Polarization Direction Of Shear Wave Research Articles

Anisotropic Characterization of the Chukchi Boardland Based on Ocean-Bottom Seismic Experiment during N11-CHINARE

Abstract The current research focus at Chukchi Boardland (CB) revolves around sediment stratification and crustal structure, but investigations into deep stress fields and mantle dynamics are limited. This article presents a study on the anisotropic characteristics of the CB. Shear-wave splitting measurements were conducted using the transverse energy minimization at six stations recovered from the 11th Chinese National Arctic Research Expedition. The observation period for these six stations ranged from 2 August 2020 to 8 September 2020. The results demonstrate significant anisotropy within the CB, with the fast shear-wave polarization direction ranging from N60°E to N70°E. The time delays between fast and slow shear waves were found to be ∼0.7 s. By comparing the anisotropy observed at the CB with that at land stations in Arctic Alaska, this study suggested that the genesis of anisotropy beneath the CB was related to the formation of the Amerasian basin. The tectonic processes of rifting during basin evolution and midocean ridge spreading led to the development of anisotropy in the lithosphere beneath the CB during expansion.

Shear-wave splitting perspectives from the intense aftershock sequence of Damasi – Tyrnavos

The area of Damasi-Tyrnavos (Thessaloniki, Central Greece), in the vicinity of Larissa, was characterized by low seismic activity during the last decades. Two strong earthquakes of Mw = 6.3 and Mw = 6.0 The area of Damasi – Tyrnavos (Thessaly, Central Greece), in the vicinity of Larissa, was characterized occurred in early March 2021, followed by an intense aftershock sequence, related to WNW-ESE to NW-SE oriented faulting. This sequence was recorded by a dense local seismological network that provided a rich dataset and a unique opportunity to investigate upper crust shear-wave splitting for the first time in the study area. A fully automated technique, employing the eigenvalues method and cluster analysis, was implemented to measure the fast shear-wave polarization direction and the time-delay between the two split-shear-waves. This procedure yielded 655 results of adequate quality grade at 9 stations, after analyzing 1602 events and applying strict selection criteria, including the shear-wave window. The measured directions revealed a complex upper crust anisotropic regime. WNW-ESE to NW-SE, in accordance both with the APE model, being parallel to the local 𝜎 Hmax direction, and the strike of the fault planes. On the other hand, stations at the central part exhibit NNW-SSE and NNE-SSW anisotropy directions. An interesting feature is that the two northern stations are characterized by larger normalized time-delay values, possibly related to the migration of seismicity to the north during the initial stage of the seismic sequence.

Spatiotemporal Properties of Seismicity and Variations of Shear-Wave Splitting Parameters in the Western Gulf of Corinth (Greece)

The Western Gulf of Corinth (WGoC) exhibits significant seismicity patterns, combining intense microseismic background activity with both seismic swarms and short-lived aftershock sequences. Herein, we present a catalogue of ~9000 events, derived by manual analysis and double-difference relocation, for the seismicity of the WGoC during 2013–2014. The high spatial resolution of the hypocentral distribution permitted the delineation of the activated structures and their relation to major mapped faults on the surface. The spatiotemporal analysis of seismicity revealed a 32-km-long earthquake migration pattern, related to pore-pressure diffusion, triggering moderate mainshock-aftershock sequences, as fluids propagated eastwards in the course of ~15 months. The anisotropic properties of the upper crust were examined through automatic shear-wave splitting (SWS) analysis, with over 2000 SWS measurements at local stations. An average fast shear-wave polarization direction of N98.8°E ± 2.8° was determined, consistent with the direction of the maximum horizontal regional stress. Temporal variations of normalized time-delays between fast and slow shear-waves imply alterations in the level of stress or microcrack fluid saturation during the long-lasting pore-pressure diffusion episode, particularly before major events. The present study provides novel insights regarding seismicity patterns, active fault structures, anisotropic properties of the upper crust and triggering mechanisms of seismicity in the WGoC.

