Temporary Array Research Articles

Imaging Karatungk Cu-Ni Mine in Xinjiang, Western China with a Passive Seismic Array

Karatungk Mine is the second-largest Cu-Ni sulfide mine in China. However, the detailed structure beneath the mine remains unclear. Using continuous waveforms recorded by a dense temporary seismic array, here we apply ambient noise tomography to study the shallow crustal structure of Karatungk Mine down to ~1.3 km depth. We obtain surface-wave dispersions at 0.1–1.5 s by calculating cross-correlation functions, which are inverted for 3D shear-wave structure at the top-most (0–1.3 km) crust by a joint inversion of group and phase dispersions. Our results show that low-velocity zones beneath Y1 ore-hosting intrusion (hereafter called Y1) at 0–0.5 km depth and northwest of the Y2 ore-hosting intrusion (hereafter called Y2) at 0–0.6 km depth are consistent with highly mineralized areas. A relatively high-velocity zone is connected with a weakly mineralized area located to the southeast of Y2 and Y3 (hereafter called Y3) ore-hosting intrusions. Two high-velocity zones, distributed at 0.7–1.3 km depth in the northernmost and southernmost parts of the study area respectively, are interpreted to be igneous rocks related to early magma intrusion. Furthermore, the low-velocity zone at 0.7–1.3 km depth in the middle of the study area may be related to: a possible channel related to initial magma transport; mine strata or a potentially mineralized area. This study demonstrates a new application of dense-array ambient noise tomography to a mining area that may guide future studies of mineralized regions.

Seismic anisotropy in the lowermost mantle beneath North America from SKS-SKKS splitting intensity discrepancies

We examined SKS-SKKS splitting intensity discrepancies for phases that sample the lowermost mantle beneath North America, which has previously been shown to exhibit seismic anisotropy using other analysis techniques. We examined data from 25 long-running seismic stations, along with 244 stations of the temporary USArray Transportable Array, located in the eastern, southeastern and western U.S. We identified 279 high-quality SKS-SKKS wave pairs that yielded well-constrained splitting intensity measurements for both phases. Of the 279 pairs, a relatively small number (15) exhibited discrepancies in splitting intensity of 0.4 s or greater, suggesting a contribution to the splitting of one or both phases from anisotropy in the lowermost mantle. Because only a small minority of SK(K)S phases examined in this study show evidence of being affected by lowermost mantle anisotropy, the traditional interpretation that splitting of these phases primarily reflects anisotropy in the upper mantle directly beneath the stations is appropriate. The discrepant pairs exhibited a striking geographic trend, sampling the lowermost mantle beneath the southern U.S. and northern Mexico, while other regions were dominated by non-discrepant pairs. We carried out ray theoretical modeling of simple anisotropy scenarios that have previously been suggested for the lowermost mantle beneath North America, invoking the alignment of post-perovskite due to flow induced by the impingement of the remnant Farallon slab on the core-mantle boundary. We found that our measurements are generally consistent with this model and with the idea of slab-driven flow, but relatively small-scale lateral variations in the strength and/or geometry of lowermost mantle anisotropy beneath North America are also likely present.

Spatiotemporal Seismic Structure Variations Associated With the 2018 Kīlauea Eruption Based on Temporary Dense Geophone Arrays

AbstractDuring the 2018 Kīlauea volcanic eruption, lava erupted from a series of new fissures in the lower East Rift Zone more than 30 km away from the summit through a dike intrusion. Between late May and early August, variations in the effusion rate at the persistent eruptive vent (Fissure 8) were observed following near‐daily summit caldera collapse events. Targeting the ongoing eruptive activity and the subsurface magma movement, we deployed a temporary dense seismic array. The observed time‐lapse changes in seismic velocity associated with the response of the summit collapse in three areas are presented in this study. The results show (1) clear spatially dependent co‐collapse velocity reductions across the newly‐intruded dike structure, (2) a gradual post‐collapse velocity increase near Fissure 8 correlated with the surge of magma supply, and (3) a gradual post‐collapse velocity increase on the summit likely associated with reservoir pressurization and crustal welding.

