Evidence for shallow and pervasive seismic anisotropy in the Wellington Region, New Zealand
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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.Keywords:
Shear wave splitting
Seismometer
Shear waves
Seismic anisotropy
Seismic anisotropy
Shear wave splitting
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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.
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Shear wave splitting
Seismic anisotropy
Shear waves
Love wave
S-wave
Rayleigh Wave
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Shear wave splitting
Seismic anisotropy
Shear waves
Indus
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An Important advantage of tele-seismic PS-waves is carrying the anisotropy information of media beneath seismic stations.This paper uses the analysis method of three-component PSwave splitting to analyze the basement PS-waves based on the high-precision tele-seismic data of Capital Area Seismic Network(CASN) from 2002 to 2003.The purpose is to acquire the PS-wave splitting parameters beneath the seismic stations,including polarization of fast PS-waves and the time delays of fast and slow PS-waves,and to reveal the shallow crust anisotropy beneath the seismic stations.The results show that this method preserves the original information of the wave field,which allows us to analyze the features of PS-wave splitting.The tele-seismic PS-waves from the basement recorded by short period stations have the advantage of less noise and higher SNR,and have splitting phenomena commonly.Besides we find the time delay of fast and slow PS-waves is beyond our expectation,which has average values of time delay 0.1~0.2 s.This research can be compared with shear-wave splitting,which would help recognize anisotropy and stress state of crust and study tectonics and seismic activity.
Shear wave splitting
Seismic anisotropy
Microseism
Dispersive body waves
Shear waves
Basement
Vertical seismic profile
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Abstract An important advantage of tele‐seismic PS‐waves is carrying the anisotropy information of media beneath seismic stations. This paper uses the analysis method of three‐component PS‐wave splitting to analyze the basement PS‐waves based on the high‐precision tele‐seismic data of Capital Area Seismic Network (CASN) from 2002 to 2003. The purpose is to acquire the PS‐wave splitting parameters beneath the seismic stations, including polarization of fast PS‐waves and the time delays of fast and slow PS‐waves, and to reveal the shallow crust anisotropy beneath the seismic stations. The results show that this method preserves the original information of the wave field, which allows us to analyze the features of PS‐wave splitting. The tele‐seismic PS‐waves from the basement recorded by short period stations have the advantage of less noise and higher SNR, and have splitting phenomena commonly. Besides we find the time delay of fast and slow PS‐waves is beyond our expectation, which has average values of time delay 0.1∼0.2 s. This research can be compared with shear‐wave splitting, which would help recognize anisotropy and stress state of crust and study tectonics and seismic activity.
Shear wave splitting
Seismic anisotropy
Microseism
Dispersive body waves
Shear waves
Basement
S-wave
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We examine upper mantle anisotropy across the Hawaiian Swell by analyzing shear wave splitting of teleseismic SKS waves recorded by the PLUME broadband land and ocean bottom seismometer deployments. Mantle anisotropy beneath the oceans is often attributed to flow‐induced lattice‐preferred orientation of olivine. Splitting observations may reflect a combination of both fossil lithospheric anisotropy and anisotropy due to present‐day asthenospheric flow, and here we address the question whether splitting provides diagnostic information on possible asthenospheric plume flow at Hawaii. We find that the splitting fast directions are coherent and predominantly parallel to the fossil spreading direction, suggesting that shear wave splitting dominantly reflects fossil lithospheric anisotropy. The signature of anisotropy from asthenospheric flow is more subtle, although it could add some perturbation to lithospheric splitting. The measured delay times are typically 1 s or less, although a few stations display larger splitting delays of 1–2 s. The variability in the delay times across the different stations indicates differences in the degree of anisotropy or in the thickness of the anisotropic layer or in the effect of multilayer anisotropy. Regions with smaller splitting times may have experienced processes that modified the lithosphere and partially erased the fossil anisotropy; alternatively, asthenospheric splitting may either constructively add to or destructively subtract from lithospheric splitting to produce the observed variability in delay times.
