Double-difference Seismic Tomography Research Articles

The 2022 Ms 5.8 and 6.0 Maerkang Earthquakes: Two Strike-Slip Events Occurred on V-Shaped Faults

ABSTRACT Within four hours on 10 June 2022, three consecutive earthquakes of Ms≥5.0 with a maximum magnitude up to Ms 6.0 struck Maerkang, Sichuan, where is supposedly less prone to earthquakes. This article uses seismic observations recorded by the Sichuan Seismic Network to relocate the earthquake sequences and refine velocity models in Maerkang using a double-difference seismic tomography method. The results show that the aftershocks align along northwest and north-northwest directions with a V-shaped pattern, and the Ms 5.8 and 6.0 earthquakes are located in the center of the sequences. The seismogenic faults of the Maerkang earthquake are the two intersecting secondary faults on the east side of the Songgang fault that dip to the northeast and southwest, respectively, with dip angles of about 80°. Analysis of the seismicity parameters and Coulomb stress changes revealed that the Ms 5.8 and 6.0 earthquakes occurred on two separate faults, and the Ms 5.8 earthquake triggered the Ms 6.0 earthquake, while the Ms 6.0 earthquake prevented the rupture of the Ms 5.8 earthquake. The apparent high-velocity anomaly in the upper crust and the low-velocity and high-conductivity layer in the middle-lower crust of the source region suggest that drag forces exerted by the ductile middle-lower crust may have caused the brittle upper crust to move under the eastward extrusion of the Tibetan plateau, leading to the stresses accumulate and release on the causative faults. The Maerkang earthquake highlighted the lateral movement of deep materials and energy redistribution in the intrablock deformation of the Bayan Har block.

High-precision seismic relocation and three-dimensional shallow crustal structure in the southern Sichuan basin

Since 2014, rates of earthquakes have increased rapidly in the Weiyuan and Changning regions of the southern Sichuan basin, where the shale gas and salt mine have been operated. We perform double-difference seismic tomography using P- and S-wave arrival times from 44,759 events recorded by the permanent seismic networks and two local networks. Data augmentation is offered by a temporary broadband array deployed recently in the Weiyuan region. Relocated seismicity is concentrated at depths of 2–8 km, and mainly occurs in and around hydraulic fracturing well pads or salt wells. Some moderate earthquakes occur relatively deeper. The distribution of M 3.0+ earthquakes clearly reveal several pre-existing faults in the Weiyuan region. Our retrieved three-dimensional shallow crustal models show that although the Vp and Vs values at earthquake locations vary between the Weiyuan and Changning shale gas demonstration zones, the Vp/Vs ratios are coherently low. Such low Vp/Vs values of the source regions suggest the presence of gas-saturated cracks with relatively large aspect ratios under high pore pressure, rather than a great number of liquid-filled cracks.

Tectonic Structure of Northern Sumatra Region Based on Seismic Tomography of P and S Wave Velocity

The tectonic setting of Sumatra Island is strongly influenced by the oblique subduction of the Indo-Australian Plate, which subducts the Eurasian Plate at a speed of 52–60 mm/year. The movement of these plates resulted in the Northern Sumatra region having seismic sources from tectonic and volcanic activity. The data used in this study is in the form of seismic wave travel-time recorded by numerous seismic stations in the research area from January 2012 to December 2020. The data comes from 5,003 earthquakes recorded by the BMKG seismic network. The inversion is a simultaneous inversion between seismic velocity models (Vp and Vs) and hypocenter parameters by applying a double-difference seismic tomography algorithm. Tomogram results in parts of Aceh (Singkil and Subulussalam) and North Sumatra (Pakpak Bharat and Dairi) at a depth of 0 km show negative perturbations in Vp and Vs values and high Vp/Vs values. The anomaly is most likely related to cracks in fluid-saturated rocks. The tomograms in the south of Lake Toba at depths of 30 km and 40 km have high Vp and Vs perturbation values and low Vp/Vs values. This anomaly indicates a magma supply line that is no longer active or has cooled for a long time. Based on the seismic tomography modeling results, the subducted Indo-Australian Plate to the Eurasian Plate is visible in the study area.