A widespread mid-crustal low-velocity layer beneath Northeast China revealed by the multimodal inversion of Rayleigh waves from ambient seismic noise

In this study, we re-examined the crustal and uppermost mantle shear velocity structure beneath Northeast China using the broadband ambient seismic noise data from several temporary and permanent arrays in this region. By employing our newly developed interferometric analysis method (F-J method) for array noises and the corresponding multi-mode dispersion curve inversion approach, we obtained an integrated crustal and uppermost mantle shear velocity model of this region. We divided the study area into 39 overlapping sub-areas and then applied the F-J method to each sub-area to obtain a 3D shear velocity model. We extracted the fundamental mode and up to 4 higher-mode dispersion curves of Rayleigh waves in frequencies ranging 0.02–0.45 Hz from the ambient seismic noise data, inverted multi-mode dispersion curves, and obtained local 1D shear velocity model for each sub-area. By integrating all 1D shear velocity models of the 39 overlapping sub-areas, we obtained a 3D crustal and upper mantle model of Northeast China. Our integrated 3D model shows a widely distributed low-velocity zone at a depth of approximately 10–20 km located beneath the northern part of Xing-Mong orogenic belt and Songliao basin and the Changbaishan Mountain. Moreover, a large high-velocity zone underneath the region around the northern edge of Songliao basin is observed at a depth of approximately 40–50 km depth. These results provide evidence to improve our understanding of the tectonics of Northeast China.

Normal faulting activated by hydraulic fracturing: A case study from the Barnett Shale, Fort Worth Basin

Surface microseismic arrays enable long-term field-scale monitoring over multiple stimulations during the life of an unconventional field. In this study, we show highly economic methods of monitoring with sparse surface arrays in the Barnett Shale and develop an alternatative method of processing to enable good vertical and horizontal resolution of located events. We show that sparse surface monitoring arrays enable not only the detection and location of high numbers of microseismic events but also source mechanism characterization. This case study illustrates how hydraulic fracturing activated normal faulting at a distance of approximately 1 mile from stimulated wells. We show that the source mechanism enables us to resolve between newly created hydraulic fractures and activated faults. The differences in source mechanisms and b-values of newly created fractures and activated faults are consistent with independently processed temporary star-like arrays, which are also deployed over the same stimulation.

Seismic tomography of compressional wave velocity and attenuation structure for Makushin Volcano, Alaska

Located in the central portion of the Alaska-Aleutian subduction zone, Makushin Volcano is among the most active volcanoes in the United States and has been classified as a very high threat by the National Volcano Early Warning System based on its eruptive history and proximity to the City of Unalaska and international air routes. In 2015, five standalone seismic stations and three mini seismic arrays of 15 stations each were deployed on Unalaska Island to supplement the Alaska Volcano Observatory permanent seismic network. This temporary array was operational for one year. Taking advantage of the increased azimuthal coverage and the array's increased earthquake detection capability, we developed body-wave Vp and Vp/Vs seismic images of the velocity structure beneath the volcano. Our results show a complex structure with the upper 4 km of the crust. The shallow high-Vp features possibly delineate remnant magma pathways or conduits. Low-Vp regions are found east of the caldera at approximately 4–8 km depth. This is in agreement with previous seismic tomographic work and InSAR-based geodetic models, which had identified this region as a possible long-term source of magma. A three-dimensional compressional wave attenuation model complemented the velocity models. In general, we observe regions of corresponding low P-wave velocity and high attenuation east of the summit caldera between 5 and 7 km depth, supporting the presence of a melt-rich zone. The absolute Vp values (5.5–6.0 km/s) in this region are consistent with the presence of a magma body with a mafic signature. We suggest that a triggering mechanism that involves mafic injections into a more evolved shallow magma chamber is representative of the current magmatic system below the volcano, similarly to what petrological studies have inferred for past eruptions.

