Sichuan-Yunnan Block Research Articles

Active Fault Interpretation in the Northern Segment of the Red River Fault Based on Multisource Remote Sensing Data

High-resolution topographic and geomorphic data are important basic data for the study of active structures. Here, multisource remote sensing data were used to reinterpret the active faults in the northern segment of the Red River Fault (China). First, we obtained airborne light detection and ranging (LiDAR) data, high-resolution GaoFen-7 (GF-7) remote sensing image data, and historical aerial photographs, and a high-resolution digital elevation model (DEM) was generated based on the airborne LiDAR data and GF-7 data. According to the remote sensing interpretation, the main active faults were identified. We subsequently verified the faults in the field and constrained the geographic locations. The current activity was confirmed to be dominantly normal faulting, with some dextral strike-slip components, and the latest active age was the Late Holocene. It reflects the coordination of structural deformation between the rotation of the secondary block and the sliding of the boundary fault within the Sichuan–Yunnan Block. The results show that airborne LiDAR and GF-7 remote sensing data have a great application value in providing high-resolution topographic and geomorphologic data for the study of active structures. The comprehensive application of multisource remote sensing data can greatly improve the reliability of active fault interpretations and provide a reference for follow-up research within the study area.

Clockwise extrusion of the Sichuan‒Yunnan block toward the Red River Fault in the southeastern Tibetan Plateau

The Xiaojiang Fault and the Red River Fault, which are located on the southeastern margin of the Tibetan Plateau, are the eastern and southern boundaries of the Sichuan–Yunnan Block, respectively. The relationship between these two faults is highly important for the study on the tectonic evolution and seismic risk of the southeastern margin of the Tibetan Plateau. Using the digital elevation model (DEM) data, we extracted and analyzed the maximum elevation, peak steepness index, maximum slope, and relatively flat surface of the Ailaoshan Shear Zone on the southwest side of the Red River Fault. The results revealed that the geomorphic indices result around Yuanjiang were significantly lower than those on the northern and southern sides of Yuanjiang. On the basis of lithology, climate and tectonics, it is inferred that tectonic activity is the main factor controlling landform development. On the basis of existing geophysical and geomorphic survey results in this area, a geodynamic model of this area was constructed: in the mid-Miocene, tectonic inversion of the Red River Fault occurred, and material from the Tibetan Plateau flowed into the Sichuan‒Yunnan Block around the East Himalayan Syntax. The resulting extrusion caused the Red River Fault to bend of ∼50 km; in the early Pliocene, the middle and lower crust broke through the barrier and entered the interior of the Ailaoshan Shear Zone. Because of the extrusion of the middle and lower crust, the role of the Red River Fault as the boundary has weakened.

Low-velocity middle-and-lower crustal materials blocked by the red river fault in the SE margin of the Tibetan plateau

The ongoing collision between the Indian and Eurasian plates propels the eastward movement of the Tibetan plateau (TP), leading to substantial crustal deformation around the southern Sichuan-Yunnan block (SYB). Using ambient noise data from multiple temporary seismic arrays and permanent stations, we construct a high-resolution regional crustal azimuthally anisotropic Vs model in the SYB. Our new model reveals two significant low-velocity anomalies with strong azimuthal anisotropy near the block boundary faults in the middle-and-lower crust. The extensive low-velocity anomalies around the middle-south segment of the Xiaojiang Fault (XJF) possibly result from partial melting due to spontaneous deformation caused by crustal thickening and increased felsic components, as well as the superimposition of shear heating faults and local upwelling asthenosphere. The N‒S trending low-velocity anomaly at the northwest end of the Red River Fault (RRF) may be associated with weak material migration from the TP, potentially serving as a conduit for mantle upwelling. The azimuthal anisotropy along the block boundary faults exhibits spatial variations linked to segmented distortion resulting from southeastward crustal movement and various geological activities. A key finding is that the crustal channelized low-velocity along the XJF is clearly blocked by the RRF, instead of going through. Notably, the azimuthal anisotropy in the E‒W direction, observed above the Moho and at depths deeper than 30 km in the intersection end, implies the potential intrusion of localized mantle materials into the lower crust. Therefore, lithospheric deformation is significantly affected by block boundary faults and the properties of the crust and mantle.

