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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 veloc...
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We present detailed 3-D tomographic images of P and S wave velocity (Vp, Vs), Poisson's ratio (ν) and Vp azimuthal anisotropy of the crust and uppermost mantle beneath the source area of the 21 May 2021 Maduo earthquake (M 7.4) in the NE Tibetan Plateau. The images are obtained by inverting a large number of P and S wave arrival-time data of 11,235...
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... The 3D P and S-wave velocity structures are inverted using the double-difference seismic tomography method (tomoDD) (Zhang and Thurber 2003), which has been extensively applied in previous studies (Allam and Ben-Zion 2012;Feng et al. 2022;Sun et al. 2021;Wu and Ma 2022;Xin et al. 2018;Zhang et al. 2004). The initial velocity model we construct refers to the seismic reference model proposed by Cui et al. (2021) and He et al. (2021) as it serves as a crucial input for the inversion of the 3D velocity structure ( Fig. 3 a and b). ...
The Jiangsu-South Yellow Sea area is located on the East Asian Continental Margin and at the junction of South and North China. It is one of the high seismicity areas in eastern China. To enhance our understanding of the relationship between disastrous earthquakes and the geophysical properties of Earth materials, P- and S-wave travel-time data over the past 20 years are inverted to image the 3D distributions of seismic velocity (VP and Vs) and Poisson’s ratio (σ). The results show that the velocity and Poisson’s ratio demonstrate congruence with the prominent geological formations present within the study area. Specifically, the velocity models for the upper crust at depths of 5, 10, and 15 km. We found that within the total study area, 70% ± 7.2% of M5+ earthquakes that occurred during the past decades initiated in the VP tomographic edge zones (TEZ). Moreover, the distribution patterns of M5+ earthquakes in the offshore sea region exhibit significant differences when compared to those in the Jiangnan Orogenic Belt. In the offshore sea region, about 66.7% ± 8% of M5+ earthquakes initiated in the velocity and Poisson’s ratio TEZ. The presence of comparatively low Vs and high Poisson’s ratio anomalies in the Wunansha Uplift suggest the potential existence of fluids. The triggering mechanism for these earthquakes can likely be ascribed to the presence of hydrothermal fluids and the reactivation of preexisting strike-slip faults, influenced by the subduction of the Pacific and Philippine Sea Plates, as well as the collision between the India and Eurasia Plates. In contrast, within the Jiangnan Orogenic Belt, M5+ earthquakes predominantly initiated in the high-velocity and low Poisson’s ratio perturbation zones. The primary factor contributing to seismotectonic features disparities are likely the variation in fluid content. We infer the rock matrix in the vicinity of the source region of the Jiangnan Orogenic Belt is presumed to be drier, more brittle, and possess greater mechanical strength.
... It is observed that the 2014 M W 5.9 Kangding earthquake caused a significant coseismic slip shallower than 8 km in depth (orange rectangle in Figure 7e) (Jiang et al., 2015), which was obviously impeded by the ZGH. Therefore, we suggest that the ZGH act as a rigid asperity Pei et al., 2014;Wang et al., 2015;Sun Q et al., 2021), which is consistent with the Tagong locked asperity identified by Yi and Fan (2005) from relocated earthquakes, whereas the ZDSL act as a weak barrier. ...
... However, the southern Selaha and the Zheduotang faults are situated along the boundaries separating the high and low velocities where the brittle-ductile transitions are present, and may have higher seismic risks Pei et al., 2014;Wang et al., 2015;Sun Q et al., 2021). Compared to the velocity variations across the southern Selaha and the Zheduotang faults, the velocity difference across the northern Selaha fault in the Bamei segment is less distinct. ...