Mapping of Eastern North Atlantic Ocean seismicity from Po/So observations at a mid-aperture seismological broad-band deep sea array

SUMMARYA mid-aperture broad-band test array (OBS array DOCTAR) was deployed from June 2011 to April 2012 about 100 km north of the Gloria fault in the Eastern North Atlantic in about 5000 m water depth. In addition arrays were installed on Madeira Island and in western Portugal mainland. For the first time in the Eastern North Atlantic, we recorded a large number of high frequency Po and So waves from local and regional small and moderate earthquakes (ML < 4). An incoherent beamforming method was adapted to scan continuous data for such Po and So arrivals applying a sliding window waveform migration and frequency–wavenumber technique. We identify about 320 Po and 1550 So arrivals and compare the phase onsets with the ISC catalogue (ISC 2015) for the same time span. Up to a distance of 6° to the DOCTAR stations all events listed in the ISC catalogue could be associated to Po and So phases. Arrivals from events in more than 10° distance could be identified only in some cases. Only few Po and/or So arrivals were detected for earthquakes from the European and African continental area, the continental shelf regions and for earthquakes within or northwest of the Azores plateau. Unexpectedly, earthquake clusters are detected within the oceanic plates north and south of the Gloria fault and far from plate boundaries, indicating active intraplate structures. We also observe and locate numerous small magnitude earthquakes on the segment of the Gloria fault directly south of DOCTAR, which likely coincides with the rupture of the 25 November 1941 event. Local small magnitude earthquakes located beneath DOCTAR show hypocentres up to 30 km depth and strike-slip focal mechanisms. A comparison with detections at temporary mid-aperture arrays on Madeira and in western Portugal shows that the deep ocean array performs much better than the island and the continental array regarding the detection threshold for events in the oceanic plates.We conclude that sparsely distributed mid-aperture seismic arrays in the deep ocean could decrease the detection and location threshold for seismicity with ML < 4 in the oceanic plate and might constitute a valuable tool to monitor oceanic plate seismicity.

Empirical Green's Function Retrieval Using Ambient Noise Source‐Receiver Interferometry

AbstractEmpirical Green's function (EGF) retrieval commonly relies on cross‐correlating the long‐term ambient seismic wavefield that is simultaneously recorded at multiple stations. Recent studies have demonstrated observationally that cross‐correlating the coda of ambient noise cross‐correlation functions enables reconstruction of the EGFs, regardless of the operating time of the stations. In this study, we examine the feasibility of using the nondiffuse energy (i.e., surface waves) of the ambient noise cross‐correlation functions to retrieve EGFs between asynchronous stations. We show that source‐receiver interferometry (SRI), which is conventionally applied to reconstruct virtual seismograms between earthquake‐station pairs, provides an effective framework to retrieve EGFs between asynchronous stations. Our implementation of SRI exploits the nondiffuse wavefield rather than the scattered coda waves that may be contaminated by incoherent energy under nonideal (e.g., sparse, noisy, and short‐duration) network configurations. We demonstrate the robustness of SRI by retrieving asynchronous EGFs and performing seismic tomography between (1) nearby stations and (2) distant temporary arrays from southern Australia. The additional raypaths from asynchronous EGFs provide better illumination of small‐scale crustal structures beneath the regional network. In the larger‐scale example, involving two asynchronous arrays, SRI offers new constraints to the sparsely sampled region along the continental margin of southern Australia. The resulting velocity model agrees well with the independent structural constraints from each seismic array study and sedimentary thickness measurements. This study demonstrates that SRI is a promising tool for integrating seismic arrays operated at different times and can greatly benefit the efforts of improving data coverage and resolution in seismic imaging.

Estimation of High-Frequency Spectral Decay (κ) for the Gyeongju Area in South Korea

ABSTRACT The high-frequency spectral decay parameter kappa (κ) is estimated for the Gyeongju area using records of the temporary seismic array in the Gyeongju area (TSAG), which operated from 2010 to 2013. Gyeongju is a city located in the southeastern part of the Korean Peninsula. Moderate earthquakes frequently occurred around the Gyeongju area in the past; the area has experienced numerous earthquakes for more than a year since an earthquake of ML 5.8 occurred in Gyeongju on 12 September 2016. It is also an important place from an engineering point of view because nuclear facilities are located in the area. The high-frequency spectral decay model of Anderson and Hough (1984) and the standardized procedure for the κ-estimation suggested by Ktenidou et al. (2013) are applied to estimate κ for the Gyeongju area. The estimation gives rise to the equation, κ=0.0219(±0.00151)+7.56×10−5(±1.45×10−5)R indicating that the site-specific κ0-value and the slope κR are 0.0219 s and 7.56×10−5 s/km, respectively, for the Gyeongju area. The κ-estimation for individual stations of TSAG was done to obtain the site-specific κ0-values of stations. Despite the close distance between TSAG stations, the site-specific κ0-values of each station vary. Other than TSAG, κ is also estimated for the permanent borehole station HDB, which has been operated by the Korea Institute of Geoscience and Mineral Resources and is located in the Gyeongju area. The estimated κ0-value of 0.0178 s for HDB is smaller than that of TSAG by 0.0219 s. This is because HDB is a borehole station, but all stations of TSAG are installed at surface. The variation of the κ0-values among the individual TSAG stations and the difference between κ0 for TSAG and that for HDB indicate that spectral decay characteristic at high frequency is affected by surficial sediment layers.