Newly Discovered NE-Striking Dextral Strike-Slip Holocene Active Caimashui Fault in the Central Part of the Sichuan-Yunnan Block and Its Tectonic Significance

The Sichuan-Yunnan block is a tectonically active region in China, with frequent large earthquakes occurring in and around it. Despite most earthquakes being concentrated along boundary faults, intraplate faults also have the potential to generate damaging earthquakes. Remote sensing makes it possible to identify these potential earthquake source faults. During an active fault investigation in the Liangshan area, a distinct lithological boundary named Caimashui fault was found. The geometric distribution and kinematic parameter of the fault is crucial for assessing seismic hazards and understanding the deformation pattern within the Sichuan-Yunnan block. The Caimashui fault is mapped with remote sensing interpretation, a field survey, and UAV measurement. Through trenching and Quaternary dating, the Late Quaternary active characteristics of the fault are studied. The fault is a Holocene active dextral strike-slip fault with a reverse component, exhibiting a dextral strike-slip rate of ~0.70 ± 0.11 mm/a. Paleoseismic investigation shows that the last surface rupture event of the Caimashui fault occurred later than 4150 ± 30a BP, with a magnitude of M ≥ 7.0. The fault may act as a secondary splitting fault, absorbing the deformation caused by various sinistral strike-slip rates of the boundary faults and the potential energy from the counterclockwise rotation of the Central Yunnan micro-block.

Interseismic deformation in the northwestern Sichuan-Yunnan block constrained by Sentinel-1 InSAR and GNSS

The northwestern Sichuan-Yunnan block (NW SYB), located in the southeastern Tibetan Plateau, is characterized by complex fault systems. Its detailed crustal deformation is crucial to comprehending the kinematics of the Tibetan expansion. In this study, we integrate the Interferometric Synthetic Aperture Radar (InSAR) and Global Navigation Satellite System (GNSS) data to obtain the high-resolution present-day deformation of the NW SYB, which provides insights into the strain partitioning, fault kinematics, and block motion characteristics in this region. Our results show that the elastic strain is not only built up around faults but also widely distributed in the off-fault areas. The Ganzi segment of the Ganzi-Yushu fault and the Luhuo segment of the Xianshuihe fault in the study area accommodate 5.31 mm/yr and 8.41 mm/yr left-lateral strike-slip motion, respectively. The small-scale faults show certain deformation, among which the left-lateral and right-lateral slip rates of the Litang and Zhongdian faults are 2.53 mm/yr and 1.97 mm/yr, respectively. The spatial patterns of the strain partitioning and block motion show that the active strike-slip faults accommodate displacements from Cenozoic block extrusion and rotation, which partially coordinate the kinematic discrepancy of the Tibetan expansion. Our geodetic measurements and existing structural observations indicate that the southeastward expansion of the Tibetan Plateau is absorbed mainly by E-W shortening and N-S extension in the NW SYB, may accommodated by continental strike-slip faults in the upper crust and distributed shear in the lower crust.

Recent Holocene activity and regional tectonic significance of the northern segment of the red river fault zone