On September 5, 2022, a strong MS6.8 earthquake struck the Luding area in the Kangding-Moxi segment of the Xianshuihe fault zone, which is the northern boundary of the Sichuan-Yunnan rhombic block, causing considerable casualties. The Bamei-Kangding segment of the Xianshuihe fault zone, which is located only tens of kilometers away from the Luding earthquake, has hosted frequent moderate to strong earthquakes in history and is a dangerous earthquake-prone zone. Therefore, it is critical to investigate the regional seismogenic environment for strong earthquakes and to evaluate the impact of the Luding earthquake in this area. For this purpose, we deployed a dense seismic array comprising over 200 short-period nodes in this region from July to August, 2022 and acquired seismic ambient noise for over 30 days. Using the collected data, we conducted surface wave tomography and obtained a high-resolution 3-D shear wave velocity model for the regional shallow crust down to 8 km in depth. The key findings include: (1) the Bamei-Kangding segment of the Xianshuihe fault zone exhibits widespread stripped low-velocity anomalies, suggesting shear movements at a relatively high temperature of the Xianshuihe fault zone; the Zheduoshan granitic pluton situated between the Zheduotang and southern Selaha faults shows a distinct low-velocity anomaly, which may be attributed to the localized high-temperature anomaly resulted by a deep magmatic heat source and the recent rapid uplift of the Zheduoshan area; (2) a ten-kilometer-wide high velocity body found below 4 km in depth near the Zhonggu area in the Bamei segment coincides with the seismic gap of moderate to strong earthquakes in this region, suggesting that the high velocity body may act as a seismic barrier; (3) the heterogeneity of the velocity structure along the Bamei-Kangding segment of the Xianshuihe fault zone corresponds to the regional changes in temperature, which reveals the reason for the spatially varying seismogenic potential in this segment; especially, the Selaha and Zheduotang faults which are located along the boundaries between the high and low velocity anomalies may possess considerable seismogenic potential; (4) the Coulomb failure stress calculations indicate that the Luding earthquake has imposed nontrivial stress loading in the Bamei-Kangding segment, and may shorten the earthquake recurrence intervals of the southern Selaha fault, the Zheduotang fault, and the Xuemenkan segment of the Xianshuihe fault zone. Thus, the Luding earthquake may potentially pose threats to the Sichuan-Xizang railway passing through this region.
... The presence of distinct magnetic and velocity boundaries can be indicative of variations in rock lithology, structural integrity, mechanical strength, and porosity (Tenthorey et al., 2003). These variations facilitate the accumulation of tectonic stress, reminiscent of seismic nucleation in the context of asperity models (Pei et al., 2014;Hardebeck and Loveless, 2017;Sun et al., 2021Sun et al., , 2022. ...
On 16 September 2021, an Ms 6.0 earthquake occurred in Luxian, Sichuan, China, breaking the historical record of no earthquake with magnitude ≥ M 6 along the Huaying Mountain fault belt. The regional geological structure is primarily controlled by the northeast-striking fault belt, but the long axis of the isoseismic line, distribution of early aftershocks and coseismic rupture plane all strike northwest, posing challenges to the seismogenic mechanism. To investigate this, we conducted a 400 km2 unmanned aerial vehicle (UAV) aeromagnetic survey near the epicenter, with a line spacing of 1 km. Through aeromagnetic analyses, combined with the spatial distribution of relocated foreshocks and aftershocks, we outline potential basement causative faults and a change in the structural trend between the shallow and deep portions of the seismic zone. We conjecture that the Luxian earthquake was triggered by the hydrofracturing-driven reactivation of a pre-existing northwest-striking and southwest-dipping basement fault, of which the upward propagation induced extrusion and dislocation at the hypocenter within the sedimentary layer. The special structural configurations for the focal area could contribute to the stress concentration and occurrence of large earthquakes.
... Tectonism in the NE margin of the Qinghai-Tibetan Plateau may depend on its lower strength (weaker rheology and perhaps higher temperatures) compared with that of the surrounding cratonic blocks (Alxa and Ordos blocks; Zhang, 1997Zhang, , 2012, material properties that have been estimated by means of some geophysical observations (Zhao et al., 2005;Guo et al., 2019;Xia et al., 2019;Xin et al., 2020;Sun et al., 2021). Pei et al. (2021) used these observations when conducting numerical simulations. ...