Crustal structure and seismicity characteristics based on dense array monitoring in northwestern Yunnan, China

Owing to the complex geological environment and frequent seismic activity in northwestern Yunnan, China, the northwest Yunnan dense array was installed. The dense array monitored the Mw6.1 Yangbi earthquake that occurred on May 21, 2021, on the western boundary of the Sichuan-Yunnan block. The earthquake exhibited a typical foreshock-mainshock-aftershock sequence and presented a clear process of earthquake migration. From April 1, 2018 to December 31, 2021, 11,623 earthquakes were recorded by 72 stations in the dense array. Based on their arrival time data, the Vp/Vs model consistency-constrained double-difference seismic tomography method was employed. We obtained high-precision crustal 3-D Vp, Vs, Vp/Vs models, and relocation results for northwestern Yunnan. A wide distribution of low-velocity anomalies was observed in the middle and upper crust of northwestern Yunnan. Based on GPS measurements, the contour of the low-Vp zone is aligned with the clockwise movement of crustal material caused by the compression of the Tibetan Plateau. A seismic cluster with a banded distribution pattern was identified in an intersection area of multiple faults. And its formation might be related to the infiltration of geothermal fluid in a fluid channel composed of unknown fractures. The geothermal fluid originates from the intersection of the fault-enriched hot springs. The flow and seepage from the fluid channels spread to the Yangbi earthquake source region, reactivating the previously existing unmapped faults, ultimately leading to the occurrence of Yangbi foreshocks. Because of the stress transmission, the Coulomb stress of the foreshock triggered the high-strength part of the main fault, which triggered the occurrence of the main earthquake.

Tomographic evidences for hydraulic fracturing induced seismicity in the Changning shale gas field, southern Sichuan Basin, China

Seismic activity near the shale gas fields in the Sichuan Basin of China has risen significantly in the past few years. Due to the lack of industrial stimulation data to the public, it is difficult to directly link the surge in seismicity near the shale gas fields with hydraulic fracturing stimulation. Meanwhile, it is also challenging to better understand the mechanisms inducing/triggering seismicity in these areas. In the absence of stimulation data, understanding subsurface fluid distribution can go a long way toward addressing these problems. Therefore, in this study, we have applied the double-difference seismic tomography method and the focal mechanism tomography method to the monitoring data of a local seismic array in the Changning shale gas field to build high-resolution three-dimensional models of seismic velocity structure and pore pressure field, then used these models to characterize fluid distributions beneath the shale gas field. From the inverted models, the correlation of overpressure regions (>12 MPa) with moderate-to-high Vp/Vs (1.75-1.90) is clearly observed. With the aid of the exploration data (including well log, active and passive seismic data), the overpressure regions are interpreted to be created by pore pressure or fluid diffusion along pre-existing faults from the shale gas reservoirs, while variations of the Vp/Vs model partly indicate water- and gas-bearing rocks. By jointly analyzing the geophysical model features and earthquake spatio-temporal characteristics, we infer that the earthquakes are primarily caused by increased pore pressures on the seismogenic faults due to the presence of water and/or gas. Besides, the poroelastic stress perturbation and aseismic slip produced by hydraulic fracturing also contribute to the generation of certain earthquake swarms.

Enhanced 3D velocity structure, seismicity relocation and basement characterization of Changning shale gas and salt mining regions in southern Sichuan basin

Current extensive seismicity in southern Sichuan Basin is ascribed to the reactivation of pre-existing faults, as a result of prolonged fluid injection for salt mining and shale gas development, respectively. However, the structural framework of the region remains poorly understood. Here, we apply Vp/Vsconsistency-constrained double-difference seismic tomography to high quality phase data from 36,314 earthquakes jointly recorded by our local array and a regional seismic network to determine high-resolution velocity models. Earthquake relocations reveal shallow hypocenters for the Ms>5.0 earthquakes and two distinct seismogenic zones corresponding to the salt mine and shale gas regions, with most induced seismic events forming widespread lineaments some of which extend to the basement and are remarkably similar to the fault and fracture trends interpreted on reflection seismic and outcrops, respectively. Our 3-D crustal velocity analyses show that seismicity beneath the Changing salt mining area is associated with a combination of relatively low Vp/Vs(1.6–1.74) and high Vp/Vs(1.75–1.86) expressions, while most of small earthquakes within the Xingwen shale gas block are associated with relatively high Vp/Vsvalues (1.77–1.87), indicating the earthquakes in these two areas are caused by unique inducing mechanisms. The two moderately strong 2018 Xingwen Ms5.7 and 2019 Gongxian Ms5.3 earthquakes in the Xingwen shale gas block are located around low Vp/Vs. zones, suggesting they could be structurally controlled. In comparison, the 2019 Changning Ms6.0 earthquake in the Changning salt mining area is associated with high Vp/Vs. expression, suggesting its occurrence is related to fluid injections. In addition, top of the crystalline early Neoproterozoic (pre-Sinian) Sichuan basement is characterized by the 6.5 km/s Vpcontour, which is new for earthquake tomographic studies in the region. Combined with outcrop analysis, we are able to construct a structural framework for induced seismicity in southern Sichuan basin, which unravels the structural architecture of induced seismicity.