The Red River Fault Zone is a large-scale right-lateral strike-slip fault zone with relatively strong activity during the Quaternary Period. This fault, located on the southeastern margin of the Qinghai‒Tibetan Plateau, plays a key role in the extrusion, rotation and escape of the continental blocks constituting the Qinghai‒Tibetan Plateau. Furthermore, this fault represents the southwestern boundary of the Sichuan–Yunnan Block, which has experienced strong deformation and frequent seismic activity. The northern segment of the Red River Fault Zone is the most active part of the whole fault. However, surface erosion and vegetation coverage have obscured the activity of the northern segment; therefore, the study of its activity has obviously been insufficient. There is still controversy over whether all the secondary faults on the northern segment are active Holocene faults. Studying the activity characteristics of the northern segment, which is densely populated, is particularly important for seismic risk prevention in this area. Based on remote sensing interpretations and field geological surveys, this paper describes the latest activity characteristics of the Cangshan Piedmont Fault, Fengyi–Dingxiling Fault and Midu Basin Margin Fault, including their spatial distributions and kinematic characteristics. According to the ages of the offset strata in profiles, the above three secondary faults were all active in the late Holocene. The latest active age of the Cangshan Piedmont Fault was later than 543-494 cal BP, and two palaeoseismic events that occurred in this section during the Holocene occurred at 2700 and 473 cal BP. The latest active age of the Fengyi–Dingxiling Fault was later than 2760–2700 cal BP; in this section, one Holocene palaeoseismic event occurred between 1777 cal BP and 2730 cal BP, another occurred between 2730 cal BP and 5664 cal BP, and the third occurred between 6449 cal BP and 8360 cal BP. The latest active age of the Midu Basin Margin Fault was later than 558-510 cal BP, and two Holocene palaeoseismic events occurred later than 2318-2114 cal BP and 558-510 cal BP. Based on the results of this paper and previous studies, we believe that the Fengyi–Dingxiling Fault on the northern segment of the Red River Fault Zone is at risk for future strong earthquakes. Additionally, abundant geological and geomorphologic evidence suggests that the northern segment is dominated by normal faults, reflecting the local strain response to secondary clockwise rotation of the Sichuan–Yunnan Block along the boundary fault. This finding is in line with the eastwards extrusion and escape of materials on the Qinghai‒Tibetan Plateau caused by the northwards and northeastwards pushing of the Indian Plate. To a certain extent, these observations reflect the tectonic deformation coordination between block rotation and boundary fault slip in the Sichuan–Yunnan Block in the context of continental block extrusion on the Qinghai–Tibetan Plateau.

Geochemical characteristics of hot springs in active fault zones within the northern Sichuan-Yunnan block: Geochemical evidence for tectonic activity

The geochemistry of the fluids is spatially closely related to faults and earthquakes that have been detected in the northern Sichuan-Yunnan (SC-YN) block. However, the processes and factors that control hydrothermal fluid circulation on a regional scale remain challenging. The geochemical circulation of the hot springs integrated with geological and geophysical data revealed the processes and controlling factors of the relationship between fluid circulation and tectonic activity. Our results showed that the hydrochemical type of the hot springs at the intersection of the Yushu-Ganzi fault (YS-GZF), Xianshuihe fault (XSHF) and Yunongxi fault (YNXF), Jinshajiang fault (JSJF) and Batang fault (BTF) as well as Ganzi-Litang fault (GZ-LTF) is composed of Na-HCO3 and Na-HCO3·Cl. It demonstrated higher reservoir temperature and circulation depth (5 − 7 km). The helium isotope (0.22 − 2.36 Ra) of the gas indicated that the mantle helium composition was up to 5 − 30 %. These hot springs may undergo water–rock-gas exchange reactions with deep fluids. Considering geological and geophysical background as well as previous studies, the geochemical characteristics of the hot spring at the intersections of faults in the north of the SC-YN block are thought to be controlled by the deep-driven upwelling lower crustal flows, mixing with shallow hot spring water and the temporal and spatial heterogeneity of the high strain fields, slip rates, and permeability of the fault intersections. Whereas, the transport of fluids in hot springs of fault may increase pore fluid pressure, and reduce the effective stress at the intersection of the fault zones, which can promote fault sliding and provoke seismic events as well as fault movements. It could result in the enhancement of regional tectonic activity. This study provides an important basis for seismic prediction with hot spring hydrochemistry stations.

Holocene activity and seismic surface rupture zone of the Abuduo fault eastern Tibetan Plateau

The Garzê–Yushu fault is a large active NW-trending strike-slip fault on the Tibetan Plateau, along which strong earthquakes have frequently occurred historically. Together with the Xianshuihe fault, the Garzê–Yushu fault constitutes the northern boundary of the Sichuan–Yunnan block, and it is also the boundary between the Bayan Har and Qiangtang blocks. The Abuduo fault is a near E–W-trending fault on the southern side of the Garzê–Yushu fault. Through remote sensing image interpretation and field seismic geological investigations, this study found a series of left-lateral displacement landforms and well-preserved seismic surface rupture zones along the Abuduo fault, extending over a distance of approximately 65 km from Abuduo through Yushu and Qinghai to Selikou Village. According to the geological and geomorphological evidence, the Abuduo fault is considered a left-lateral strike-slip fault with Holocene activity, which has tectonic conditions suitable for breeding and generating strong earthquakes. Additionally, the eastward extension of the Abuduo fault is likely to intersect the Garzê–Yushu fault and thereby decompose its horizontal sliding deformation.