Insights into the structure and dynamics of the upper mantle beneath Bass Strait, southeast Australia, using shear wave splitting

We investigate the structure of the upper mantle using teleseismic shear wave splitting measurements obtained at 32 broadband seismic stations located in Bass Strait and the surrounding region of southeast Australia. Our dataset includes ∼366 individual splitting measurements from SKS and SKKS phases. The pattern of seismic anisotropy from shear wave splitting analysis beneath the study area is complex and does not always correlate with magnetic lineaments or current N-S absolute plate motion. In the eastern Lachlan Fold Belt, fast shear waves are polarized parallel to the structural trend (∼N25E). Further south, fast shear wave polarization directions trend on average N25–75E from the Western Tasmania Terrane through Bass Strait to southern Victoria, which is consistent with the presence of an exotic Precambrian microcontinent in this region as previously postulated. Stations located on and around the Neogene-Quaternary Newer Volcanics Province in southern Victoria display sizeable delay times (∼2.7 s). These values are among the largest in the world and hence require either an unusually large intrinsic anisotropy frozen within the lithosphere, or a contribution from both the lithospheric and asthenospheric mantle. In the Eastern Tasmania Terrane, nearly all observed fast directions are approximately NW-SE. Although part of our data set strongly favours anisotropy originating from “fabric” frozen in the lithospheric mantle, a contribution from the asthenospheric flow related to the present day plate motion is also required to explain the observed splitting parameters. We suggest that deviation of asthenospheric mantle flow around lithospheric roots could be occurring, and so variations in anisotropy related to mantle flow may be expected. Alternatively, the pattern of fast polarisation orientations observed around Bass Strait may be consistent with radial mantle flow associated with a plume linked to the recently discovered Cosgrove volcanic track. However, it is difficult to characterise the relative contributions to the observed splitting from the lithospheric vs. asthenospheric upper mantle due to poor backazimuthal coverage of the data.

Contemporary crustal tectonic movement in the southern Sichuan-Yunnan block based on dense GPS observation data

We analyzed 360 permanent and campaign GPS data from 1999 to 2017 in the southern Sichuan-Yunan block, and obtained crustal horizontal deformation in this region. Then, we derived the strain rate using a multi-scale spherical wavelet method. Results reveal a complex pattern of tectonic movement in the southern Sichuan-Yunnan block. Compared to the stable Eurasian plate, the maximum rate of the horizontal deformation in the southern Sichuan-Yunnan block is approximately 22 mm/a. The Xiaojiang fault shows a significantly lower deformation—a left-lateral strike-slip movement of 9.5 mm/a. The Honghe fault clearly shows a complex segmental deformation from the north to south. The northern Honghe fault shows 4.3 mm/a right strike-slip with 6.7 mm/a extension; the southern Honghe fault shows 1.9 mm/a right strike-slip with 1.9 mm/a extension; the junction zone in the Honghe and Lijiang–Xiaojinhe faults shows an obvious clockwise-rotation deformation. The strain calculation results reveal that the maximum shear-strain rate in this region reaches 70 nstrain/a, concentrated around the Xiaojiang fault and at the junction of the Honghe and Lijiang–Xiaojinhe faults. We note that most of the earthquakes with magnitudes of 4 and above that occurred in this region were within the high shear strain-rate zones and the strain rate gradient boundary zone, which indicates that the magnitude of strain accumulation is closely related to the seismic activities. Comparison of the fast shear-wave polarization direction of the upper-crust with the upper-mantle anisotropy and the direction of the surface principal compressive strain rate obtained from the inversion of the GPS data reveals that the direction of the surface principal compressive strain is basically consistent with the fast shear-wave polarization direction of the upper crust anisotropy, but different from the polarization direction of the upper mantle. Our results support the hypothesis that the principal elements of the deformation mechanism in the southern Sichuan-Yunnan block are decoupling between the upper and lower crust and ductile flow in the lower crust.

