Interstation Paths Research Articles

Local station correlation: large N-arrays and DAS

The use of cross-correlation between seismic stations has had widespread applications particularly in the exploitation of ambient seismic noise. We here show how the effects of a non-ideal noise distribution can be understood by looking directly at correlation properties and show how the behaviour can be readily visualised for both seismometer and DAS configurations, taking into account directivity effects. For sources lying in a relatively narrow cone around the extension of the inter-station path, the dispersion properties of the correlation relate directly to the zone between the stations. We illustrate the successful use of correlation analysis for both a large-N array perpendicular to a major highway and DAS cable along a busy road. For correlation work, the co-array consisting of the ensemble of inter-station vectors provides an effective means of assessing the behaviour of array layouts, supplementing the standard plane-wave array response. When combined with knowledge of the suitable correlation zones for noise sources, the co-array concept provides a useful way to design array configurations for both seismometer arrays and DAS.

Analysis of Bus Line Operation Reliability Based on Copula Function

To promote the development level of urban sustainability, more and more cities have been paying attention to the improvement of public transportation. City managers intend to attract people from private cars to public transport by improving the service level of urban public transport. The operational reliability of bus lines plays a crucial role in maintaining high-level service of public transportation. Previous studies have focused on the service level of the whole line by investigating the overall stability of departure punctuality, line running time, and punctuality. This study aims to clarify the relationship between the whole line and sites in terms of the influence on the operational reliability of bus lines. This study proposes a bus route reliability evaluation method, based on copulas connect function, and the actual line data are taken as a case study. The results show that the method reveals the relationship between the overall line and the interstation paths in the context of operational reliability. This study finds the interstation path, which is more critical to the whole line. This method can provide a good reference for the diagnosis and evaluation of line operation and the optimization of bus routes. The results can support the sustainable development of public transportation.

Карты распределения скоростей волн Релея на территории Кавказа по данным кавказской сети CNET

Ambient noise surface wave tomography is a widely used method for determining the velocity structure of the upper layers of the Earth. It is based on the fact that the cross-correlation function (CCF) of noise at two stations, averaged over a long time interval, determines the Green's function of the surface wave. This allows us to estimate the group and phase velocities of surface waves on the paths between stations. The method was applied to the records of the vertical components of 67 seismic stations of the Caucasian network CNET network, which were obtained during 2018. The cross-correlation functions for all interstation paths were calculated. The dispersion curves of group and phase Rayleigh velocities for periods from 5 to 30 s were obtained by means of frequency-time analysis. The lateral distribution of the velocities was received for periods from 7 to 22 s, which correlate with velocity structure at depths of 5-25 km. The group and phase velocity maps for Rayleigh wave for periods 7, 10, 12, 14, 17, 22 s are presented.

Shear wave velocity structure beneath Bandung basin, West Java, Indonesia from ambient noise tomography

Summary We investigated the seismic shear wave velocity structure of the upper crust beneath the Bandung area in West Java, Indonesia, using ambient seismic noise tomography. We installed 60 seismographs to record ambient seismic noise continuously in the city of Bandung and its surrounding area for 8 months. After inter-station cross-correlation of recordings of ambient seismic noise, we obtained empirical Green's functions for Rayleigh waves. Group velocity dispersion curves for Rayleigh waves between periods of 1 s and 8 s were measured on each inter-station path by applying the multiple filter analysis method with phase-matched processing. The spatial variation of group velocities shows a good correlation with the geological structure of the Bandung Basin. The Rayleigh wave dispersion maps were inverted to obtain the 1D shear wave velocity profiles beneath each station, which were interpolated to infer a pseudo-3D structure under the study region. The results show that the Bandung Basin has a thick layer of sediment. Along the northern, eastern and southern mountains surrounding the Bandung Basin there is high-velocity structure, except to the west of the Tangkuban Parahu volcano, where a massive low-velocity structure extending throughout the upper crust might indicate the presence of fluids or partial melts.