... km/s) significantly lower than those (~6.4 km/s) of the close-vicinity Ordos and Alxa blocks (Guo et al., 2019;Xin et al., 2020;Sun et al., 2021). The tomography slices through the arcuate structural belts and Alxa block also show that there is a fair horizontal variation in seismic velocity between the two blocks throughout the crust, and the wave velocity of the Alxa block is modestly larger (Xia et al., 2021). ...
... The similarities of seismograms before and after the reconstructions in both waveforms and spectra suggest that the reconstruction should be reliable ( Supplementary Fig. 17). The main slip area is well associated with a high-velocity anomaly zone along the fault plane ( Fig. 3), which is geomechanically stronger and can accumulate more stresses compared to surrounding areas [47][48][49] , and potentially acts as an asperity for developing strong earthquakes. Similar to the Xingwen M L 5:7 earthquake, the Gongxian M L 5:3 earthquake also nucleated at the boundary between the high and low velocities, and propagated unilaterally into the high-velocity anomaly zone, which acts as an asperity for the strong earthquake. ...
Moderate to strong earthquakes have been induced worldwide by shale gas development, however, it is still unclear what factors control their behaviors. Here we use local seismic networks to reliably determine the source attributes of dozens of M > 3 earthquakes and obtain a high-resolution shear-wave velocity model using ambient noise tomography. These earthquakes are found to occur close to the target shale formations in depth and along high seismic velocity boundaries. The magnitudes and co-seismic slip distributions of the 2018 Xingwen ML5.7\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${M}_{{{{{{\rm{L}}}}}}}5.7$$\end{document} and 2019 Gongxian ML5.3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${M}_{{{{{{\rm{L}}}}}}}5.3$$\end{document} earthquakes are further determined jointly by seismic waveforms and InSAR data, and the co-seismic slips of these two earthquakes correlate with high seismic velocity zones along the fault planes. Thus, the distribution of high velocity zones near the target shale formations, together with the stress state modulated by hydraulic fracturing controls induced earthquake behaviors and is critical for understanding the seismic potentials of hydraulic fracturing.
... There are many methods for earthquake research; the most direct is to understand the internal structure of the underground and the physical properties of the fault. Seismic tomography technology is widely used in the study of the earth's internal structure and has achieved fruitful research results [4][5][6][7][8][9][10][11]. Seismic tomography is a geophysical method [12][13][14][15] that reverses important information such as velocity structure and other physical parameters of underground media by analyzing the observed data of kinematic (such as travel time, ray path) and dynamic (such as waveform, amplitude) characteristics of various seismic phases of seismic waves. ...
To obtain an accurate one-dimensional velocity model, we developed the EA_VELEST method based on the evolutionary algorithm and the VELEST program. This method can quickly generate a suitable 1D velocity model and finally input it into the 3D velocity inversion process using the TomoDD method. We adopt TomoDD methods to inverse the high-resolution three-dimension velocity structure and relative earthquake hypocenters for this sequence. This system processing flow was applied to the Sichuan Maerkang earthquake swarm in 2022. By collecting the seismic phase data of the Maerkang area between 1 January 2009 and 15 June 2022, we relocated the historical earthquakes in the area and obtained accurate 3D velocity imaging results. The relocated hypocenters reveal a SE-trending secondary fault, which is located ~5 km NW of the Songgang fault. In the first ten-hour of the sequence, events clearly down-dip migrated toward the SE direction. The inverted velocity structure indicates that the majority of earthquakes during the sequence occurred along the boundaries of the high and low-velocity zones or high and low-VP/VS anomalies. Especially both the two largest earthquakes, MS 5.8 and MS 6.0, occurred at the discontinuities of high and low-velocity zones. The EA_VELEST method proposed in this paper is a novel method that has played a very good enlightenment role in the optimization of the one-dimensional velocity model in geophysics and has certain reference significance. The 3D velocity results obtained in this paper and the analysis of tectonic significance provide a reference for the seismogenic environment of this Maerkang earthquake and the deep 3D velocity of the Ganzi block.