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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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Shear wave splitting is a robust tool to infer the direction and strength of seismic anisotropy in the lithosphere and underlying asthenosphere. Previous shear wave splitting studies in the Afar Depression and adjacent areas concluded that either Precambrian sutures or vertical magmatic dikes are mostly responsible for the observed anisotropy. Here we report results of a systematic analysis of teleseismic shear wave splitting using all the available broadband seismic data recorded in the Afar Depression, Main Ethiopian Rift (MER), and Ethiopian Plateau. We found that while the ∼450 measurements on the Ethiopian Plateau and in the MER show insignificant azimuthal variations with MER‐parallel fast directions and thus can be explained by a single layer of anisotropy, the ∼150 measurements in the Afar Depression reveal a systematic azimuthal dependence of splitting parameters with a π /2 periodicity, suggesting a two‐layer model of anisotropy. The top layer is characterized by a relatively small (0.65 s) splitting delay time and a WNW fast direction that can be attributed to magmatic dikes within the lithosphere, and the lower layer has a larger (2.0 s) delay time and a NE fast direction. Using the spatial coherency of the splitting parameters obtained in the MER and on the Ethiopian Plateau, we estimated that the optimal depth of the source of anisotropy is centered at about 300 km, i.e., in the asthenosphere. The spatial and azimuthal variations of the observed anisotropy can best be explained by a NE directed flow in the asthenosphere beneath the MER and the Afar Depression.
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Asthenosphere
Seismic anisotropy
Dike
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SUMMARY Determinations of seismic anisotropy, or the dependence of seismic wave velocities on the polarization or propagation direction of the wave, can allow for inferences on the style of deformation and the patterns of flow in the Earth’s interior. While it is relatively straightforward to resolve seismic anisotropy in the uppermost mantle directly beneath a seismic station, measurements of deep mantle anisotropy are more challenging. This is due in large part to the fact that measurements of anisotropy in the deep mantle are typically blurred by the potential influence of upper mantle and/or crustal anisotropy beneath a seismic station. Several shear wave splitting techniques are commonly used that attempt resolve seismic anisotropy in deep mantle by considering the presence of multiple anisotropic layers along a raypath. Examples include source-side S-wave splitting, which is used to characterize anisotropy in the deep upper mantle and mantle transition zone beneath subduction zones, and differential S-ScS and differential SKS-SKKS splitting, which are used to study anisotropy in the D″ layer at the base of the mantle. Each of these methods has a series of assumptions built into them that allow for the consideration of multiple regions of anisotropy. In this work, we systematically assess the accuracy of these assumptions. To do this, we conduct global wavefield modelling using the spectral element solver AxiSEM3D. We compute synthetic seismograms for earth models that include seismic anisotropy at the periods relevant for shear wave splitting measurements (down to 5 s). We apply shear wave splitting algorithms to our synthetic seismograms and analyse whether the assumptions that underpin common measurement techniques are adequate, and whether these techniques can correctly resolve the anisotropy incorporated in our models. Our simulations reveal some inaccuracies and limitations of reliability in various methods. Specifically, explicit corrections for upper mantle anisotropy, which are often used in source-side direct S splitting and S-ScS differential splitting, are typically reliable for the fast polarization direction ϕ but not always for the time lag δt, and their accuracy depends on the details of the upper mantle elastic tensor. We find that several of the assumptions that underpin the S-ScS differential splitting technique are inaccurate under certain conditions, and we suggest modifications to traditional S-ScS differential splitting approaches that lead to improved reliability. We investigate the reliability of differential SKS-SKKS splitting intensity measurements as an indicator for lowermost mantle anisotropy and find that the assumptions built into the splitting intensity formula can break down for strong splitting cases. We suggest some guidelines to ensure the accuracy of SKS-SKKS splitting intensity comparisons that are often used to infer lowermost mantle anisotropy. Finally, we suggest a new strategy to detect lowermost mantle anisotropy which does not rely on explicit upper mantle corrections and use this method to analyse the lowermost mantle beneath east Asia.
Shear wave splitting
Seismic anisotropy
Core–mantle boundary
Shear waves
Seismogram
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