Crustal and upper mantle structure beneath Abaga area in Inner Mongolia revealed by Rayleigh-wave phase velocity tomography

There are strong interactions between the crust and the mantle in the Abaga area. To study the structure of the crust and upper mantle in that area, the fundamental-mode Rayleigh-wave phase velocity dispersion along 3,331 inter-station paths were estimated by the continuous wavelet transformation method. The dispersion data were used to construct the first 2-D phase velocity maps for waves with periods of 12–80 s and a horizontal resolution of 0.5°×0.5° by applying a linear inversion. The results show that the short-period phase velocity distributions (12–20 s) are affected by exposed Cenozoic volcanoes and the thickness of the sedimentary layer. Phase velocity maps for waves with periods of 30–40 s indicate that the direction of the Solonker suture zone is in good agreement with the extension of the high-speed anomalous connection in the study area. It is concluded that the Solonker suture zone extends throughout the entire lithosphere. For periods of 30–80 s, stable low-velocity anomalies were observed near the Holocene volcanoes in the northeast and south of the study area, suggesting that the low-velocity anomaly is related to volcanic activity. Moreover, the two low-velocity anomalies may be connected, suggesting that the Darigan Volcano may have the same thermal source as the Honggeertu Volcano and even the Datong Volcano. And the thermal source of the two low-velocity anomalies is speculated to be upwelling of hot mantle material. In addition, the velocities in the crust and upper mantle are lower than those from the AK135 model and are close to the velocity for the destroyed North China Craton, and very similar to that for the East African Rift Valley. This may indicate that the lithospheric structure in the Abaga area is active.

Seismic Velocity Changes in the Backarc Continental Crust After the 2011 Mw 9.0 Tohoku‐Oki Megathrust Earthquake

AbstractThe 2011 Mw 9.0 Tohoku‐Oki megathrust earthquake accompanied coseismic and postseismic displacements around the eastern Eurasian continental plate. Noise cross correlations produced transient seismic waveforms along interstation paths in the Korean Peninsula. We measured the traveltime changes of the fundamental mode Rayleigh waves over the range of 0.03–0.08 Hz after the megathrust earthquake. The temporal seismic velocity changes in the lower crust were assessed from the traveltime changes. The traveltimes increased instantly after the megathrust earthquake and were gradually recovered over several hundreds to thousands of days. The instant shear wave velocity decreases ranged between 0.731 (±0.057)% and 4.068 (±0.173)%. The temporal medium perturbation might be caused by the transient uniaxial tensional stress due to the coseismic and postseismic displacements. The medium properties may be recovered by progressive stress field reconstruction.

PHASE VELOCITY TOMOGRAPHY OF RAYLEIGH WAVE IN QINLING‐DABIE AND ITS ADJACENT AREAS USING AMBIENT SEISMIC NOISE

AbstractThe Qinling‐Dabie orogenic belt is located between the northeastern margin of the Tibetan Plateau and the Tan‐Lu fault; it was formed by the collision of Yangtze and North China blocks. We obtain the phase velocity of Rayleigh wave at the periods of 8∼35 s in the Qinling‐Dabie orogenic belt using ambient seismic noise recorded at 160 broad‐band stations from China digital seismic network. 24 month data have been cross‐correlated to yield the empirical Rayleigh wave Green's functions. Phase velocity dispersion curves are measured for each interstation path by frequency‐time analysis. The Rayleigh wave phase speed maps agree well with each other and show clear correlations with major tectonic structures. The Dabie is characterized by high velocity anomaly at 8 s but slow velocity at 14 s, indicating the influence of high pressure (HP)/ultrahigh pressure (UHP) metamorphic regions in the upper crust. At 25 s, the velocity varies from slow in the west to high in the east across the gravity gradient zone in the Taihang‐Wuling Mountains. This pattern mainly reflects the effect of crust thickness which is thicker in the west and thin in the east. The southern segment of Tan‐Lu fault shows different features across it at 14∼35 s, suggesting that the fault zone may extend down to the crust‐mantle boundary. The slow velocity close to the fault is probably caused by the hot material upwelling. More constraints are needed by further study. Obvious slow velocities at periods of 14∼25 s in South Qinglin and northeast Sichuan basin are observed. We could not determine whether this low velocity zone is due to the Tibetan lower crustal flow and/or the delamination of the South Qinglin now. Study of the 3‐D shear wave velocity structure in the crust and upper mantle is necessary to constrain the geodynamics of this region in the future.