Imaging sub-caldera structure with local seismicity, Okataina Volcanic Centre, Taupo Volcanic Zone, using double-difference seismic tomography

We examine sub-caldera structure and seismicity in and around the Okataina Volcanic Centre (OVC), Taupo Volcanic Zone, New Zealand, using seismic data collected over 4+ years with a temporary array of broadband and short-period seismometers, supplementing data from the permanent New Zealand seismometer network. We derive a new 3-D image of P-wave seismic velocity for the upper crust in the region, using double-difference seismic tomography and utilising waveform cross-correlations. We subsequently relocate 6989 earthquakes which occurred in the region over the 2010–2021 time period, using the 3D velocity model. The seismicity distribution shows spatial clusters west of Lake Rotomahana, as well as beneath Haroharo and the Makatiti Dome, inside the Okataina caldera. Beneath Makatiti Dome 90% of the events are shallower than 7.7 +/− 0.5 km. Outside of the Okataina caldera event relocations highlight short (∼3–4 km long) streaks of activity in the Ngakuru graben, part of the active Taupo Rift southwest of Okataina caldera. Inside the OVC the relocated seismicity beneath Makatiti appears closely associated with low (∼10%) P-wave velocity anomalies, which we resolve in the ∼5-to-8-km depth range beneath the Okataina caldera, and which are likely related to partial melt and/or fluid-volatile pathways. Moment tensor analyses for two larger-magnitude events (ML4.5 and ML4.9) near Haroharo indicate normal faulting, with NNE-SSW fault strike, but with positive CLVD and positive isotropic components when allowing for a full moment tensor, consistent with a magmatic environment with degassing and/or fluid migration.

Seismogenesis of the 2021 Mw 7.1 earthquake sequence near the northeastern Japan revealed by double-difference seismic tomography

Recently, a Mw 7.1 earthquake struck the Pacific coast of northeastern Japan on 2021/02/13. To investigate the cause of this earthquake and its aftershocks, we applied a Vp/Vs model-consistency constrained double-difference (DD) seismic tomography method to the earthquake arrival time data from the dense seismic networks in Japan to image the velocity structures at the hypocenter areas. Compared to the conventional DD tomography, the new method can determine the Vp/Vs model with high resolution and precision. The reliability of the seismic tomography results is testified by checkerboard resolution test. The velocity profiles show that the oceanic crust and uppermost part of the oceanic mantle are characterized by low Vs and high Vp/Vs due to hydration reaction, while the lower oceanic mantle is associated with high Vs and low Vp/Vs anomalies that are likely caused by serpentine dehydration. The mainshock of the Mw 7.1 earthquake sequence is located at the boundary between the low velocity layer and underlying high velocity zone. The aftershocks are mainly distributed within the low velocity layer. The rupture plane inferred from the aftershock distribution and mainshock focal mechanism solution has penetrated ∼10 km into the slab and extended ∼50 km along a direction nearly parallel to the trench. These observations have suggested that the Mw 7.1 earthquake sequence occurs as a result of reactivation of a pre-existing fault likely created at the outer-rise before subduction. The association of the mainshock with the velocity anomaly edge suggests that this earthquake sequence is probably caused by dehydration of the mantle serpentines, which increases the pore pressure on the fault. Besides, the variation of rock mechanics at the hypocenter area of the mainshock also facilitates the nucleation of the whole earthquake sequence.