Crustal anisotropy and state of stress at Uturuncu Volcano, Bolivia, from shear-wave splitting measurements and magnitude–frequency distributions in seismicity

The physical signatures of unrest in large silicic magma systems are commonly observed in geophysical surveys, yet the interactions between magmatic processes and crustal stresses are often left unconstrained. Stresses in the mid and upper crust exert a strong control on the propagation and stalling of magma, and magma ascent can in turn change the magnitude and orientation of these stresses, including those associated with hydrothermal systems. This study assesses the state of stress at the restless Uturuncu Volcano in the Bolivian Andes with space, depth and time using observations of seismic anisotropy and the magnitude–frequency distributions of local earthquakes. Shear-wave splitting measurements are made for 677 events in the upper crust (1–25 km below sea level) between June 1, 2009 and March 10, 2012, and b-values are calculated using the Aki maximum likelihood method for a range of catalog subsets in the entire crust (−5 to 65 km below sea level). The b-value of the crustal events is unusually low (b=0.66±0.09), indicating that the seismogenic region features strong material with high stresses that are released with limited influence from hydrothermal fluids. The 410 good quality shear-wave splitting results have an average delay time of 0.06 ± 0.002 s and an average percent anisotropy ranging from 0.25 ± 0.04% to 6.2 ± 0.94% with a mean of 1.70 ± 0.32%. Fast shear-wave polarization directions are highly variable and appear to reflect a combination of tectonic and magmatic stresses that overprint the regional E–W compressive stress associated with the convergence of the Nazca and South American Plates. The shear-wave splitting results and b-values suggest that the upper crust beneath Uturuncu (∼0–7 km below the summit) is characterized by a weak and localized hydrothermal system in a poorly developed fracture network. We conclude that stresses imposed by crustal flexure due to magmatic unrest above the Altiplano-Puna Magma Body activate crack opening on a pre-existing fault beneath the volcano, generating seismicity and a spatially variable 1–10% anisotropy above the brittle–ductile transition zone. These results suggest that strong stresses in relatively unfractured upper crustal rocks may locally inhibit fluid migration in large silicic magma systems, leading to pluton emplacement and effusive volcanism rather than explosive eruptions.

Seismic anisotropies of the Songshugou peridotites (Qinling orogen, central China) and their seismic implications

Though extensively studied, the roles of olivine crystal preferred orientations (CPOs or fabrics) in affecting the seismic anisotropies in the Earth's upper mantle are rather complicated and still not fully known. In this study, we attempted to address this issue by analyzing the seismic anisotropies [e.g., P-wave anisotropy (AVp), S-wave polarization anisotropy (AVs), radial anisotropy (ξ), and Rayleigh wave anisotropy (G)] of the Songshugou peridotites (dunite dominated) in the Qinling orogen in central China, based on our previously reported olivine CPOs. The seismic anisotropy patterns of olivine aggregates in our studied samples are well consistent with the prediction for their olivine CPO types; and the magnitude of seismic anisotropies shows a striking positive correlation with equilibrium pressure and temperature (P–T) conditions. Significant reductions of seismic anisotropies (AVp, max. AVs, and G) are observed in porphyroclastic dunite compared to coarse- and fine-grained dunites, as the results of olivine CPO transition (from A-/D-type in coarse-grained dunite, through AG-type-like in porphyroclastic dunite, to B-type-like in fine-grained dunite) and strength variation (weakening: A-/D-type → AG-type-like; strengthening: AG-type-like → B-type-like) during dynamic recrystallization. The transition of olivine CPOs from A-/D-type to B-/AG-type-like in the forearc mantle may weaken the seismic anisotropies and deviate the fast velocity direction and the fast S-wave polarization direction from trench-perpendicular to trench-oblique direction with the cooling and aging of forearc mantle. Depending on the size and distribution of the peridotite body such as the Songshugou peridotites, B- and AG-type-like olivine CPOs can be an additional (despite minor) local contributor to the orogen-parallel fast velocity direction and fast shear-wave polarization direction in the orogenic crust such as in the Songshugou area in Qinling orogen.

Structurally controlled seismic anisotropy above small earthquakes in crustal rocks beneath the Rwenzori region, Albertine Rift, Uganda