Crustal Velocity Structure of the Sichuan–Yunnan Block Revealed by High-Quality Crustal Phase Travel Time

Abstract The Sichuan–Yunnan block is located at the southeastern margin of the Tibetan Plateau, which is the key area as a transition belt from the active plate extrusion zone to the stable Yangtze Craton. Using a semiautomatic measuring method based on a graphical interface, we pick 81,585 precise travel times from 449 local earthquake records and finally obtain a crustal 3D P-wave velocity model of the Sichuan–Yunnan block. The model reveals an unexpected velocity contrast between the shallower and deeper crusts. It is summarized as weakly perturbed low-velocity belts encircling a high-velocity zone in the upper crust and strongly perturbed low-velocity anomalies in the mid-lower crust, respectively. The weak low-velocity anomalies are revealed along the major strike-slip faults, and their small perturbations may imply a slip-driven mechanism. The strong low-velocity anomalies are distributed extensively in the Sichuan–Yunnan block, and their great perturbations may be related to the partial melting of weak material extruded from Tibet. Besides, our result shows noticeable high-velocity anomalies in the core zone of the Emeishan Large Igneous Province (ELIP), which may be an indication of magma solidification from the ancient mantle plume. The result further exhibits an interesting pattern that the strong low-velocity anomalies are partially separated by the high-velocity anomalies in the ELIP. Such a specific pattern probably reflects that the stable zone in the ELIP leads to the bifurcation of weak Tibetan material.

Present-Day Tectonic Stress Evolution in Southern Yunnan Based on Focal Mechanisms.

Tectonic extrusion bypassing the eastern Himalayan syntaxis results in a significant increase in regional stress instability and the associated frequent occurrence of earthquakes in southern Yunnan, China. However, the stress field, and the relationship between the focal mechanism of earthquakes and stress evolution in southern Yunnan, remain enigmatic. In this paper, using a modified grid point test method, we calculated the focal mechanism of ML ≥ 2.5 earthquakes in southern Yunnan (22-25° N, 100-104° E) from January 2009 to June 2023. Utilizing the solutions of historical earthquake focal mechanisms, we obtained the present-day regional tectonic stress field in southern Yunnan via inversion. The results indicate complex and diverse seismic focal mechanisms, and the main types of earthquakes are strike-slip events, followed by normal fault and reverse fault events. The orientations of the maximum and minimum principal stress axes rotate in a clockwise direction from northeast to southwest. The internal stress orientation distribution of the rhombic Sichuan-Yunnan block in the study area is consistent, and the block boundary zone is the site where stress deflection occurs, and the regional tectonic stress field is influenced by the interaction among different blocks. The distribution of R-value in the Lamping-Simao block gradually increases from north to south, indicating that the compressive stress required for material transport becomes relatively small. Combined with the geological and tectonic background of the study area, our results suggest that the speed of block movement gradually decreases from north to south; the distribution of R-value in the South China block is significantly smaller than that of the interior of the Sichuan-Yunnan rhombus, and the proportion of compressive stresses is larger, indicating a stronger extrusion in this region, which may be related to the fact that the Sichuan-Yunnan rhombus is strongly resisted by the South China block in the east.

Rigid widths of active block boundary faults and crustal layered anisotropy in the intersection of faults Honghe and Xiaojiang in the SE margin of the Tibetan Plateau