Structure-controlled asperities of the 1920 Haiyuan M8.5 and 1927 Gulang M8 earthquakes, NE Tibet, China, revealed by high-resolution seismic tomography

Detailed crustal structure of large earthquake source regions is of great significance for understanding the earthquake generation mechanism. Numerous large earthquakes have occurred in the NE Tibetan Plateau, including the 1920 Haiyuan M8.5 and 1927 Gulang M8 earthquakes. In this paper, we obtained a high-resolution three-dimensional crustal velocity model around the source regions of these two large earthquakes using an improved double-difference seismic tomography method. High-velocity anomalies encompassing the seismogenic faults are observed to extend to depths of 15 km, suggesting the asperity (high-velocity area) plays an important role in the preparation process of large earthquakes. Asperities are strong in mechanical strength and could accumulate tectonic stress more easily in long frictional locking periods, large earthquakes are therefore prone to generate in these areas. If the close relationship between the aperity and high-velocity bodies is valid for most of the large earthquakes, it can be used to predict potential large earthquakes and estimate the seismogenic capability of faults in light of structure studies.

Velocity structure of the upper crust beneath the Jiaonan uplift and its adjacent areas, China

The Jiaonan uplift and its adjacent areas (JUAA) are the result of collision and amalgamation of the North China plate and Yangtze plate. In this area, the tectonic environment is complex and crustal deformation is strong. However, detailed and in-depth study of the upper crustal structure and medium properties in the JUAA has not previously been performed. The high-resolution three-dimensional crustal velocity structure of the JUAA is helpful to analyze the characteristics of the crustal structure in this area and is important for evaluating the tectonic environment and medium properties of the JUAA. We obtained the three-dimensional crustal velocity structure in the JUAA using the double-difference seismic tomography technique, and found that the Rizhao area and the sea areas to its southwest in the Jiaonan uplift are characterized by a high-velocity structure, and that most of the high-velocity anomaly area is located in the sea area. The crustal velocity values of the southern end of the Yishu fault zone are also high. The high crustal velocity anomaly areas in the JUAA are considered to be caused by the upwelling of mantle material. The velocity structure of the upper crust beneath the depression structures shows notable low-velocity anomalies, which are closely related to loose sediments in the depression structures. The existence of abnormally high-velocity and low-velocity structures in the Jiaonan uplift indicates that there are substantial differences in the properties of the crustal media in the Jiaonan uplift. Crust-mantle interaction in the Jiaonan uplift is mainly concentrated in the Rizhao area and sea areas to its southwest.

Double-difference seismic attenuation tomography method and its application to The Geysers geothermal field, California

SUMMARY Knowledge of attenuation structure is important for understanding subsurface material properties. We have developed a double-difference seismic attenuation (DDQ) tomography method for high-resolution imaging of 3-D attenuation structure. Our method includes two main elements, the inversion of event-pair differential ${t^*}$ ($d{t^*}$) data and 3-D attenuation tomography with the $d{t^*}$ data. We developed a new spectral ratio method that jointly inverts spectral ratio data from pairs of events observed at a common set of stations to determine the $d{t^*}$ data. The spectral ratio method cancels out instrument and site response terms, resulting in more accurate $d{t^*}$ data compared to absolute ${t^*}$ from traditional methods using individual spectra. Synthetic tests show that the inversion of $d{t^*}$ data using our spectral ratio method is robust to the choice of source model and a moderate degree of noise. We modified an existing velocity tomography code so that it can invert $d{t^*}$ data for 3-D attenuation structure. We applied the new method to The Geyser geothermal field, California, which has vapour-dominated reservoirs and a long history of water injection. A new Qp model at The Geysers is determined using P-wave data of earthquakes in 2011, using our updated earthquake locations and Vp model. By taking advantage of more accurate $d{t^*}$ data and the cancellation of model uncertainties along the common paths outside of the source region, the DDQ tomography method achieves higher resolution, especially in the earthquake source regions, compared to the standard tomography method using ${t^*}$ data. This is validated by both the real and synthetic data tests. Our Qp and Vp models show consistent variations in a normal temperature reservoir that can be explained by variations in fracturing, permeability and fluid saturation and/or steam pressure. A prominent low-Qp and Vp zone associated with very active seismicity is imaged within a high temperature reservoir at depths below 2 km. This anomalous zone is likely partially saturated with injected fluids.