... By including RFs in the joint inversion, the Vs structure across the Moho can be better determined. As a result, more accurate ray paths for body wave first arrivals, especially for the head waves, can be computed (Sun et al., 2021). In addition, the new joint inversion algorithm also considers topographic variations, which are often ignored in previous joint inversion algorithms of body wave and surface wave data (e.g., Fang et al., 2016;Zhang et al., 2014). ...
We have developed a new joint inversion method that incorporates body wave arrival times, surface wave dispersion and receiver functions to simultaneously update earthquake locations and constrain three‐dimensional P‐wave (Vp) and S‐wave velocity (Vs) models. Due to complementary sensitivities of the three types of data, the proposed joint inversion algorithm can reduce the intrinsic non‐uniqueness of inversions using fewer types of data and better determine smooth velocity variations and velocity discontinuities. Synthetic tests demonstrate the advantages of this new joint inversion algorithm in resolving velocity structures, especially in constraining velocity gradients across the Moho interface. We have applied the proposed joint inversion algorithm to image the lithosphere velocity structure of south China. The inverted Vp and Vs models fit body wave arrival times, surface wave dispersion and receiver functions well. We further analyzed the distribution of Vs gradients across the Moho interface in detail, which helps us better understand tectonics in south China.
... Thus, in view of the average incident angle of 25° (Figure 2b), the minimum cross-fault velocity contrast is approximately 2%, as H should be smaller than the crustal thickness of 50 km (Deng et al., 2018;Shi et al., 2021). Alternatively, if we set velocity contrast thickness H = 15 km (Sun et al., 2021), then Vp averaged over the top 15 km is approximately 6 km/s (Zhang et al., 2011), which yields a much larger cross-fault velocity contrast (approximately 5%). Of course, if the velocity contrast is in a shallower depth, the value will be larger. ...
... Mw 6.9 Yushu earthquake . An average contrast value of 5% would be estimated if the velocity contrast is limited to the upper 15 km of the crust (Sun et al., 2021). Similar velocity contrast of 5%-10% is observed along the Parkfield section of the SAF (e.g., Ben-Zion & Malin, 1991;Zhao et al., 2010), the Calaveras Fault (e.g., Zhao & Peng, 2008), the Hayward Fault (e.g., Allam et al., 2014), and the North Anatolian Fault in Turkey (e.g., Bulut et al., 2012;Ozakin et al., 2012). ...
High‐resolution imaging of fault zone structures is essential for understanding earthquake physics and fault mechanics. As a major left‐lateral strike‐slip fault in northeastern Tibet, fine structures of the damage zone in the creeping (Laohushan) section of the Haiyuan fault remain unclear. To resolve geometry and velocity reduction of the damage zone, we deployed a dense temporary network of 110 seismic stations around the creeping section of the Haiyuan fault. Travel time delays from teleseismic P arrivals suggest an approximately 1‐km‐wide low‐velocity zone, likely illuminating a broader damage zone around the Haiyuan fault. A catalog is constructed for local earthquakes based on phase picks identified from a machine learning technique. The amplification of waveforms from these local events and waveform modeling of fault zone trapped waves indicate a narrower inner damage zone with depth‐dependent width (ranging from 150 to 50 m) that extends to a depth of approximately 4 km. These values are generally consistent with those found on other noncreeping faults in California, suggesting that these damage zone properties are not affected by fault slip behaviors at shallow depth. In addition, a clear bi‐material velocity contrast across the fault is revealed by the analysis of teleseismic P arrivals. Assuming the contrast extends to a depth of 15 km, we find that P‐wave velocity is approximately 5% slower in the crustal block north of the fault. Our study shows that a temporary dense seismic network is effective in illuminating cross‐fault velocity contrast and fault geometry.