SUMMARYThe distribution of block boundary faults, both horizontally and vertically, has significant implications for crustal movements and structural variations. In the SE margin of the Tibetan Plateau, the south part of the Sichuan–Yunnan block is surrounded by the first-order block boundaries of the Honghe faults (HHF) trending NW–SE and the Xiaojiang faults (XJF) trending N–S. Crustal anisotropy provides valuable insights into the ongoing deformation and evolution. This study investigates the upper-crustal anisotropy at 26 stations, using local seismic waveforms from the temporary linear Honghe–Xiaojiang seismic broad-band Array (i.e. the HX Array) in 2017–2019 and regional permanent stations in 2012–2019. The PFS (i.e. polarization of fast S wave) shows an abrupt change in the Yimen region, which is the middle of the HX Array survey line between two large faults. The NW–SE dominant PFS orientations across the HHF agree well with the strike of faults and the direction of regional principal compressive stress, illustrating cracks or minerals aligned with the HHF, influenced by regional stress. The NE–SW dominant PFS orientations crossing the XJF differ from the faults strike or regional stress orientation, suggesting alignment with microcracks caused by the in situ principal compressive stress. The comparison between the upper-crustal and whole-crustal anisotropy demonstrates a noticeable PFS consistency across the two large faults, the HHF and the XJF. This observation implies that these two large faults may tectonically control a wide zone, with wide rigid widths of approximately 150 and 200 km, respectively. This study presents a new pattern indicating the control of deep physical properties on the boundary faults and uncovering the layered deformation in the upper and lower crust.

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.

Seismic anisotropy in the upper crust around the north segment of Xiaojiang faults in the SE margin of Tibetan Plateau

The Xiaojiang faults located in the SE margin of the Tibetan Plateau is a fault system of left-lateral strike-slip, striking NS, between the 2nd-order Sichuan-Yunnan block and the 1st-order South China block. The Xiaojiang faults and the surrounding areas are characterized by strong tectonic movements and intense seismic activities. Using seismic data from January 2013 to November 2020 recorded at the stations of the temporary QiaoJia seismic Array (QJ Array), deployed by the Institute of Geophysics, China Earthquake Administration, this study investigates the upper crustal anisotropy by the shear-wave splitting analysis on small local earthquakes, discusses the deformation patterns in the upper crust in the north segment of the Xiaojiang faults, evaluates the stress distribution in the study area, and analyzes its relationship with the regional tectonic structure. Adopting the data processing technique of shear-wave splitting, a total of 875 effective records were obtained at 50 stations. The mean direction of polarizations of fast shear-wave (PFS) is 162° ± 44° in the study areaand the mean normalized time-delay is 4.96 ± 2.38 ms/km. Based on the spatial distribution of the PFS and the regional geologic structure, the study area is divided into two zones: the zone N and the zone S. The PFS in the zone N is scattered, but the dominant PFS direction is in NNW, which is consistent with the direction of the regional maximum principal compressive stress. In the zone N, there are a few smaller local areas (i.e., subzones A, B, C, and D) in which the orientations of the PFS are quite different from the surrounding area. In the zone S, the dominant directions at most stations are in nearly NS, consistent with the strike of the Xiaojiang fault. It reveals the detailed spatial distribution of seismic anisotropy in the upper crust, as well as in situ principal compressive stress, indicating the influence of the regional stress, the complex tectonic environment, and maybe also the impact of the South China block. It also reveals that there also might be an upper-crust scale of tectonic line at near 26°20′N under Xiaojiang faults, which coincides with the north-south tectonic boundary in the lithospheric anisotropy.

Active faulting of the Nanhe Fault and relation to the Anninghe Fault zone in the late Quaternary, eastern Tibetan Plateau

Faults along the boundaries of active tectonic blocks are the main structures that are responsible for major earthquakes in mainland China. Investigating the geometric distribution, rupture behavior, and paleoseismic history of these faults is the prerequisite for constraining geodynamic models and regional seismic hazard analyses. The Nanhe Fault, located at the eastern boundary of the Sichuan–Yunnan Block near Mianning County, has been paid less attention so far due to insufficient historical records of major earthquakes. In this paper, we focused on the Nanhe Fault and conducted satellite imagery interpretation, field investigations, and trench excavations. Our findings indicate that the Nanhe Fault initiates north of Mianning County; the north segment of the fault is connected with the Anninghe Fault; and it extends for about 70 km south-westward and terminates southwest of Ermaga Village. The fault has been faulting in the late Late Pleistocene with a left-lateral strike-slip rate of 2.40–2.56 mm/yr, while in the late Holocene, the left-lateral strike-slip and vertical slip rates are 2.50–2.60 mm/yr and about 0.60 mm/yr, respectively. Three paleoseismic events (5373–4525 BC, AD 1193–1576, and AD 1496–1843) were identified by excavating trenches at the Nanhe Fault. A comparative analysis of paleoseismic events between the Nanhe and the Anninghe fault indicates that both faults may have induced cascade rupture or triggered earthquakes—such related events may have occurred in 1496–1627. Additionally, by comparing the kinematic relationship of the faults at the eastern boundary of the Sichuan–Yunnan Block, we propose that the Nanhe Fault takes part in strain partitioning along the boundary. This interpretation reasonably explains the loss of the sliding rate between the Anninghe and Zemuhe faults, which also supports the GPS inversion results, and the discontinuous deformation model for the eastern margin of the Tibetan Plateau.