Seismic imaging of the double seismic zone in the subducting slab in Northern Chile

Double seismic zones are commonly observed in the subducting slabs in a global scale, serving as ideal examples for studying the seismogenetic mechanism of the intermediate-depth earthquakes. In this study, we relocate earthquakes and determine seismic velocity models using the double-difference seismic tomography method in the northern Chile subduction zone where a double seismic zone exists. The results suggest that the double seismic zone in northern Chile is located at about 50–140 ​km depth, with an interval of approximately 20 ​km between the two zones. The upper seismic zone is characterized by relatively low Vp (~7.8–8.0 ​km/s), low Vs (~4.4–4.5 ​km/s) and high Vp/Vs (~1.85) above the depth of ~90 ​km, while the region below ~90 ​km is distinguished by relatively high Vp (~8.2 ​km/s), high Vs (~4.8 ​km/s) and slightly high Vp/Vs (~1.75), which may be related to a series of dehydration reactions of hydrous minerals in the subducted oceanic crust. In comparison, the lower seismic zone is featured by the anomaly of low Vp/Vs (~1.7), although some local areas may consist of relatively high Vp/Vs values (~1.8), possibly due to the dehydration reaction of serpentine. Based on the Vp, Vs, Vp/Vs anomalies combined with previous petrological experiments and thermodynamic models, it can be derived that intermediate-depth earthquakes are mainly related to the dehydration of various hydrous minerals in the subducting slab. The dehydration process of hydrous minerals releases water into the subducting slab and subsequently leads to the increase of pore fluid pressure and the decrease of effective normal stress, thus causing the occurrence of brittle failure and intermediate-depth earthquakes in subduction zones. The imaging results of the northern Chile subduction zone further indicate that the existence of the double seismic zone is related to the dehydration process of different hydrous minerals.

Upper crustal velocity and seismogenic environment of the <em>M</em>7.0 Jiuzhaigou earthquake region in Sichuan, China

On August 8, 2017, a magnitude 7.0 earthquake occurred in Jiuzhaigou County, Sichuan Province, China. The deep seismogenic environment and potential seismic risk in the eastern margin of Tibetan Plateau have once again attracted the close attention of seismologists and scholars at home and abroad. The post-earthquake scientific investigation could not identify noticeable surface rupture zones in the affected area; the complex tectonic background and the reason(s) for the frequent seismicity in the Jiuzhaigou earthquake region are unclear. In order to reveal the characteristics of the deep medium and the seismogenic environment of the <i>M</i>7.0 Jiuzhaigou earthquake region, and to interpret the tectonic background and genesis of the seismicity comprehensively, in this paper, we have reviewed all available observation data recorded by the regional digital seismic networks and large-scale, dense mobile seismic array (China Array) for the northern section of the North–South Seismic Belt around Jiuzhaigou earthquake region. Using double-difference seismic tomography method to invert the three-dimensional P-wave velocity structure characteristics of the upper crust around the Jiuzhaigou earthquake region, we have analyzed and discussed such scientific questions as the relationship between the velocity structure characteristics and seismicity in the Jiuzhaigou earthquake region, its deep tectonic environment, and the ongoing seismic risk in this region. We report that: the P-wave velocity structure of the upper crust around the Jiuzhaigoug earthquake region exhibits obvious lateral inhomogeneity; the distribution characteristics of the shallow P-wave velocity structure are closely related to surface geological structure and formation lithology; the <i>M</i>7.0 Jiuzhaigou earthquake sequence is closely related to the velocity structure of the upper crust; the mainshock of the <i>M</i>7.0 earthquake occurred in the upper crust; the inhomogeneous variation of the velocity structure of the Jiuzhaigou earthquake area and its surrounding medium appears to be the deep structural factor controlling the spatial distribution of the mainshock and its sequence. The 3D P-wave velocity structure also suggests that the crustal low-velocity layer of northeastern SGB (Songpan–Garzê Block) stretches into MSM (Minshan Mountain), and migrates to the northeast, but the tendency to emerge as a shallow layer is impeded by the high-velocity zone of Nanping Nappe tectonics and the Bikou Block. Our results reveal an uneven distribution of high- and low-velocity structures around the Tazang segment of the East Kunlun fault zone. Given that the rupture caused by the Jiuzhaigou earthquake has enhanced the stress fields at both ends of the seismogenic fault, it is very important to stay vigilant to possible seismic hazards in the large seismic gap at the Maqu–Maqên segment of the East Kunlun fault zone.