Complex deformation in the Caucasus region revealed by ambient noise seismic tomography

Cross-correlation of 3years of ambient seismic noise recorded at 35 seismic stations deployed in Caucasus region yields hundreds of short-period surface-wave phase-speed dispersion curves on inter-station paths. We inverted these measurements using two techniques to construct tomographic images of the principal geological units of Caucasus. High-resolution isotropic and azimuthally anisotropic phase-velocity maps (at periods between 5 and 20s) and shear-velocity tomographic maps between 5 and 30km are generated.The resulting maps show a velocity dichotomy between the Caucasus region and the surrounding that is interpreted in term of changes in crustal thickness. There is also a strong dichotomy in the anisotropic pattern between the eastern part and the western part of the Caucasus. This difference in both amplitudes and directions of the 2ψ anisotropy is linked to the tectonic regime changes in the region. These observations suggest a good correlation between the tomographic models and the geology of the region. It was also possible to identify the early stage of the indentation of the Arabian Plate into the Eurasian plate, as well as to detect the possible magma chamber responsible for the Javakheti highland.

Shear-wave velocity structure of the Tongariro Volcanic Centre, New Zealand: Fast Rayleigh and slow Love waves indicate strong shallow anisotropy

Models of the velocity structure of volcanoes can help define possible magma pathways and contribute to calculating more accurate earthquake locations, which can help with monitoring volcanic activity. However, shear-wave velocity of volcanoes is difficult to determine from traditional seismic techniques, such as local earthquake tomography (LET) or refraction/reflection surveys. Here we use the recently developed technique of noise cross correlation of continuous seismic data to investigate the subsurface shear-wave velocity structure of the Tongariro Volcanic Centre (TgVC) of New Zealand, focusing on the active Ruapehu and Tongariro Volcanoes. We observe both the fundamental and first higher-order modes of Rayleigh and Love waves within our noise dataset, made from stacks of 15min cross-correlation functions. We manually pick group velocity dispersion curves from over 1900 correlation functions, of which we consider 1373 to be high quality. We subsequently invert a subset of the fundamental mode Rayleigh- and Love-wave dispersion curves both independently and jointly for one dimensional shear-wave velocity (Vs) profiles at Ruapehu and Tongariro Volcanoes. Vs increases very slowly at a rate of approximately 0.2km/s per km depth beneath Ruapehu, suggesting that progressive hydrothermal alteration mitigates the effects of compaction driven velocity increases. At Tongariro, we observe larger Vs increases with depth, which we interpret as different layers within Tongariro's volcanic system above altered basement greywacke. Slow Vs, on the order of 1–2km/s, are compatible with P-wave velocities (using a Vp/Vs ratio of 1.7) from existing velocity profiles of areas within the TgVC, and the observations of worldwide studies of shallow volcanic systems that used ambient noise cross-correlation methods.Most of the measured group velocities of fundamental mode Love-waves across the TgVC are 0.1–0.4km/s slower than those of fundamental mode Rayleigh-waves in the frequency range of 0.25–1Hz. First-higher mode Love-waves are similarly slower than first-higher mode Rayleigh waves. This is incompatible with synthetic dispersion curves we calculate using isotropic, layered velocity models appropriate for Ruapehu and Tongariro, in which Love waves travel more quickly than Rayleigh waves of the same period. The Love-Rayleigh discrepancy is likely due to structures such as dykes or cracks in the vertical plane having increased influence on surface-wave propagation. However, several measurements at Ruapehu have Love-wave group velocities that are faster than Rayleigh-wave group velocities. The differences between the Love- and Rayleigh-wave dispersion curves also vary with the azimuth of the interstation path across Ruapehu and Tongariro Volcanoes. Significant azimuthal dependence of both Love and Rayleigh-wave velocities are also observed. This suggests azimuthal anisotropy within the volcanic structures, which coupled with radial anisotropy, makes the Vs structures of Ruapehu and Tongariro Volcanoes anisotropic with orthorhombic or lower order symmetry. We suggest that further work to determine three-dimensional volcanic structures should include provisions for such anisotropy.