We investigated shear-wave splitting of local earthquakes recorded by a temporary network of stations deployed in the Rwenzori region (Albertine Rift) from February 2006 to September 2007. Over 21,700 local earthquakes with magnitudes ranging between −0.5 and 5.1 were recorded. Shear-wave splitting analyses were made on only local crustal earthquakes with incidence angles of less than 34°. About 7000 earthquakes were available for analysis of their particle motions. The fast shear-wave polarization directions (φ) and the delay times (δt) between the fast and the slow shear-wave were calculated using the covariance and cross-correlation methods respectively. Exactly 309 shear-wave splitting pairs showed evidence of shear-wave splitting due to the abrupt changes in their shear-wave polarization directions as shown on their polarization diagrams and were available for interpretation. Visual inspection of our results revealed a heterogeneous pattern of φ directions throughout the entire study area, although a more-or-less rift parallel pattern of φ directions dominated at stations in the southern part of our study area. The orientation patterns of fault planes, foliation planes and the directions of maximum compressional stress were observed to be aligned with the φ directions in the study area. The extensive dilatancy anisotropy hypothesis was considered to be the most likely explanation for the observed φ directions. In general, the results do not show a correlation of δt with the ray-path length. We therefore argued that the observed seismic anisotropy probably has its origin at shallow crustal depths. An average δt of about 0.04s was observed, however, relatively large values of δt, ranging between 0.1s and 0.26s, occur in the region northwest of the Rwenzori Mountains. We interpreted these large values to be caused by stress-aligned pockets of melt within the uppermost crust.

Elastic properties of transparent nano-polycrystalline diamond measured by GHz-ultrasonic interferometry and resonant sphere methods

The sound velocities and elastic moduli of transparent nano-polycrystalline diamond (NPD) have been determined by GHz-ultrasonic interferometry on three different bulk samples, and by resonant spectroscopy on a spherically fabricated NPD sample. We employ a newly-developed optical contact micrometer to measure the thickness of ultrasonic samples to ±0.05μm with a spatial resolution of ∼50μm in the same position of the GHz-ultrasonic measurements, resulting in acoustic-wave sound velocity measurements with uncertainties of 0.005–0.02%. The isotropic and adiabatic bulk and shear moduli of NPD measured by GHz-ultrasonic interferometry are KS0=442.5 (±0.5)GPa and G0=532.4 (±0.5)GPa. By rotating the shear-wave polarization direction, we observe no transverse anisotropy in this NPD. Using resonant sphere spectroscopy, we obtain KS0=440.3 (±0.5)GPa and G0=532.7 (±0.4)GPa. For comparison, we also measured by GHz-ultrasonic interferometry the elastic constants of a natural single-crystal type-IA diamond with about one-half the experimental uncertainty of previous measurements. The resulting Voigt–Reuss–Hill averaged bulk and shear moduli of natural diamond are KS0=441.8 (±0.8)GPa and G0=532.6 (±0.5)GPa, demonstrating that the bulk-elastic properties of transparent NPD are equivalent to natural single-crystal diamond as calculated from polycrystalline averaging of its elastic constants.

Deformation in Rutford Ice Stream, West Antarctica: measuring shear-wave anisotropy from icequakes

AbstractIce streams provide major drainage pathways for the Antarctic ice sheet. The stress distribution and style of flow in such ice streams produce elastic and rheological anisotropy, which informs ice-flow modelling as to how ice masses respond to external changes such as global warming. Here we analyse elastic anisotropy in Rutford Ice Stream, West Antarctica, using observations of shear-wave splitting from three-component icequake seismograms to characterize ice deformation via crystal-preferred orientation. Over 110 high-quality measurements are made on 41 events recorded at five stations deployed temporarily near the ice-stream grounding line. To the best of our knowledge, this is the first well-documented observation of shear-wave splitting from Antarctic icequakes. The magnitude of the splitting ranges from 2 to 80 ms and suggests a maximum of 6% shear-wave splitting. The fast shear-wave polarization direction is roughly perpendicular to ice-flow direction. We consider three mechanisms for ice anisotropy: a cluster model (vertical transversely isotropic (VTI) model); a girdle model (horizontal transversely isotropic (HTI) model); and crack-induced anisotropy (HTI model). Based on the data, we can rule out a VTI mechanism as the sole cause of anisotropy – an HTI component is needed, which may be due to ice crystal a-axis alignment in the direction of flow or the alignment of cracks or ice films in the plane perpendicular to the flow direction. The results suggest a combination of mechanisms may be at play, which represent vertical variations in the symmetry of ice crystal anisotropy in an ice stream, as predicted by ice fabric models.