Two Unconnected Low-Velocity Zones in the Eastern Boundary of the Sichuan-Yunnan Block Revealed with High-Resolution Ambient Noise Tomography

The Anninghe-Zemuhe-Xiaojiang fault zone (AZXFZ) is an important boundary fault zone on the southeastern margin of the Tibetan Plateau, with frequent strong earthquakes. Previous studies have imaged widespread low-velocity zones in this area. However, there are still many disputes on the connectivity and genesis of the low-velocity zones. In this study, we obtain the Rayleigh wave phase velocity dispersion curves at 4–25-s periods using observations from 378 broadband stations located near the AZXFZ. The new 3D S-wave velocity model has a lateral resolution of about 30 km in 0–35-km depth and is obtained by direct inversion of surface wave dispersion data. The new results clearly image two low-velocity zones and a high-velocity zone in the middle crust of the study region. The low-velocity zone on the western side of the Lijiang-Xiaojinhe fault is related to the eastward flow of crustal material and the movement of the left lateral strike-slip faults in the Tibetan Plateau, while the low-velocity anomaly distributed along the Daliangshan fault and Xiaojiang fault is the superimposition effect of shear heating of the faults and upwelling of mantle material. The uplift of Gongga Shan is a combination of the continuous accumulation of crustal material in the middle and lower crust of the southeastern margin of the Tibetan Plateau as well as the bending and compression of the Sichuan-Yunnan Block and the Xianshuihe-Anninghe fault zone.

Deep seismogenic tectonics of Yangbi MS6.4 on 21 May 2021 in the SE margin of the Tibetan plateau from earthquake sequence relocation, stress field and seismic anisotropy

The MS6.4 earthquake occurred in Yangbi County, Yunnan Province, China, on 21 May, 2021. The earthquake is located on the west side of the Red River fault, the western boundary of the Sichuan-Yunnan block in the SE margin of the Tibetan Plateau. The Yangbi MS6.4 earthquake has aroused great concern due to its unique geographical location and frequent strong earthquakes in Yunnan. In this study, a total of 4695 local earthquakes of the Yangbi MS6.4 earthquake sequence are relocated using the double-difference location method, including the mainshock source location of 99.879°E, 25.687°N, and depth of 9.67 km. The mainshock has a moment magnitude of 6.1, and one of the fault-plane solutions is 137°, 83° and − 163° in strike, dip, rake respectively. Combined with the relocation and focal mechanism solutions results, it is suggested that the seismogenic fault of the Yangbi MS6.4 earthquake is a NW-SE trending fault on the southwest side of the Weixi-Qiaohou fault (WQF), with high dip of SW, which is a previously-unknown blind fault. The regional 2D stress field shows that the principal compressive stress around the Yangbi earthquake is NNW-SSE and nearly N-S direction, which appears to be deflected counterclockwise by a small angle on its east side and changed to N-S and NNE-SSW direction on its west side, while both the north and south sides of the Yangbi earthquake show directional coherence. Strong differential deformation distorts in the upper crust to the northeast of the seismic source as reflected by tectonic stress field and the upper crust seismic anisotropy, indicating strong influence of Lijiang-Xiaojinhe fault (LXF). The stress field in the source area is jointly controlled by southeastward movement of southeastern margin of Tibetan Plateau, the northeastward movement of Indian plate and the differential movement between the different tectonic blocks.