Transdimensional Love-wave tomography of the British Isles and shear-velocity structure of the East Irish Sea Basin from ambient-noise interferometry

We present the first Love-wave group velocity and shear velocity maps of the British Isles obtained from ambient noise interferometry and fully non-linear inversion. We computed interferometric inter-station Green's functions by cross-correlating the transverse component of ambient noise records retrieved by 61 seismic stations across the UK and Ireland. Group velocity measurements along each possible inter-station path were obtained using frequency-time analysis and converted into a series of inter-station traveltime datasets between 4 and 15 seconds period. Traveltime uncertainties estimated from the standard deviation of dispersion curves constructed by stacking randomly-selected subsets of daily cross-correlations, were observed to be too low to allow reasonable data fits to be obtained during tomography. Data uncertainties were therefore estimated again during the inversion as distance-dependent functionals. We produced Love-wave group velocity maps within 8 different period bands using a fully non-linear tomography method which combines the transdimensional reversible-jump Markov chain Monte Carlo (rj-McMC) algorithm with an eikonal raytracer. By modelling exact raypaths at each step of the Markov chain we ensured that the non-linear character of the inverse problem was fully and correctly accounted for. Between 4 and 10 seconds period, the group velocity maps show remarkable agreement with the known geology of the British Isles and correctly identify a number of low-velocity sedimentary basins and high-velocity features. Longer period maps, in which most sedimentary basins are not visible, are instead mainly representative of basement rocks. In a second stage of our study we used the results of tomography to produce a series of Love-wave group velocity dispersion curves across a grid of geographical points focussed around the East Irish Sea sedimentary basin. We then independently inverted each curve using a similar rj-McMC algorithm to obtain a series of one-dimensional shear velocity profiles. By merging all 1D profiles, we created a fully three-dimensional model of the crust beneath the East Irish Sea. The depth to basement in this model compares well with that averaged from seismic reflection profiles. This result is the first 3-dimensional model in the UK with fully quantified uncertainties: it shows basin depths and basement structures, and their concomitant uncertainties.

Layered anisotropy within the crust and lithospheric mantle beneath the Sea of Japan

Continental rifting during the Oligocene to mid-Miocene caused the opening of the Sea of Japan and the separation between the Japanese Islands and the Eurasian Plate. The tectonic evolution in the Sea of Japan is important for understanding the evolution of back-arc regions in active convergent margins. Here, we use data from the seismic stations surrounding the Sea of Japan to map the Rayleigh-wave azimuthal anisotropy in the crust and lithospheric mantle beneath the Sea of Japan. We explore the variations of Rayleigh-wave phase-velocity beneath the Sea of Japan in a broad period range (30–80s). Rayleigh-wave dispersion curves are measured by the two-station technique for a total of 231 interstation paths using vertical-component broad-band waveforms at 22 seismic stations around the Sea of Japan from 1411 global earthquakes. The resulting maps of Rayleigh-wave phase velocity and azimuthal anisotropy allow the examination of azimuthal anisotropy at specific periods. They exhibit several regions with different isotropic and anisotropic patterns: the Japan Basin displays fast velocities at shorter periods (30 and 40s) with NNE-SSW anisotropy, whereas at 60s and longer, the velocities become slow even if the anisotropy remains NE-SW; the East China Sea shows fast velocities at all periods (30–80s) with constant NW-SE anisotropy. Trench-normal anisotropy beneath the Japanese Islands is found at short periods (30–40s) and become trench-parallel at periods of 60s and longer. Overall, our model resolves two layers of anisotropy, the shallowest and deepest layers being potentially related to frozen deformation due to recent geodynamic events, and asthenospheric flow, respectively.