The 2017 ML 5.4 Pohang earthquake sequence, Korea, recorded by a dense seismic network

The 2017 ML 5.4 Pohang Earthquake in southeastern Korea, which was induced by fluid injection from an enhanced geothermal system, and its foreshock–aftershock sequence has been recorded by a dense and portable temporary seismic array. The hypocentral distribution of the earthquake sequence reveals the reactivation of a complex subsurface fault system that was previously unmapped. Aftershock seismicity propagated to the southwest and revealed previously unknown fault segments. The aftershocks tend to concentrate along the tips of the main rupture, with a lack of aftershocks around the mainshock hypocenter. This pattern suggests that the stress in the mainshock area was almost completely released during the mainshock and early aftershock phase, and that the recurrence of moderate to large earthquakes in the hypocentral area is unlikely compared with the marginal areas. A new rupture occurred along the southwestern margin of the main rupture after the largest aftershock (ML 4.6).

Temporal changes in the distinct scattered wave packets associated with earthquake swarm activity beneath the Moriyoshi-zan volcano, northeastern Japan

We investigated temporal changes in the waveforms of S-coda from triggered earthquakes around the Moriyoshi-zan volcano in northeastern Japan. Seismicity in the area has drastically increased after the 2011 off the Pacific coast of Tohoku earthquake, forming the largest cluster to the north of the volcano. We analyzed distinct scattered wave packets (DSW) that are S-to-S scattered waves from the mid-crust and appeared predominantly at the high frequency range. We first investigated the variation of DSW for event groups with short inter-event distances and high cross-correlation coefficients (CC) in the time window of direct waves. Despite the above restriction, DSW showed temporal changes in their amplitudes and shapes. The change occurred gradually in some cases, but temporal trends were much more complicated in many cases. We also found that the shape of DSW changed in a very short period of time, for example, within ~ 12 h. Next, we estimated the location of the origin of the DSW (DSW origin) by applying the semblance analysis to the data of the temporary small-aperture array deployed to the north of the largest cluster of triggered events. The DSW origin is located between the largest cluster within which hypocentral migration had occurred and the low-velocity zone depicted by a tomographic study. This spatial distribution implies that the DSW origin was composed of geofluid-accumulated midway in the upward fluid movement from the low-velocity zone to the earthquake cluster. Though we could not entirely exclude the possibility of the effect of the event location and focal mechanisms, the temporal changes in DSW waveforms possibly reflect the temporal changes in scattering properties in and/or near the origin. The quick change in DSW waveforms implies that fast movement of geofluid can occur at the depth of the mid-crust.

Imaging the Fault Damage Zone of the San Jacinto Fault Near Anza With Ambient Noise Tomography Using a Dense Nodal Array

AbstractWe apply the double‐beamforming tomography to a monthlong temporary dense seismic array to obtain high‐resolution images of the San Jacinto Fault's damage zone. We obtain Rayleigh waves between 0.3‐ and 0.8‐s periods via vertical‐vertical noise cross correlation, apply double beamforming to obtain phase velocities, and apply a piecewise 1‐D least squares inversion to obtain shear velocities in the top 300 m. We observe a ~200‐m‐wide low‐velocity zone that narrows with depth, which we interpret as the main damage zone in addition to two other ~100‐m‐wide subsidiary zones corresponding to secondary damaged structures, agreeing with the distribution of fault zone trapped waves produced by local earthquakes. The primary damage zone asymmetry indicates that materials on the northeast side of the fault are stiffer at seismogenic depth and suggests that large San Jacinto earthquakes tend to nucleate to the southeast and propagate to the northwest.