Crustal azimuthal anisotropy in the lateral collision zone of the SE margin of the Tibetan Plateau and its tectonic implications

SUMMARYResolving the detail of crustal deformation of the seismically and technically active Sichuan-Yunnan block located in the southeast margin of the Tibetan Plateau is crucial for understanding the lateral extrusion of the Tibetan Plateau. Using seismic data from two regional seismograph stations of the China Earthquake Networks Center and twenty recently deployed broad-band temporary stations from the MCD Array (Middle ChuanDian block seismic array), we constrained crustal thickness and seismic anisotropy at the west boundary of the Sichuan-Yunnan block, near the east Himalayan syntaxis by receiver function techniques including CCP stacking and sinusoidal moveout of PmS conversions. Crustal thickness varies significantly across major faults inside the block which increases by >20 km from west to east. East of the Chenghai Fault, a clear positive phase lies above the Moho and is interpreted to be caused by the magma intrusion related to a late Permian Emeishan large igneous province (ELIP). The crustal anisotropy shows strong zoning features and clockwise rotation with respect to due north. To the west of the Lijiang-Xiaojinhe and the Honghe Faults, anisotropic fast orientations are near NS, indicating strong crustal deformation and movement of southward crustal flow along the Nujiang-Lancangjiang Fault; to the east of this fault, anisotropic fast orientations are near NE. The variation of the anisotropic fast orientations indicates that the southeastward crustal flow from the Tibetan Plateau may be prevented by fast velocity anomalies beneath the ELIP. These observations provided new evidence for strong crustal deformation along the tectonic boundary in the southeastern Tibetan Plateau with the expansion of the Tibetan Plateau.

Relationship between hydrogeochemical characteristics of hot springs and seismic activity in the Jinshajiang fault zone, Southeast Tibetan Plateau

Significant anomalous hydrogeochemical changes in hot spring water are detected during strong seismic cycles. It is now necessary to clarify the relationship between tectonic movements, earthquakes and the evolution of hot springs. In this paper, laboratory analyses of major, trace elements, δD, δ18O and 87Sr/86Sr values of 28 hot spring waters in the Jinshajiang fault zone (JSJFZ) in the northwestern boundary of the Sichuan-Yunnan block were conducted. The results showed that the primary source of water for JSJFZ hot springs was atmospheric precipitation. The geothermal reservoir temperature variation based on the silicon enthalpy mixing model ranged from 73 to 272°C. And the circulation depth range was 1.2–5.4 km. The segmentation characteristics of the 87Sr/86Sr values were related to the influence of source rocks on groundwater cycle processes. A conceptual model of the hydrologic cycle of hot springs explained the spatial distribution of earthquakes associated with tectonic movements. The Batang segment had the strongest water-rock reaction, the highest reservoir temperature and the deepest circulation depth; meanwhile, it was also an earthquake prone area. The fluid circulation of the JSJFZ corresponds well with the seismicity, which indicates that the hydrological characteristics of the hot spring water in a fracture zone play a crucial role in receiving information on seismic activity.

Coseismic deformation of the 2022 Luding <italic>M</italic><sub>S</sub>6.8 earthquake and seismic potential along adjacent major faults