A new growth model of the northeastern Tibetan Plateau from high-resolution seismic imaging by improved double-difference tomography

High-resolution 3-D P and S wave velocity (Vp and Vs) models of the crust and uppermost mantle beneath the northeastern (NE) Tibetan Plateau and the surrounding areas were obtained by applying an improved double-difference seismic tomography method on abundant body wave travel-time data of local and regional earthquakes collected from 1985 to 2018. The improved method not only increases constraints from observed Moho depth information but also incorporates the later-arriving Pg and Sg phases to improve the ray coverage in the middle-lower crust and uppermost mantle. The results show significant low-velocity anomalies in the crust beneath the NE Tibetan Plateau and high-velocity features beneath the surrounding stable Alxa and Ordos blocks. The areas near the boundaries of the cratonic blocks show high velocity in the upper crust and uppermost mantle beneath the Hexi Corridor, and high velocity in the upper crust beneath the Liupan Shan belt. Both of those regions have significant low-velocity anomalies in the middle-lower crust similar to the Tibetan Plateau, which suggests they probably were parts of the Alxa and Ordos blocks respectively, and were cut off from the two cratonic blocks during the lateral expansion of the Tibetan Plateau because of middle-lower crustal ductile deformation and asthenospheric flow. We propose a new tectonic model controlled by middle-lower crustal ductile deformation and asthenospheric flow to illuminate the tectonic evolution and growth mechanism of the NE Tibetan Plateau.

Upper crustal velocity structure and geological significance of southwest Shandong Province, China: insights from double-difference seismic tomography

As an important part of the North China Craton extension system, the southwest of Shandong Province comprises several uplift and fault depression structures. However, the tectonic activity there is controversial, and more studies are required for the upper crustal velocity structure. In the present study, the three-dimensional upper crustal P wave velocity structure in southwestern Shandong Province was obtained using double-difference seismic tomography. The findings show that southwestern Shandong Province has many vertical high-velocity structures, indicating that mantle-magma upwelling was strong there and formed the uplift structures. It is reasoned that Taishan Mountain is still undergoing uplift caused by magma upwelling that is concentrated mainly below a depth of 20 km. The shallow–middle low-velocity structures in southwestern Shandong Province correspond strongly to the uplift depression structures. The tectonic activity in southwestern Shandong Province can be divided mainly into (i) brittle-ductile deformation of uplift depression structures at depths of 0–10 km and (ii) mantle-magma upwelling under the uplift structures. The formation of uplift structures promoted the formation of the fault depression structures, and the fault depression and uplift structures are closely related in genesis.

3D Crustal Structure and Seismicity Characteristics of Changning–Xingwen Area in the Southwestern Sichuan Basin, China

ABSTRACTUsing seismic data recorded on permanent and temporary stations around the Changning area in the Sichuan basin, the high-resolution 3D crustal VP, VS, VP/VS models and earthquake locations in the Changning–Xingwen area are obtained using the VP/VS model consistency-constrained double-difference seismic tomography method. The results show that crustal structures in the source area of the 2019 Ms 6.0 Changning earthquake have significant variations, especially in the depth of 0–7 km. Seismic activity in the Shuanghe and Yutan anticline areas before the Ms 6.0 Changning earthquake outlined several northeast-trending stripes, implying pre-existing small-scale faults that are perpendicular to the major northwest-striking faults in the Changning–Shuanghe anticline system. We found that the Ms 6.0 Changning earthquake broke through these pre-existing small-scale faults and extended from the Shuanghe to the Yutan anticlines. Both the rupture process and aftershock activity were influenced by the pre-existing small-scale faults. Most earthquakes within the Changning area are located in a slant zone that gradually deepens from the Shuanghe anticline on the east to the Yutan anticline on the west with the maximum depth from 5 to 10 km, which are associated with obvious high-VS and low-VP/VS features. The relocated seismic clusters in the Luochang–Jianwu syncline area have different strikes and dips, which are mainly located at the edge of low-velocity anomaly bodies and correspond to the low-VP/VS area.