Estimating near‐surface shear wave velocities in Japan by applying seismic interferometry to KiK‐net data

We estimate shear wave velocities in the shallow subsurface throughout Japan by applying seismic interferometry to the data recorded with KiK‐net, a strong motion network in Japan. Each KiK‐net station has two receivers; one receiver on the surface and the other in a borehole. By using seismic interferometry, we extract the shear wave that propagates between these two receivers. Applying this method to earthquake‐recorded data at all KiK‐net stations from 2000 to 2010 and measuring the arrival time of these shear waves, we analyze monthly and annual averages of the near‐surface shear wave velocity all over Japan. Shear wave velocities estimated by seismic interferometry agree well with the velocities obtained from logging data. The estimated shear wave velocities of each year are stable. For the Niigata region, we observe a velocity reduction caused by major earthquakes. For stations on soft rock, the measured shear wave velocity varies with the seasons, and we show negative correlation between the shear wave velocities and precipitation. We also analyze shear wave splitting by rotating the horizontal components of the surface sensors and borehole sensors and measuring the dependence on the shear wave polarization. This allows us to estimate the polarization with the fast shear wave velocity throughout Japan. For the data recorded at the stations built on hard rock sites, the fast shear wave polarization directions correlate with the direction of the plate motion.

Seismic evidence for reverse activation of a paleo-rifting system in the East Sea (Sea of Japan)

The Japanese Islands were separated from the Eurasian plate due to continental rifting during the Oligocene to mid-Miocene, which caused the opening of the East Sea (Sea of Japan). Such tectonic evolution in the East Sea is important for understanding the evolution of back-arc regions with active convergent margins. To understand the evolution of the paleo-rifted back-arc region, we investigate seismicity, crustal seismic anisotropy, focal mechanism solutions and ambient stress field around the Korean Peninsula. The Korean Peninsula displays diffused seismicity with small and moderate earthquakes. Shallow earthquakes rarely occur in the central East Sea. The crustal fast shear-wave polarization directions in the Korean Peninsula are observed to vary in azimuth between 40° and 90°. The focal mechanism solutions are calculated by long period waveform inversions. The ambient stress field is calculated from the focal mechanism solutions. The compressional stress field in the Korean Peninsula is observed to be in ENE-WSW, which is consistent with the fast shear-wave polarization directions. The compressional stress directions in the East Sea progressively change from ENE to SE with increasing longitude. The rapid change of compressional directions in the central East Sea prohibits accumulation of stress, causing rare shallow seismicity. High seismicity of reverse faulting events is observed at the fringes of the East Sea, in particular, around the east coast of the Korean Peninsula and the west coast of Japanese Islands, which correspond to paleo-rifted margins where compressional stresses are accumulated. The compressional stress field and active thrustal events suggest reverse activation of paleo-normal faults that were developed during the opening of the East Sea.

Crustal shear-wave splitting in the epicentral zone of the 2001 Mw 7.7 Bhuj earthquake, Gujarat, India

Abstract We report here crustal shear-wave anisotropy, ranging from 1% to 10.76% with an average of 2.4% in the aftershock zone of the 2001 Bhuj earthquake, Gujarat, India, from a study of leading shear-wave polarization directions (LPSDs), which vary on average from NNW–SSE to E–W with a delay of 0.07–0.14 s. The delays in the NNW–SSE to NE–SW directions observed at seven stations, near the seismogenic fault, suggest cracks parallel to the direction of the maximum horizontal regional compressional stress prevailing in the region, suggesting a dilatancy-induced anisotropy resulting from approximately stress-aligned parallel vertical micro-cracks. In contrast, the LPSDs at Ramvav, Rapar and Vondh stations, away from the seismogenic fault, are fault parallel, approximately E–W and almost orthogonal to the stress-aligned polarizations inferred elsewhere. The maximum average time delay of 0.14 s is observed at Lodai, where the fast polarization direction is found to be N338°W. This has been observed from anisotropic poro-elastic (APE) modelling and observations that these are 90° flips in shear-wave polarization, resulting from propagation through micro-cracks containing fluids at critically high pore-fluid pressure surrounding the hypocenter of the 2001 mainshock. The presence of high pore-fluid pressure in the seismogenic fault zone could also explain the observed scatter in shear-wave time delays. Further, the coincidence of the N–S trending intrusive bodies (as inferred from tomographic studies in the area) with the N–S direction of regional maximum horizontal compressional stress supports the interpretation of stress-aligned vertical extensive-dilatant anisotropic (EDA) cracks. The depth distribution of the estimated anisotropy (1–10.76%), b -values and stress drop values suggests an increase at 18–30 km depths, which could be attributed to high pore-fluid pressures resulting from a fluid-filled fractured rock matrix or open micro-cracks (characterized by high crack density and high porosity) coinciding with a low velocity zone (at 18–30 km depths) as delineated from tomographic studies in the area.