<p indent="0mm">The 2022 Luding (Sichuan) <italic>M</italic>6.8 earthquake occurred on the Moxi segment of the Xianshuihe fault, one of the most seismically active faults in mainland China. Since 1900, six moderate to strong earthquakes (&gt; <italic>M</italic>6.5) have ruptured different sections of the Xianshuihe fault. This Moxi segment has a complex tectonic setting as it forms an intersection zone of the Anninghe, Daliangshan, Xiaojin, and Longmenshan faults. In this study, we investigated the distribution of coseismic deformation of the 2022 Luding earthquake in order to improve understanding of the seismogenic fault and gain insight into the kinematic behaviors of regional faults and seismogeneity of future strong earthquakes. We collected continuous global positioning system (GPS) and strong motion data within 200 and <sc>50 km</sc> from the epicenter, respectively, and performed high-precision processing to extract the coseismic deformation field of this earthquake. We also collected Sentinel-1 and Advanced Land Observing Satellite-2 (ALOS-2) ScanSAR model descending data covering the epicenter area, and processed these data using differential interferometric synthetic aperture radar (D-InSAR) technology. The GPS measurements confirm that deformation from this earthquake occurred as far as <sc>200 km</sc> from the fault. This coseismic deformation was distributed in four quadrants, indicating that the focal mechanism of the Luding earthquake was left-lateral strike-slip. A strong motion station <sc>16 km</sc> from the epicenter recorded a significant displacement waveform; peak ground displacement was <sc>14 cm</sc> and the permanent coseismic displacement was <sc>12 cm.</sc> InSAR deformation maps show significant coseismic deformation in a 30 km×<sc>30 km</sc> area along the fault. The maximum line-of-sight displacement reached <sc>15 cm</sc> to the west of the fault. In addition, significant local deformation (&gt; <sc>15 cm)</sc> was observed near the Wajiao township. Field investigations suggest that surface ruptures developed along the fault segment between Ertaizi and Aiguocun. However, it is difficult to identify rupture information from the InSAR maps owing to a loss of coherence in the near-field of the seismogenic fault. Detailed field investigations are needed to determine if coseismic slip reached the surface. Finally, we inverted interseismic coupling along the northeastern boundary faults of the Sichuan–Yunnan block, and quantitatively calculated the seismic moment budget on these faults. Based on the historical earthquake distribution of the past <sc>300 years</sc> and present-day fault locking conditions along the Xianshuihe fault, the Luding earthquake accommodated clockwise vertical-axis rotation of the expanding Qinghai-Tibetan Plateau relative to the South China Block. The earthquake fully released the strain accumulated on the Moxi segment since the 1786 Moxi <italic>M</italic>7³/₄ earthquake; as such, it is unlikely that large earthquakes will occur on the Moxi segment in the near future. However, we also performed a comprehensive investigation of seismic hazard along the Xianshuihe-Xiaojiang fault system based on the mapped seismic gap, asperity distribution, seismic moment deficit, and Coulomb failure stress caused by surrounding earthquakes. We found that the seismic moment accumulated on the Anninghe, Zemuhe, Daliangshan, and southern Longmenshan faults can be balanced by <italic>M</italic>_w7.0–7.7 earthquake. The Luding earthquake caused significant Coulomb stress loading (&gt; <sc>0.01 MPa)</sc> on the Daliangshan, Anninghe, and southern Longmenshan faults, bringing those faults closer to failure. In particular, Coulomb stress loading at the northern end of the Anninghe fault exceeded the conventional triggering threshold <sc>(0.1 MPa)</sc> for earthquakes; therefore, the risk of future strong earthquakes on the Anninghe fault deserves special attention.

Characteristics of stress field and crustal kinematics of the southern region of Sichuan-Yunnan block, southern Tibet Plateau

The Red River fault is a massive tectonic shear zone of the Sichuan-Yunnan block, southern Tibet Plateau. The target area of this study is the Red River fault's middle-northern section and adjacent areas. Considering the spatial difference of tectonic structures and the inhomogeneity of focal mechanism solutions, we divided the target area into three structural units: the northern extension area, the middle strike-slip movement area, and the west complex rupture area. The study is carried out on the basis of zoning. Using the CAP method to invert the focal mechanism of moderate earthquakes and the principal compressive and tensile stress. Restoring the seismic source spectrum of small and medium earthquakes, then inversion the stress drops in three sub-units based on Brune model using digital waveform records. Our study suggests that: (1) Many high-stress drop earthquakes occurred in the transitional areas from the Weixi-Qiaohou fault to the Red River fault and from the Chuxiong fault to the Qujiang fault, also occurred in the Changning-Baoshan-Yongping area, revealing a large accumulation of local tectonic stress. (2) Some events with Δσ ≥ 20 MPa have occurred in the prominent low-speed anomaly area, indicating that a passive deformation crossed the Red River fault and extended to the west. Yangbi MS 6.4 earthquake (2021) occurred in a strongly locked area with high potential earthquake risk. (3) In the north of the study region, the movement of the Sichuan-Yunnan block to SE-SSE orientation was governed by the dextral strike-slip movement of the NW strike fault; in the middle region, the SE-orientated movement of the block relative to the South China block was governed by the sinistral slip movement of the SN-strike faults; and in the west region, the tectonic stress was governed by clockwise rotation of the block and the sinistral slip movement of the NE strike faults. We suggest that strike slip and rotation of the Red River fault's middle section may drive and aggravate passive movement in the north section, resulting in locally geophysical field responses. Therefore, the potential risk of a large earthquake in the middle-northern section of the Red River fault should be monitored.