Velocity structure of the upper crust and its correlation with earthquake swarms activity in Laizhou Bay and its adjacent areas, China

This study used the double-difference seismic tomography method to obtain the upper crustal velocity structure in Laizhou Bay and its adjacent areas (LBAA). We found that the horizontal and vertical distributions of the earthquakes after relocation in earthquake swarms are more concentrative than that before relocation. The lateral heterogeneity of crustal velocity structure near the surface in the LBAA is weak, and the low-velocity sedimentary caprocks are developed in most areas except for the Jiaobei uplift. The velocity structure in 5–25 km depth range is highly heterogeneity, which is closely related to different tectonic units. The local uplift tectonic units in Jiyang depression show high-velocity anomalies and should have deep crustal media properties. The high-velocity anomalies under the crust of Tancheng–Lujiang fault zone and the northern Laizhou Bay indicate the properties of upwelling mantle material media. Earthquake swarms activity in the study area are mainly concentrated in the high-velocity interlayer of the upper crust and its periphery. Compared with the relatively stable high-velocity body at the bottom crust, the location of high-velocity interlayer in the study area is more prone to earthquakes under the combined action of gravity and tectonic stress.

Structure of crust and upper mantle beneath NW Himalayas, Pamir and Hindukush by multi-scale double-difference seismic tomography

The India-Asia collision and subsequent subduction initiated the evolution of major tectonic features in the Western Syntaxis. The complex tectonic structure and shallow to deep seismicity have attracted geoscientists over the past two decades. The present research is based on a 3D tomographic inversion of P-wave arrival time data to constrain the crustal and upper mantle structure beneath the NW Himalayas and Pamir-Hindukush region using the Double-difference tomography. We utilized a very large multi-scale dataset comprising 19,080 earthquakes recorded at 397 local and regional seismic stations from 1950 to 2017. The northward dipping seismic zone coinciding with the low velocity anomaly suggests the subduction of the Indian lower crust beneath the Hindukush. The extent of the northward advancing Indian slab increases from east to west in this region. We observed no signs of northward subduction of the Indian plate under the Hindukush beyond 71°E longitude. The Indian plate overturns due south after interacting with the Asian plate beneath the southern Pamir, which correlates with the counter-clockwise rotation of the Indian plate. The Asian plate is also imaged as a southward subducting seismic zone beneath the southern Pamir. In the NW Himalayas, the northward subducting Indian plate appears as a gently dipping low velocity anomaly beneath the Karakoram Block. The stresses caused by the collision and subduction along the Shyok Suture and Indus Suture are translated to the south. The crustal scale seismicity and high velocity anomalies indicate an intense deformation in the crust, which is manifested by syntaxial bends and thrust faults to the south of the Main Mantle Thrust.

Back azimuth constrained double-difference seismic location and tomography for downhole microseismic monitoring

We have developed a new seismic tomography method, back azimuth constrained double-difference (DD) seismic tomography, which is suitable for downhole microseismic monitoring of hydraulic fracturing. The new method simultaneously locates microseismic events and determines three-dimensional (3D) Vp and Vs models for the fracturing zone using differential arrival times from pairs of events and event back azimuths in addition to absolute arrival times. Compared to the existing DD location and tomography method, our method incorporates back azimuth information to better constrain microseismic event locations in the case of poor spatial station coverage such as the linear downhole seismic array generally used for microseismic monitoring. By incorporating the relative arrival time and back azimuth information of events, the extended DD method can provide better relative event locations, and thus can better characterize the fracture distribution. In addition to microseismic locations, seismic velocity anomalies determined around the fracturing zone may also provide valuable information for fracture development. Due to the existence of fractures and fluids, the seismic velocity is expected to be lower in the fractured zone compared to the surrounding regions. Therefore the area of low seismic velocity anomaly may be used as a proxy for the stimulated reservoir volume. We have applied the new method to a downhole microseismic dataset from shale gas hydraulic fracturing. The microseismic events are more accurately relocated than the conventional grid search location method, and they are generally associated with low velocity anomalies.

Wavelet-based double-difference seismic tomography with sparsity regularization

Wavelet-based double-difference seismic tomography with sparsity regularization