Continent–arc collision in the Banda Arc imaged by ambient noise tomography

The tectonic configuration of the Banda region in southeast Asia captures the spatial transition from subduction of Indian Ocean lithosphere to subduction and collision of the Australian continental lithosphere beneath the Banda Arc. An ongoing broadband seismic deployment funded by NSF is aimed at better understanding the mantle and lithospheric structure in the region and the relationship of the arc–continent collision to orogenesis. Here, we present results from ambient noise tomography in the region utilizing this temporary deployment of 30 broadband instruments and 39 permanent stations in Indonesia, Timor Leste, and Australia. We measure dispersion curves for over 21,000 inter-station paths resulting in good recovery of the velocity structure of the crust and upper mantle beneath the Savu Sea, Timor Leste, and the Nusa Tenggara Timur (NTT) region of Indonesia. The resulting three dimensional model indicates up to ∼25% variation in shear velocity throughout the plate boundary region; first-order velocity anomalies are associated with the subducting oceanic lithosphere, subducted Australian continental lithosphere, obducted oceanic sediments forming the core of the island of Timor, and high velocity anomalies in the Savu Sea and Sumba. The structure in Sumba and the Savu Sea is consistent with an uplifting forearc sliver. Beneath the island of Timor, we confirm earlier inferences of pervasive crustal duplexing from surface mapping, and establish a link to underlying structural features in the lowermost crust and uppermost mantle that drive upper crustal shortening. Finally, our images of the volcanic arc under Flores, Wetar, and Alor show high velocity structures of the Banda Terrane, but also a clear low velocity anomaly at the transition between subduction of oceanic and continental lithosphere. Given that the footprint of the Banda Terrane has previously been poorly defined, this model provides important constraints on tectonic reconstructions that formerly have lacked information on the lower crust and uppermost mantle.

Phase velocities of Rayleigh and Love waves in central and northern Europe from automated, broad-band, interstation measurements

The increasingly dense coverage of Europe with broad-band seismic stations makes it possible to image its lithospheric structure in great detail, provided that structural information can be extracted effectively from the very large volumes of data. We develop an automated technique for the measurement of interstation phase velocities of (earthquake-excited) fundamental-mode surface waves in very broad period ranges. We then apply the technique to all available broad-band data from permanent and temporary networks across Europe. In a new implementation of the classical two-station method, Rayleigh and Love dispersion curves are determined by cross-correlation of seismograms from a pair of stations. An elaborate filtering and windowing scheme is employed to enhance the target signal and makes possible a significantly broader frequency band of the measurements, compared to previous implementations of the method. The selection of acceptable phase-velocity measurements for each event is performed in the frequency domain, based on a number of fine-tuned quality criteria including a smoothness requirement. Between 5 and 3000 single-event dispersion measurements are averaged per interstation path in order to obtain robust, broad-band dispersion curves with error estimates. In total, around 63,000 Rayleigh- and 27,500 Love-wave dispersion curves between 10 and 350 s have been determined, with standard deviations lower than 2 per cent and standard errors lower than 0.5 per cent. Comparisons of phase-velocity measurements using events at opposite backazimuths and the examination of the variance of the phase-velocity curves are parts of the quality control. With the automated procedure, large data sets can be consistently and repeatedly measured using varying selection parameters. Comparison of average interstation dispersion curves obtained with different degrees of smoothness shows that rough perturbations do not systematically bias the average dispersion measurement. They can, therefore, be treated as random but they do need to be removed in order to reduce random errors of the measurements. Using our large new data set, we construct phase-velocity maps for central and northern Europe. According to checkerboard tests, the lateral resolution in central Europe is ≤150 km. Comparison of regional surface-wave tomography with independent data on sediment thickness in North-German Basin and Polish Trough confirms the high-resolution potential of our phase-velocity measurements. At longer periods, the structure of the lithosphere and asthenosphere around the Trans-European Suture Zone (TESZ) is seen clearly. The region of the Tornquist-Teisseyre-Zone in the southeast is associated with a stronger lateral contrast in lithospheric thickness, across the TESZ compared to the region across the Sorgenfrei-Tornquist-Zone in the northwest.

Lg attenuation tomographic models of Himalaya and southern Tibet

We investigate the Lg attenuation structure of the crust beneath the tectonically complex Himalaya and southern Tibet regions adopting a tomographic regionalization method. A total of 1671 earthquake waveforms registered at 38 seismic stations operated in the region are selected for the initial LgQ measurements using the standard two-station method. Q0 (1Hz LgQ) values of 76 high quality interstation paths are finally considered as input for the tomographic inversion. The estimates of Q0 exhibit distinct variations in the crustal attenuation from north to south across the whole region. The zones of lowest Q0 values (<50) are observed along the Main Central Thrust (MCT) in Sikkim, Lesser Himalaya and some parts of the Greater Himalaya. Southern Tibet is characterized by low Q0 (<100) values. These observations are consistent with the results of body wave attenuation and velocity structure reported for the region. We interpret the variations in the attenuation characteristics in terms of both the intrinsic and scattering contributions caused by thermal effects, presence of aqueous fluids as well as heterogeneities present below these seismically active regions. Our results are found to be comparable with the other parts of Himalaya and Tibet.