... Over these years, most research studies on the 1927 Gulang earthquake have focused on surface ruptures (Hou, 1998;Hou et al., 1998;Xu et al., 2010), fault structures (Zheng et al., 2004;Liu et al., 2015;Liu et al., 2018;Deng et al., 2020). and seismogenic mechanism (Hou et al., 1999;Dong and Liu, 2017;Guo et al., 2019;Guo et al., 2020;Sun et al., 2021). However, profound studies of the landslides triggered by the Gulang earthquake have not been conducted until about 70 years after the earthquake. ...
The 1927 Gulang Mw8.0 earthquake was one of the largest earthquakes in Gansu, China, which triggered lots of loess landslides. However, the loess of the study area is atypical and different from that of the Loess Plateau. Meanwhile, there are few systematic research studies on the characteristics of these seismic landslides. Combined with previous studies and field investigations, the landslide distribution has been revealed through the visual interpretation of remote sensing images of the study area. The relationships between landslides and various influencing factors have been explored through spatial analysis of geographic information science (GIS). Furthermore, comparisons have been made between the Gulang earthquake landslides and other seismic landslides in the Loess Plateau from macroscopic and microcosmic perspectives. A total of 807 earthquake landslides have been interpreted, with a total landslide area of 256.14 km². The region with the seismic intensity of Ⅹ is the dominant area of landslide distribution. Also, most landslides are distributed at slope angles of (10° and 40°). The relative elevation difference of (100 m and 300 m) is the high-incidence range of landslides. Compared with seismic landslides in the Loess Plateau, the slope angle and the relative elevation difference of most landslides in the study area are larger. The strength of the loess structure in the study area is stronger than that in the Loess Plateau from the view of physical properties and microstructure.
... It is more sensitive to the existence of fluids and melts in cracks and pores than the v P and v S values (Kuster and Toksöz, 1974;Takei, 2002). High resistivity, high seismic wave velocities, and low-v P /v S generally correspond to high density, low porosity, and high strength (Sun Q et al., 2021;Tenthorey et al., 2003). Based on the threedimensional electrical structure of the Sichuan-Yunnan region (Cheng YZ et al., 2017), Li CL et al. (2022) discovered that the source region of the 2021YB M6.4 earthquake is entirely located in a high resistivity zone, and its maximum foreshock occurred in a low resistivity layer. ...
The Yangbi MS6.4 earthquake occurred on May 21, 2021 in western Yunnan, China, where moderate earthquakes strike frequently. It exhibited a typical “foreshock-mainshock-aftershock” sequence and did not occur on a pre-existing active fault. The seismogenic environment and mechanism of this earthquake have aroused considerable research attention. In this study, we obtain the three-dimensional vP, vS and vP/vS images using the vP/vS consistency-constrained double-difference tomography method, which improves the accuracy of vP/vS models. We focus on characteristics of vP/vS images in areas with a lateral resolution of 0.1°, and reveal the seismogenic environment of the Yangbi MS6.4 earthquake. The conclusions are as follows: (1) Low velocity and high-vP/vS anomalies are revealed at different depths around the northern segment of the Red River fault. vS and vP/vS images along the Weixi-Qiaohou-Weishan fault and the buried faults on its west show obviously segmented feature. (2) The source region of the Yangbi MS6.4 earthquake is located in a low-vP/vS zone implying high medium strength. High-vP/vS anomalies in its NW direction indicate cracks development and the existence of fluids or partial melts, which are unfavorable for stress accumulation and triggering large earthquakes. Such conditions have also prevented the earthquake sequence from extending northwestward. (3) With the southeastward extrusion of materials from the Tibetan Plateau, fluid migration was blocked by the low-vP/vS body in the source region. The high-vP/vS anomaly beneath the source region may implies that the fluids or partial melts in the middle and lower crust gradually weakened medium strength at the bottom of the seismogenic layer, and preparing the largest foreshock in the transition zone of high to low vP/vS. Meanwhile, tectonic stress incessantly accumulated in the brittle upper crust, eventually led to the MS6.4 earthquake occurrence.
