Crustal anisotropy from tectonic tremor under Washington State in the Cascadia
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Abstract We present new observations of crustal shear wave anisotropy extracted from nonvolcanic tremor in Cascadia under Washington State. Measurements of crustal anisotropy are extremely sparse and limited in this area mainly due to low level of seismicity. Abundance of tremor activity during slow earthquakes offers a unique opportunity to measure anisotropy parameters of the continental crust using tremor signal. To accomplish this, polarization and splitting analyses of nonvolcanic tremor are performed using three‐component broadband seismic stations. Splitting times measurements range between 0.08 and 0.17 s and similar to the splitting magnitude typically observed in the continental crust. Fast direction of shear wave anisotropy generally trends ESE‐WNW. Fast polarization directions are, in general, perpendicular to the prevailing maximum compressive stress field but tend to be parallel to several mapped EW and ESE‐WNW trending faults in this area. The observed spatial pattern of anisotropy is likely controlled by faulting that accommodates NS compression resulting from the tectonic movement of the Oregon block toward north. Existence of several EW trending crustal faults and source parameters of crustal earthquakes at depth, consistent with the regional stress regime, indicate that these faults may be the dominant factor causing the observed pattern of shear wave anisotropy.Keywords:
Shear wave splitting
Seismic anisotropy
Terra Nova, 25, 57–64, 2013 Abstract Characterizing the interaction of a fault with its surroundings is vital to fully understand the tectonic processes involved and predict future behaviour. Regional and local stress orientations affect different fracture length scales, manifested by numerous associated fault, fracture and crack structures. We use seismic anisotropy to constrain the dominant orientation of aligned rupture planes of various length scales. In particular, we study shear‐wave splitting of regional seismic events in Trans‐Alboran Shear Zone (TASZ), south‐east Spain. The TASZ consists of three major left‐lateral strike‐slip faults and numerous secondary strike‐slip and thrust faults. The observed orientations for S‐waves vary from roughly N–S in the northern segment of TASZ, to E–W in the centre, to NNW–SSE and NNE–SSW in the south. We show that the strikes of fast polarizations reflect both structural and lithological differences, indicating complex interactions of principal and secondary faults within the crust to accommodating tectonic stresses.
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Seismic anisotropy can be caused by the presence of aligned fractures. The imaging of fracture sets using anisotropy is a valuable tool for guiding drilling strategies. Shear wave splitting (SWS) is probably the most unambiguous indicator of anisotropy. C
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Shear wave splitting
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The analysis of data of seismic crustal studies in the USSR, obtained from waves propagating at different azimuths, reveals considerable horizontal and vertical inhomogeneity of the crust. Against this background it is difficult to predict what kind of velocity anisotropy can be expected in the continental crust. The rare cases of disagreement in velocities on intersecting profiles can be attributed both to anisotropy and to horizontal crustal inhomogeneity. There is a definite disagreement in layer velocities measured by reflected waves: fine layers in the crust and upper mantle have been found to have anomalously high velocities. The role of anisotropy in these events is not clear. The frequently observed splitting of S-wave with different polarization, however, positively implies anisotropy in the Earth's crust.
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Abstract We have conducted a systematic shear wave splitting analysis using 1000 selected aftershocks with M > 2 from the 2013 Ms 7.0 Lushan earthquake along the Longmenshan fault system in southwest China. Polarization directions of fast shear waves show a bimodal distribution with one dominant direction approximately parallel to the fault strike and the other close to the regional maximum horizontal compressive stress direction. This indicates that in this area mechanisms causing crustal seismic anisotropy are both stress induced and fault zone structure controlled. Delay times between fast and slow shear waves do not show a clear trend of increase for deeper events, suggesting the anisotropic zone is mostly above the aftershocks, which are generally located below 8 km. We further applied a shear wave splitting tomography method to measured delay times to characterize the spatial distribution of seismic anisotropy. The three‐dimensional anisotropic percentage model shows strong anisotropy above 8 km but low anisotropy below it. The mainshock slip zone and its aftershocks are associated with very low or negligible anisotropy and high velocity, indicating that the zones with high anisotropy and low velocity above 8 km are mechanically weak and it is difficult for stress to accumulate there. The main and back reverse fault zones are associated with high anisotropic anomalies above ∼8 km, likely caused by shear fabric or microfractures aligned parallel to the fault zone.
