Evidence from shear-wave splitting for the restriction of seismic anisotropy to the upper crust
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Shear wave splitting
Seismogram
Seismic anisotropy
Upper crust
Shear waves
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Shear wave splitting
Seismogram
Seismic anisotropy
Upper crust
Shear waves
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Shear wave splitting
Seismic anisotropy
Upper crust
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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
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Shear wave splitting
Seismic anisotropy
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Indus
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To discern spatial and explore possible existence of temporal variations of upper crustal anisotropy in an ~15 km section of the San Jacinto Fault Zone (SJFZ) that is composed of the Buck Ridge and Clark faults in southern California, we conduct a systematic shear wave splitting investigation using local S-wave data recorded by three broadband seismic stations located near the surface expression of the SJFZ. An automatic data selection and splitting measurement procedure is firstly applied, and the resulting splitting measurements are then manually screened to ensure reliability of the results. Strong spatial variations in crustal anisotropy are revealed by 1694 pairs of splitting parameters (fast polarization orientation and splitting delay time), as reflected by the dependence of the resulting splitting parameters on the location and geometry of the raypaths. For raypaths traveling through the fault zones, the fast orientations are dominantly WNW-ESE which is parallel to the faults and may be attributed to fluid-filled fractures in the fault zones. For non-fault-zone crossing raypaths, the fast orientations are dominantly N-S which are consistent with the orientation of the regional maximum compressive stress. A three-dimensional model of upper crustal anisotropy is constructed based on the observations. An apparent increase in the raypath length normalized splitting times is observed after the 03/11/2013 M4.7 earthquake, which is largely attributable to changes in the spatial distribution of earthquakes before and after the M4.7 earthquake rather than reflecting temporal changes of upper crustal anisotropy.
Shear wave splitting
Seismic anisotropy
Upper crust
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Shear wave data from four nine component VSP's from the Iatan East Howard field, Mitchell County, Texas, have been analysed to determine the nature and extent of shear wave anisotropy. Oil production in this field is from Permian age Clearfork Dolomites. Cores indicate the presence of vertical fractures. Shear-wave splitting was observed on all VSP's and polarization of the leading split shear wave has been used to infer fracture orientation. The two anisotropic parameters, qS1 polarization and time delay between qS1 and qS2, were measured using two anisotropic estimation techniques. These measurements were confirmed by visual examination of seismograms and particle motions and then used to define an anisotropic model for the rockmass in the vicinity of the VSP. Synthetic seismograms were generated for the model, which gave a good match with observed seismograms and results.
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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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[1] We use an automated method to analyze shear wave splitting from local earthquakes recorded by the Southern California Seismic Network between 2000 and 2005. The observed fast directions of upper crustal anisotropy generally are consistent with the direction of maximum horizontal compression σHmax, suggesting that one major mechanism of anisotropy in the top 20 km of crust under southern California is regional stress. However, at other stations, fast directions are aligned with the trends of regional faulting and local alignment of anisotropic bedrock. Splitting delay times range widely within 0.2 s. These upper crustal anisotropy observations, together with previous studies of SKS shear wave splitting, surface waves, and receiver functions, suggest different mechanisms of anisotropy at different depths under southern California. Anisotropy in the upper crust appears to be in response to the current horizontal maximum compression σHmax, which differs from the cause of anisotropy in the lower crust and mantle. We also explore possible temporal variations in upper crustal anisotropy associated with preearthquake stress changes or stress changes excited by surface waves of great earthquakes but do not observe any clear temporal variations in fast directions or time delays.
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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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