Lithospheric structure beneath the East China Sea revealed by Rayleigh-wave phase velocities

We explore the variations of Rayleigh-wave phase-velocity beneath the East China Sea in a broad period range (5–200s). Rayleigh-wave dispersion curves are measured by the two-station technique for a total of 373 interstation paths using vertical-component broad-band waveforms at 32 seismic stations around the East China Sea from 6891 global earthquakes.The resulting maps of Rayleigh-wave phase velocity and azimuthal anisotropy provide a high resolution model of the lithospheric mantle beneath the East China Sea. The model exhibits four regions with different isotropic and anisotropic patterns: the Bohai Sea, belonging to the North China Craton, displays a continental signature with fast velocities at short periods; the Yellow Sea, very stable unit associated with low deformation, exhibits fast velocities and limited anisotropy; the southern part of the East China Sea, with high deformation and many fractures and faults, is related to slow velocities and high anisotropic signature; and the Ryukyu Trench shows high-velocity perturbations and slab parallel anisotropy.

Crustal and uppermost mantle structure of SE Tibetan plateau from Rayleigh-wave group-velocity measurements

A shear-wave velocity model of the crust and uppermost mantle beneath the SE Tibetan plateau was derived by inverting Rayleigh-wave group-velocity measurements of periods between 10 and 70 s. Rayleigh-wave group-velocity dispersions along more than 3,000 interstation paths were measured based on analysis of teleseismic waveform data recorded by temporary seismic stations. These observations were then utilized to construct 2D group-velocity maps in the period range of 10–70 s. The new group-velocity maps have an enhanced resolution compared with previous global and regional group-velocity models in this region because of the denser and more uniform data coverage. The lateral resolution across the region is about 0.5° for the periods used in this study. Local dispersion curves were then inverted for a 3D shear-wave velocity model of the region by applying a linear inversion scheme. Our 3D shear-wave model confirms the presence of low-velocity zones (LVZs) in the crust beneath the northern part of this region. Our imaging shows that the upper-middle crustal LVZ beneath the Tengchong region is isolated from these LVZs beneath the eastern and northern part of this region. The upper–middle crustal LVZ may be regarded as evidence of a magma chamber in the crust beneath the Tengchong Volcanoes. Our model also reveals a slow lithospheric structure beneath Tengchong and a fast shield-like mantle beneath the stable Yangtze block.

Anisotropic Rayleigh wave phase velocity maps of eastern China

AbstractWe explore the variations of Rayleigh wave phase velocity beneath eastern China in a broad period range (20–200 s). Rayleigh wave dispersion curves are measured by the two‐station technique for a total of 734 interstation paths using vertical component broadband waveforms at 39 seismic stations in eastern China from 466 global earthquakes. In addition, 599 waveform inversion interstation measurements were added to this data set. The interstation dispersion curves are then inverted for high‐resolution isotropic and azimuthally anisotropic phase velocity maps at periods between 20 and 200 s. At shorter periods sampling the crustal depth range, phase velocities are higher in the southeastern part of the region, reflecting the thinner crust there. The Jiangnan Belt separates Cathaysia from the Yangtze Craton, the latter with thicker crust and a deep, high‐velocity cratonic root. The eastern part of Yangtze Craton, however, east of 115–116°E, does not display a deep root and has a thin lithosphere. Azimuthal anisotropy at long periods (&gt;120 s) shows fast propagation directions broadly similar to that of the absolute plate motion. Beneath Cathaysia and eastern Yangtze Craton, anisotropy in the asthenosphere is strong and suggests coast‐perpendicular flow. Asthenospheric flow from beneath China's thick continental lithosphere toward the thinner lithosphere of the margin and the resulting decompression melting may be the fundamental causes of the intraplate basaltic volcanism along the eastern coast of China.