Multiscale spatial distribution of crustal seismic anisotropy beneath the northeastern margin of the Tibetan plateau and tectonic implications of the Haiyuan fault

The Haiyuan fault is a large strike-slip fault in the northeastern (NE) margin of the Tibetan plateau. Using new data recorded by a temporary seismic array, this study obtains crustal seismic anisotropy measurements in different scales across the Haiyuan fault by analyzing splitting of shear-waves from local earthquakes. Parameters of shear-wave splitting (SWS) indicate clear zoning characteristics in seismic anisotropy in the upper crust. The study area is divided into two different anisotropic subzones by the Haiyuan fault. The dominant orientation of fast polarizations is NNE or NE north of the faults and WNW to EW south of the fault. This pattern of spatial variation indicates the controlling impact of Haiyuan fault to observed seismic anisotropy. With a zone of dozens of kilometers from the Haiyuan fault, the dominant orientations of fast polarizations are WNW, generally coincidental with the strike of the Haiyuan fault except for several stations. It indicates that the influence of the fault on stress could reach to about 10 km from the fault distance. Spatial pattern of anisotropy indicates multiple effects by stress, faults and local tectonics. It suggests different anisotropic mechanisms including stress-induced anisotropy and tectonic (or structural) anisotropy. The dominant orientation of fast polarizations can indicate in situ maximum horizontal principal compressive stress although there is always influence of tectonics.The normalized time-delays are higher around the Haiyuan fault than those away from the fault, suggesting stronger crustal seismic anisotropy in a narrow belt around the strike-slip fault zone. The SWS parameters suggest that the Haiyuan fault is the actual crustal boundary of NE Tibetan plateau, about 200 km north of the reported boundary of the Tibetan plateau block. Combining with other studies in the crust and the lithosphere, this paper infers possible two-layer crustal anisotropy beneath the Yinchuan graben, north of the Haiyuan fault. The dominant orientation of fast polarizations are NNE or NE in the upper crust, probably originated from crack-induced anisotropy, but become WNW or NW in the middle-lower crust, possibly originated from deformation anisotropy.

Preliminary analysis of crustal shear‐wave splitting in the Sanjiang lateral collision zone of the southeast margin of the Tibetan Plateau and its tectonic implications

ABSTRACTThe collision of the Indian and Eurasian plates, to the east of the eastern Himalayan syntaxes, forms the Sanjiang lateral collision zone in the southeast margin of the Tibetan Plateau, where there are intense crustal deformation, active faults, earthquakes, as well as a metallogenic belt. Given the lack of adequate seismic data, shear‐wave splitting in this area has not been studied. With seismic data from a temporary seismic linear array, as well as permanent seismic stations, this paper adopts the identification on microseismic event to pick more events and obtains shear‐wave splitting parameters from local earthquakes. From the west to the east, the study area can be divided into three subzones. The “fast” polarization (i.e. the polarization of the fast shear wave) varies gradually from NNW to NS to NNE in these three subzones. The time delay of the slow shear wave (i.e. the time difference between the two split shear waves) also increases in the same direction, indicating the presence of seismic anisotropy above 25 km in the crust. Both shear‐wave splitting parameters are closely related to stress, faults and tectonics. The scatter and the “dual” (i.e. two) dominant orientations of the fast polarizations at several stations indicate strong distortions caused by nearby faults or deep tectonics. The anisotropic parameters are found to be related to some degree to the metallogenic belt. It is worth to further analyse the link between the anisotropic pattern and the metallogenic area, which suggests that shear‐wave splitting could be applied to study metallogeny. This paper demonstrates that the identification on microseismic event is a useful tool in detecting shear‐wave splitting details and exploring its tectonic implications.