Core-log integration studies in hole-A of Taiwan Chelungpu-fault Drilling Project

SUMMARY Taiwan Chelungpu-fault Drilling Project (TCDP) was initiated to understand the physical mechanisms involved in the large displacements of the 1999 Taiwan Chi-Chi earthquake. Continuous measurements of cores (including laboratory work) and a suite of geophysical downhole logs, including P- and S-wave sonic velocity, gamma ray, electrical resistivity, density, temperature, electrical borehole images and dipole-shear sonic imager, were acquired in Hole-A over the depth of 500–2003 m. Integrated studies of cores and logs facilitate qualitative and quantitative comparison of subsurface structures and physical properties of rocks. A total of 10 subunits were divided on the basis of geophysical characteristics. Generally, formation velocity and temperature increase with depth as a result of the overburden and thermal gradient, respectively. Gamma ray, resistivity, formation density, shear velocity anisotropy and density-derived porosity are primarily dependent on the lithology. Zones with changes of percentage of shear wave anisotropy and the fast shear polarization azimuth deduced from Dipole Shear-Imager (DSI) are associated with the appearance of fractures, steep bedding and shear zones. The fast shear wave azimuth is in good agreement with overall dip of the bedding (approximately 30° towards SE) and maximum horizontal compressional direction, particularly in the Kueichulin Formation showing strong shear wave velocity anisotropy. Bedding-parallel fractures are prevalent within cores, whereas minor sets of high-angle, NNW–SSE trending with N- and S-dipping fractures are sporadically distributed. The fault zone at depth 1111 m (FZA1111) is the Chi-Chi earthquake slip zone and could be a fluid conduit after the earthquake. The drastic change in fast shear wave polarization direction across the underlying, non-active Sanyi thrust at depth 1710 m reflects changes in stratigraphy, physical properties and structural geometry.

Upper mantle anisotropy beneath central and southwest Japan: An insight into subduction-induced mantle flow

Analysis of seismic anisotropy in the crust and mantle wedge above subduction zones gives much information about the dynamic processes inside the Earth. For this reason, we measure shear wave polarization anisotropy in the crust and upper mantle beneath central and southwestern Japan from local shallow, intermediate, and deep earthquakes occurring in the subducting Pacific slab. We analyze S phases from 198 earthquakes recorded at 42 Japanese F-net broadband seismic stations. This data set yields a total of 980 splitting parameter pairs for central and southwestern Japan. Dominant fast polarization directions of shear waves obtained at most stations in the Kanto–Izu–Tokai areas are oriented WNW–ESE, which are sub-parallel to the subduction direction of the Pacific plate. However, minor fast polarization directions are oriented in NNE–SSW directions being parallel to the strike of the Japan Trench, especially in the north of Izu Peninsula and the northern Tokai district. Generally, fast directions obtained at stations located in Kii Peninsula and the Chubu district are oriented ENE–WSW, almost parallel to the Nankai Trough, although some fast directions have NW–SE trends. The fast directions obtained at stations in northern central Honshu are oriented N–S. Delay times vary considerably and range from 0.1 to 1.25 s depending on the source depth and the degree of anisotropy along the ray path. These lateral variations in splitting character suggest that the nature of anisotropy is quite different between the studied areas. Beneath Kanto–Tokai, the observed WNW–ESE fast directions are probably caused by the olivine A-fabric induced by the corner flow. However, the slab morphology in this region is relatively complicated as the Philippine Sea slab is overriding the Pacific slab. This complex tectonic setting may induce lateral heterogeneity in the flow and stress state of the mantle wedge, and may have produced NNE–SSW orientations of fast directions. The ENE–WSW fast directions in Kii Peninsula and the Chubu district are more coherent and may be partly induced by the subduction of the Philippine Sea plate. The N–S fast directions in northern central Honshu might be produced by the trench-parallel stretching of the wedge due to the curved slab at the arc–arc junction.