Procedure to construct three-dimensional models of geothermal areas using seismic noise cross-correlations: application to the Soultz-sous-Forets enhanced geothermal site

The aim of this work is to assess the feasibility of the noise-cross correlation tomography method for imaging and detecting potential geothermal reservoirs even in highly urbanised areas. We tested the noise correlation method to reconstruct the shape of the Soutlz-sous- Forˆets enhanced geothermal system (northern Alsace, France). We inverted Rayleigh waves (RWs) reconstructed from cross-correlations of 15 months of ambient seismic noise recorded by a seismic array installed around the Soutlz geothermal power plant. By correlating noise records between 22 receivers, we reconstructed RWs with sufficient signal-to-noise ratio for 231 interstation paths. The reconstructed waveforms were used to estimate group velocity dispersion curves at periods between 1.0 and 5.0 s. The results were inverted for 2-D group velocity maps, and finally for a 3-D S-wave velocity model from 0 to 5.2 km depth. Our results clearly show the presence of low velocity bodies in the crystalline basement below the Soultz power plant at depth of 4-5 km, and at shallower depth (2.5-3.5 km) beneath the Rittershoffen and Woerth villages. These observations, in agreement with some previous studies, confirm that our procedure is suitable for geothermal exploration. Furthermore, the model presented here provides some suggestions to improve the existing geothermal power plant and inferences for further explorations in the area.

Seismic Attenuation Characteristics along the Hi-CLIMB Profile in Tibet from Lg Q Inversion

We investigate the Q of crustal Lg waves along the Himalayan-Tibetan Continental Lithosphere during Mountain Building (Hi-CLIMB) profile in Tibet using a reliable two-station method. Four earthquakes of M ≥5:5 recorded along a 800-km- long densely-spaced seismic profile are used to extract Lg-wave spectra. One hundred seven high quality interstation paths that allow us to use the standard two-station method for Lg Q measurement are selected from 2369 possible pairs. By using those 107 interstation Q0 (1 Hz Lg Q) values as input, lateral variations in attenuation char- acteristics are derived through inversion. The region has Q0 estimates ranging from 88 5 to 165 15, with the lowest value in the Qiangtang terrane and the highest across some part of the lower Himalaya. The lowest value is attributed to the high Poisson's ratio, higher temperatures, as well as partial melts in the crust. The results do not contain any detectable variation in Q0 either across the Himalaya-Tibet collision zone (i.e., the Indus-Tsangpo suture zone) or along the major Bangong- Nujiang suture zone (BNSZ). Consistent low Q0 values along the Hi-CLIMB profile manifest the presence of omnipresent partial melting of the crust throughout the plateau.

Temporal changes of seismic velocity associated with the 2006 Mw 6.1 Taitung earthquake in an arc‐continent collision suture zone

To detect temporal changes of elastic properties associated with the 2006 Mw 6.1 Taitung earthquake in southeast Taiwan, whereby the collision between the Luzon arc riding on the Philippine Sea plate and the Asian margin is taking place, we construct the Green's functions from auto‐ and cross‐correlation function (ACF and CCF) of continuous ambient noise between stations. Time lapse changes in the retrieved coda arrivals are estimated for monitoring spatiotemporal variations of seismic velocities around the ruptured fault zones. To the south of the main shock epicenter where the earthquake may have ruptured along two perpendicularly‐intersecting fault planes resulting in intense coseismic slip and widely‐dispersed aftershocks, the ACFs (2–8 Hz) at nearby stations reveal a large, sudden velocity drop of over 1% after the event occurrence. The CCFs (0.1–0.9 Hz) also show moderate reduction about 0.1% for the interstation paths traveling through the southeast quadrant of the focal sphere which has experienced the maximum peak ground acceleration and dilatational strain change. The intense seismic shaking combined with the rupture‐induced damage near the junction of the two faults is the most plausible cause for such localized, but significant coseismic velocity reduction. The response of groundwater levels to precipitation is in‐phase correlated with velocity variations over 3 years of investigation. Unlike the postseismic surface displacements gradually relaxed, the velocity remained slow until October 2006 due to the typhoon‐induced heavy rains between May and September.