Mantle earthquakes beneath Fogo volcano, Cape Verde: Evidence for subcrustal fracturing induced by magmatic injection

Fogo volcano belongs to the Cape Verde hotspot and its most recent eruption occurred from November 2014 to February 2015. From January to December 2016 we operated a temporary seismic network and array on Fogo and were able to locate 289 earthquakes in total. Array analysis shows that most of the events occur within the crust at distances >25km near the neighboring island of Brava. However, on 15th August 2016 the network recorded an isolated cluster of >20 earthquakes, 13 of which could be located beneath the southern part of Fogo. The differences between S- and P-wave arrival times at steep incidence clearly indicate focal depths between approximately 38 and 44km whereas receiver-function analyses place the Moho discontinuity at depths between 11 and 14km. Thus, the earthquakes are located well within the upper mantle directly beneath Fogo. In view of the elevated upper-mantle temperatures within a hotspot regime, we propose that fracturing induced by magmatic injection is the most likely cause for the observed deep earthquakes.

3-D fluid channel location from noise tremors using matched field processing

SUMMARY Presently ongoing geodynamic processes within the intracontinental lithospheric mantle give rise to different natural phenomena in the NW Bohemia/Vogtland region (Czech Republic, Germany), amongst others: earthquake swarms, mineral springs and degassing zones of mantle-derived fluids as well as highly concentrated CO2 (mofettes). Their interaction mechanisms and relations are not yet fully understood, but fluid pathways within the crust are assumed, that allow efficient fluid transport between the main hypocentral swarm quake region and the degassing areas at the surface. Here, we focus on the location of the presumed fluid channels as well as on the investigation of their near-surface spatio-temporal variability, targeting a depth of a few hundreds of metmetres. We applied a 3-D matched field processing (MFP) approach in the frequency band of 10–20 Hz considering the fluid flow as seismic noise source. Within three campaigns in 2015/2016, we recorded continuous seismic noise data on the Hartoušov Mofette Field within the Cheb Basin (NW Bohemia, CZ), which is a key site to study fluid flow as it is characterized by strong and continuous surface degassing of CO2. We used temporary arrays varying in extent (70-600 m aperture) and in the amount of stations (25–95 units). Assuming a homogeneous velocity model and applying conventional MFP phase-matching over a 3-D grid search, we located two channel-like structures beneath the test site, which could be traced down to a common source area down to 2000 m depth. We thereby evaluated the influence of amplitude normalization of the measured noise signal on the MFP location considering water-filled or dry mofette channels. Additionally, a spatio-temporal analysis using time windows with a length of 10 min during 5 hr of noise record shows variability of fluid flow activity in space and time and hence, its migration beneath the test site on a short timescale.

Source mechanism of a lower crust earthquake beneath the Himalayas and its possible relation to metamorphism

The nature of deep-crustal, intermediate and deep-focus earthquakes and their relation to metamorphic reactions is a topic of debate. Here we seek to better understand a possible link between the earthquake process and metamorphism by analyzing the mechanism of ongoing deep-crustal earthquakes. We focus on a region in the Himalayas with observed seismicity at depths expected to experience active eclogite-facies metamorphism and dehydration reactions. There are few permanent seismic stations in the region, therefore we use waveform data from a temporary seismic array deployment. We find two earthquakes with magnitude and station coverage adequate for moment tensor inversion. For a given earthquake we estimate its seismic full moment tensors (and magnitude) together with uncertainties using all available waveforms. For the largest earthquake (Mw 3.7) we obtain a best-fitting moment tensor and uncertainties that show a double-couple with a tensional crack component. In the context of geological records that document similar processes, and of laboratory experiments conducted at spatial scales that are 5–6 orders of magnitude smaller, this mechanism may be related to dehydration-driven stress changes triggering slight crack opening, and ambient stresses favoring slip along a fault.