Distinct lateral variation of lithospheric thickness in the Northeastern North China Craton

A detailed knowledge of the thickness of the lithosphere in the northeastern North China Craton (NCC) is important for understanding the significant tectonic reactivation of the craton in the Mesozoic and Cenozoic time. We achieve this goal by applying the newly proposed wave equation-based migration technique to the S-receiver functions recently collected in the region. Distinct negative signals are identified below the Moho in all the S-receiver function-migrated images and stacks, which we interpret as representing the S-to-P conversions from the lithosphere–asthenosphere boundary (LAB). The imaged LAB is as shallow as ∼60–70 km in the southeast basin and coastal areas and deepens to no more than 140 km in the northwest mountain ranges and continental interior. These observations indicate widespread lithospheric thinning in the study region in comparison with the >180-km lithospheric thicknesses typical of most cratonic regions. The revealed topography of the LAB generally agrees with the lateral variation in upper mantle seismic anisotropy previously measured through SKS splitting analysis. In particular, a sharp LAB step of ∼40 km is detected at the triple junction of the basin and mountains, at almost the same place where an abrupt change from NW–SE to NE–SW in fast polarization direction of shear waves was found. These findings suggest a close correlation between the seismic anisotropy and hence deformation of the upper mantle, the lithospheric thickness, and the surface tectonics of the northeastern NCC. While the thinned lithosphere and the NW–SE fast shear wave polarizations in the east areas probably are related to the dominant NW–SE tectonic extension in the late Mesozoic–Cenozoic time, the thicker lithosphere and the NE–SW fast polarization direction in the west mountain ranges may reflect earlier contractional deformations of the region. Synthetic tests indicate that the LAB beneath the northeastern NCC is a well-defined zone 10–20 km thick. Combined with seismic tomography results and geochemical and petrological data, this suggests that complex modification of the lithosphere probably accompanied significant lithospheric thinning during the tectonic reactivation of the old craton.

Subsurface structure, physical properties, fault-zone characteristics and stress state in scientific drill holes of Taiwan Chelungpu Fault Drilling Project

Continuous cores and a suit of geophysical measurements were collected in two scientific drill holes to understand physical mechanisms involved in the large displacements during the 1999 Chi-Chi earthquake. Physical properties obtained from wire-line logs including P- and S-wave sonic velocity, gamma ray, electrical resistivity, density and temperature, are primarily dependent on parameters such as lithology, depth and fault zones. The average dip of bedding, identified from both cores and FMI (or FMS) logs, is about 30° towards SE. Nevertheless, local azimuthal variations and increasing or decreasing bedding dips appear across fault zones. A prominent increase of structural dip to 60°–80° below 1856 m could be due to deformation associated with propagation of the Sanyi fault. A total of 12 fault zones identified in hole-A are located in the Plio-Pleistocene Cholan Formation, Pliocene Chinshui Shale and Miocene Kueichulin Formation. The shallowest fault zone occurs at 1111 m depth (FZ1111). It is a 1 m gouge zone including 12 cm of thick indurate black material. We interpreted this zone as the slip zone during Chi-Chi earthquake. FZ1111 is characterized by: 1) bedding-parallel thrust fault with 30-degree dip; 2) the lowest resistivity; 3) low density, V p and V s, 4) high V p/ V s ratio and Poisson's ratio; 5) low energy and velocity anisotropy, and low permeability within the homogeneous 1 m gouge zone; 6) increasing gas (CO 2 and CH 4) emissions, and 7) appearance of smectite within the primary slip zone. In situ stresses at the drill site were inferred from leak-off tests, borehole breakouts and drilling-induced tensile fractures from borehole FMS/FMI logs, and shear seismic wave anisotropy from DSI logs. The dominant fast shear-wave polarization direction is in good agreement with regional maximum horizontal stress axis, particularly within the strongly anisotropic Kueichulin Formation. A conjugate set of secondary directions are parallel to microcrack orientations. A drastic change of orientation of fast shear-wave polarization across the Sanyi thrust fault at the depth of 1712 m reflects the change of stratigraphy, physical properties and structural geometry.

