained by the presence of stress-aligned microcracks or stress-opened fluid-filled cracks and fractures within the sedimentary coverage, even if the role of structural anisotropy cannot be completely ruled out since the maximum horizontal stress is subparallel to the major structural features of the area (main thrusts and normal faults). The peculiar spatio-temporal evolution of the seismic sequence gives us also the opportunity to investigate temporal variations of anisotropic parameters. We analyse those seismograms whose ray paths sample the crustal volume containing two of the major fault zones, before and after the occurrence of normal faulting mainshocks (Mw > 5). We observe variations of the anisotropic parameters during the days before and after the occurrence of mainshocks and we interpret them in terms of temporal variations of anisotropic parameters. This interpretation is consistent with temporal variations of the local stress condition and of the fluid pressure in the studied crustal volume proposed in the literature. However, since the spatial sampling of the selected ray paths varies with time, we cannot exclude the contribution of spatial variations of anisotropic parameters.
Shear wave splitting
Seismogram
Seismic anisotropy
Sequence (biology)
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