Ambient noise multimode Rayleigh and Love wave tomography to determine the shear velocity structure above the Groningen gas field

SUMMARYThe Groningen gas field is one of the largest gas fields in Europe. The continuous gas extraction led to an induced seismic activity in the area. In order to monitor the seismic activity and study the gas field many permanent and temporary seismic arrays were deployed. In particular, the extraction of the shear wave velocity model is crucial in seismic hazard assessment. Local S-wave velocity-depth profiles allow us the estimation of a potential amplification due to soft sediments.Ambient seismic noise tomography is an interesting alternative to traditional methods that were used in modelling the S-wave velocity. The ambient noise field consists mostly of surface waves, which are sensitive to the Swave and if inverted, they reveal the corresponding S-wave structures.In this study, we present results of a depth inversion of surface waves obtained from the cross-correlation of 1 month of ambient noise data from four flexible networks located in the Groningen area. Each block consisted of about 400 3-C stations. We compute group velocity maps of Rayleigh and Love waves using a straight-ray surface wave tomography. We also extract clear higher modes of Love and Rayleigh waves.The S-wave velocity model is obtained with a joint inversion of Love and Rayleigh waves using the Neighbourhood Algorithm. In order to improve the depth inversion, we use the mean phase velocity curves and the higher modes of Rayleigh and Love waves. Moreover, we use the depth of the base of the North Sea formation as a hard constraint. This information provides an additional constraint for depth inversion, which reduces the S-wave velocity uncertainties.The final S-wave velocity models reflect the geological structures up to 1 km depth and in perspective can be used in seismic risk modelling.

GPU accelerated interferometric SAR processing for Sentinel-1 TOPS data

Sentinel-1 (S-1) TOPS data are widely applied in InSAR applications to monitor earthquakes and landslides. The large S-1 coverage however, leads to a high computational cost when executing InSAR techniques. Thus, we develop a GPU accelerated S-1 InSAR processing method implemented on a personal desktop. In the proposed method, computationally expensive modules including geometric coregistration, resampling, Enhanced Spectral Diversity, and coherence estimation, are implemented using CUDA. In addition, several optimizations are employed to enhance the efficiency of these modules. We select an efficient approximation method in the geometric coregistration module, and improve the GPU memory access efficiency in the resampling module through the GPU texture memory, temporary register array, and a configuration with more L1 cache. We develop a novel GPU-based parallel coherence estimation algorithm in ESD and coherence estimation modules, and use the asynchronous data transfer technology to hide the costs of CPU-GPU data transfer for resampling, ESD, and coherence estimation modules. After several optimizations, our GPU-accelerated modules (considering CPU-GPU transmission costs) achieves speedup ratios up to 157x, 166x, 145x, and 168x with respect to their single-threaded CPU counterparts. For a full frame S-1 image, our method reduces the computation time from 1415.32s to 8.59s. Moreover, our method is also validated in two case studies of the 2016 Mw6.2 Central Italy earthquake and 2018 Mw6.9 Leilani Estates earthquake caused by the Kilauea eruption in Hawaii.

Upper Crustal Weak Zone in Central Tibet: An Implication From Three‐Dimensional Seismic Velocity and Attenuation Tomography Results

AbstractA series of conjugate strike‐slip faults is the most prominent geologic feature in central Tibet and is considered to accommodate east‐west extension and coeval north‐south contraction. The development mechanism of the conjugate strike‐slip fault system is under debate because of unclear crustal physical properties and compositional variations. P and S wave arrivals from 414 local earthquakes recorded by the temporary Seismic Array Integrated Detection for a Window of Indian Continental Head array and the permanent China National Seismic Network were used for the velocity tomography, with additional P and S wave arrivals from 12 shots of the International Deep Profiling of Tibet and the Himalaya III reflection/refraction profile. The local earthquakes were simultaneously relocated with the updated velocity models. We also inverted for a three‐dimensional upper crustal Qp model with the same earthquake data set. The Vp structure near the surface shows that low‐Vp anomalies generally correspond to sedimentary basins and high‐Vp anomalies are related to exhumed metamorphic blocks in the study area. Relatively low Vp/Vs ratios in the upper crust indicate widely distributed quartz‐rich rocks. The low‐Vp zone from 0‐ to 10‐km depth (resolving depth limit) is spatially correlated with the Bangong‐Nujiang suture, possibly reflecting the compositional difference along the ophiolitic mélange belt accompanied by twin volcanic arcs from a double‐sided subduction. This interpretation is supported by relatively heterogeneous Qp values. This low‐velocity zone also implies relatively uniform stress and continuous deformation in the upper crust of central Tibet. The relatively weak materials in at least the upper crust would result in strain concentration and help the development of the conjugate strike‐slip fault system along the Bangong‐Nujiang suture.