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Shear wave splitting
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It is believed that seismic anisotropy represents dynamics of the Earth's interior, directly reflecting either instantaneous stress, cumulative strain, or deformation of in-situ rocks. A shear wave passing through an anisotropic elastic medium splits into two orthgonally polarized quasi-shear waves with different propagation speeds. This phenomenon is called shear wave splitting, and is useful to understand the Earth's anisotropic fabric, with potential advantages as high lateral resolving power and relatively low sensitivity to seismic wave velocity heterogeneities. During the past decade, a variety of anisotropy-induced shear wave splitting has been observed in many different fields of seismology, indicating that anisotropy is an ubiquitous feature in the Earth's crust and upper mantle. In this review I summarize recent observations of shear wave splitting, with special emphases on their geophysical implications. I also discuss several problems concerned with shear wave splitting analyses, which are expected to be solved in the near future.
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Seismic anisotropy is critical to infer geodynamics such as mantle convection, plate tectonics, and the evolutionary processes of the crust. S-wave splitting analysis has been extensively used in the anisotropy study of the Earth's interior, including Japan subduction zone, but the resolution of the splitting data is too poor to investigate the detailed vertical structure. To obtain 3D anisotropic velocity structure beneath the Japan Islands, anisotropic tomography has been applied with P-wave travel-time data obtained by seismic networks deployed over the Japan Islands. The resultant 3D anisotropic structure shows that the P-wave anisotropy exists not only in the upper crust and mantle wedge but also in the lower crust and subducting slabs, where anisotropy has not been thoroughly studied. Although the P-wave anisotropic structure is basically consistent with S-wave anisotropy obtained from S-wave analyses, disagreement between P-and S-wave anisotropies is seen in the upper mantle beneath northeast Japan and the upper crust of the region along the Itoigawa-Shizuoka Tectonic Line. This result requires further studies to construct an anisotropy model that allows disagreement between P-and S-wave anisotropies, and/or revisiting the S-wave anisotropy structure with a sophisticated method that could provide a high-resolution structure of S-wave anisotropy.
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We systematically analysed shear wave splitting (SWS) for seismic data observed at a temporary array and two permanent networks around the San Andreas Fault (SAF) Observatory at Depth. The purpose was to investigate the spatial distribution of crustal shear wave anisotropy around the SAF in this segment and its temporal behaviour in relation to the occurrence of the 2004 Parkfield M 6.0 earthquake. The dense coverage of the networks, the accurate locations of earthquakes and the high-resolution velocity model provide a unique opportunity to investigate anisotropy in detail around the SAF zone. The results show that the primary fast polarization directions (PDs) in the region including the SAF zone and the northeast side of the fault are NW–SE, nearly parallel or subparallel to the SAF strike. Some measurements on the southwest side of the fault are oriented to the NNE–SSW direction, approximately parallel to the direction of local maximum horizontal compressive stress. There are also a few areas in which the observed fast PDs do not fit into this general pattern. The strong spatial variations in both the measured fast PDs and time delays reveal the extreme complexity of shear wave anisotropy in the area. The top 2–3 km of the crust appears to contribute the most to the observed time delays; however substantial anisotropy could extend to as deep as 7–8 km in the region. The average time delay in the region is about 0.06 s. We also analysed temporal patterns of SWS parameters in a nearly 4-yr period around the 2004 Parkfield main shock based on similar events. The results show that there are no appreciable precursory, coseismic, or post-seismic temporal changes of SWS in a region near the rupture of an M 6.0 earthquake, about 15 km away from its epicentre.
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