Shear wave polarization anisotropy in and around the focal region of the 2005 West off Fukuoka Prefecture earthquake

Abstract Crustal shear wave polarization anisotropy is caused by the alignment of vertical microcracks. Leading shear wave polarization directions (LSPDs) are presumed to be consistent with the maximum horizontal compressional axis in many cases. We analyzed shear wave polarization anisotropy in and around the focal region of the 2005 West off Fukuoka Prefecture earthquake. Almost all of the LSPDs are oriented in the E-W direction, which is consistent with the maximum horizontal compressional axis inferred from the mechanism of the main shock. These E-to W-oriented LSPDs are caused by the alignment of stress-induced microcracks. Crack densities at most stations are estimated to be 0.02. Little spacial stress variation around focal region is suspected

Anisotropy in the Shallow Crust Observed around the San Andreas Fault Before and After the 2004 M 6.0 Parkfield Earthquake

Local seismic arrays were deployed at two locations along the San An- dreas fault (SAF) near Parkfield, California, before and after the 2004 M 6.0 Parkfield earthquake. Using local earthquakes we determine the anisotropic field within 1- 2 km of the main trace of the SAF at the two array locations separated by 12 km. The initial array, near the SAFOD site, was deployed for six weeks in October and November 2003, and the second array, located near the town of Parkfield, was de- ployed for 3 months following the 28 September 2004 M 6.0 Parkfield earthquake. We find the fast shear-wave polarization direction nearly fault-parallel (N40W) for stations on the main fault trace and within 100 m to the southwest of the SAF at both array locations. These fault-parallel measurements span the 100- to 150-m-wide zone of pervasive cracking and damage interpreted from fault-zone-trapped waves associated with the main fault core (Li et al., 2004, 2006). Outside of this zone, the fast orientations are scattered with some preference for orientations near N10E, roughly parallel to the regional maximum horizontal compressive stress direction (rh). In addition, fast directions are preferentially oriented parallel to a northern branch of the SAF recorded on stations in the 2004 Parkfield deployment. The measured anisotropy is likely due to a combination of stress-aligned micro- cracks away from the fault and shear fabric within the highly evolved fault core. The majority of our measurements are taken outside of the main fault core, and we es- timate the density of microcracks from the measured delay times. Apparent crack densities are approximately 3%, with large scatter. The data suggest weak depth dependence to the measured delay times for source depths between 2 and 7 km. Below 7-km source depth, the delay times do not correlate with depth suggesting higher confining pressure is forcing the microcracks to close. No coseismic variation in the anisotropic parameters is observed, suggesting little to no influence on measured splitting due to the 2004 M 6.0 Parkfield earthquake. However, the premainshock and postmainshock data presented here are from arrays separated by 12 km, limiting our sensitivity to small temporal changes in anisotropy.

Anisotropy and temporal variations in the fast azimuth of the fast shear wave beneath southern Kamchatka during 1993–2002

Parameters of split shear waves from local earthquakes in the area of the PET IRIS station (town of Petropavlovsk-Kamchatski) were measured over the period 1993–2002 for the study of anisotropic properties of rocks in the subduction zone and variations in the fast azimuth of the fast shear wave (ϕ). The dominating fast shear wave polarization directions were oriented in 1993–2002 along N90°E ± 20° in agreement with the direction of the Pacific plate motion. The normalized shear wave delay times δt SS increase to a depth of 150 km. The values of δt SS are largest (up to 20 ms/km) for earthquakes at depths of 50–60 and 90–150 km and smallest (up to 6 ms/km) for earthquakes at depths greater than 200 km. The fast azimuths for events with H < 80 km are described in terms of a horizontal transversely isotropic (HTI) model of the medium, with the axis oriented northward. Temporal variations in the fast azimuths with an amplitude of up to 90° and a predominant period of about 400–600 days are observed for events at depths of 80–120 km. The anisotropy of rocks is described by effective models of the orthorhombic and HTI symmetries. The predominant fast shear wave fast azimuths from events at depths of 120–310 km vary with time: the polarization axis was oriented to the north in 1993–1995, to the north and east in 1996–1998, to the east in 1999–2000, and to the northeast and southeast in 2001–2002. The anisotropy of rocks can be described in terms of the HTI model with the symmetry axis subparallel to the focal zone dip.