Lisheng Xu

The 2008 M_w7.9 Wenchuan earthquake ruptured the middle and northeastern segments of the Longmenshan Fault Zone (LMSFZ), and the 2013 M_w6.6 Lushan earthquake ruptured a 50km-long fault in the southwestern segment. Subsequently, an M_w5.8 earthquake occurred approximately 10 km distant from the Mw6.6 Lushan earthquake. Therefore, the potential risk for larger earthquakes (>Mw6.6)on the southwestern section must be considered. This study collects the latest seismological and GPS data to construct an integrated seismotectonic model for the two neighboring earthquake sequences. The model integrates the fault planes involved, the mainshock rupture processes, the mainshock-caused Coulomb stress perturbation, the aftershock distribution and the 3-D velocity structure of the source region, providing information for seismic risk evaluation. We find that three fault planes were involved, two of which were related to the mainshocks, and the third was generated by the aftershocks following the first mainshock. The mainshocks were caused by nearly pure thrust faulting on the two planes with dip angles of approximately 45° and almost opposite dipping directions, thereby forming a conjugate angle of around 90°. The third plane was located between the two mainshocks, approximately parallel to the second mainshock's fault plane. Each of the mainshocks primarily ruptured a single asperity, displaying simple time history. The Coulomb stress change of the first mainshock facilitated the generation of the second mainshock and the third fault plane, and the second mainshock increased the stress on the first mainshock's fault plane. The aftershocks were distributed within stratified materials by spatially varying interfaces and characterized by high Vp and Vs velocity and a low Vp/Vs ratio. The atypical dip angles of approximately 45° for thrust faults and the conjugate angle of approximately 90° are indicative of high stress state. The single asperity rupture implies simple stress accumulation. The mainshock-caused Coulomb stress change did not reduce the seismic risk in the source region. The varying interfaces are interpreted as a consequence of long-term horizontal compression. All of these characteristics suggest that the two earthquake sequences were generated by the breakage of three immature faults under strong compression by background stress, and the high stress state remains within the southwestern LMSFZ.

We used near-field and regional GNSS (Global Navigation Satellite System) data to quantify the deformation and locking ratio of the Xiaojiang fault (XJF) in southeastern Tibet. The inversion based on the dislocation model shows that the slip rate of the XJF is 9–11 mm/a; the locking depths of the northern, central, and southern segments are 25.5 km, 12 km, and 22.5 km, respectively. The inversion with DEFNODE program shows that the locking of the northern segment is the strongest above a depth of 20 km, while the locking between 20 km and 26 km is intermediate, and the weakest locking is found below 26 km. In the central segment, the depths of the interface are 6 km and 12 km. Additionally, a locked asperity that has the potential of generating an Mw 7.2 earthquake along the northern segment is delineated. The asperity and the shallow locking zone are basically consistent with the rupture area of the 1733 M 7.8 Dongchuan earthquake and the 1833 M 8 Songming earthquake, respectively. Both the activity of the historical strong earthquakes and the seismicity of the microearthquakes recorded over recent years seem to suggest that a potential earthquake is imminent.

The Xiaojiang fault zone (XJF), located at the southeastern margin of the Tibetan plateau, has been frequently struck by destructive earthquakes throughout history. Some studies showed that repeating earthquakes may be used to estimate fault slip rates and even to monitor the pre-slip prior to large asperity. This study focuses on the repeating earthquakes recorded by a high-density seismic array in the northern section of the XJF, a segment ruptured by the 1733 Dongchuan M7.8 earthquake. Using the waveform cross-correlation and relocation techniques, we processed the continuous seismic recordings from seismic stations of the array and confirmed 27 clusters of repeating earthquakes. The repeaters are located in three relatively independent sub-areas, areas A, B, and C. Area A is close to the source areas of the 2014 Ludian Ms6.5 and the 2020 Qiaojia Ms5.0 earthquakes; by analyzing the recurrence intervals of repeating clusters, we found that the Ludian earthquake had an insignificant triggering effect on surrounding events, while the triggering effect from the Qiaojia earthquake was obvious. The clusters in area B were located very close to the northernmost tip of the XJF, which should be associated with an asperity preparing for a large earthquake on the northern XJF. Together with a repeating cluster identified in a previous study and the seismicity pattern revealed by our seismic array, we determined the geometry of the asperity and slip rate, which further helps to estimate a potential Mw7.1 earthquake for the asperity, where no M>7 earthquakes have occurred since the 1733 Dongchuan M7.8 earthquake. The clusters in area C are more like human-induced earthquakes because they are concentrated during the daytime when local people are at work, and, thus, they cannot be used for the study of tectonic deformation. This study clearly indicates that the repeating earthquakes can be triggered by a moderate earthquake nearby and also that repeating earthquakes can be used to estimate fault slip rates and outline locked asperities.

Human has been eagerly expecting to know some information that can indicate a large earthquake’s impending1,2,3,4,5. Decades of efforts showed that the impending of large earthquakes was usually accompanied by a process of regional weakening manifested with multiple temporal and spatial scaled deformation, and such a deformation was often accompanied by clusters or swarms of micro-earthquakes6, which further caused observable change in anisotropy of nearby rocks7,8,9,10. Here we show that the anisotropy information of crustal rocks may be extracted from the seismological garbage, the travel-time residuals produced by dense seismic stations which record clusters or swarms of micro-earthquakes, by means of a new concept, the slowness deviation tensor expressing 3-axis slowness anisotropy. In particular, we present a real example to justify that the slowness deviation tensor with arbitrarily inclined axis may be successfully extracted from the travel-time residual data, and meanwhile to demonstrate that this anisotropy reflected by the extracted tensor mainly originated from the local stress action or deformation. Undoubtedly, this work provides a completely new methodology to link the clusters or swarms of micro-earthquakes with the weakening process which occur before large earthquakes, which essentially contributes a promising direction toward the information that human has been expecting for.

As early as in 1733, an M~7.8 earthquake (1733DC-M7.8 earthquake) ruptured the 110km-long northern segment of the Xiaojiang fault zone, on the southeastern boundary of Tibetan plateau, and 100 years later, in 1833, another M~8.0 earthquake (1833SM-M8.0 earthquake) broke the 130 km-long middle segment. Here we show a new insight into the source of the 1733DC-M7.8 earthquake through the latest regional GNSS measurements plus our near-fault seismic and GNSS observations since February 2012. An inversion of the regional GNSS measurements resolved an asperity for the 1733DC-M7.8 earthquake that is bearing more slip deficit in comparison with the 1833SM-M8.0 earthquake, implying an Mw7.2+ earthquake, and the near-fault GNSS and seismic observations not only justified this asperity with localized deformation and multi-scaled seismicity but also are declaring an impending rupture of the asperity. More importantly, this work contributes a typical example to the recently proposed model for the generation of large earthquakes.

Fault geometry is widely recognized as one of the most important factors that affect the rupture process and damages of earthquakes. However, there have been few earthquake cases in which the close relation between the fault geometry and rupture process is resolved from inversions of seismic and geodetic observations. In this study, we develop an approach to simultaneously estimate the rupture process and dip-angle variation on the fault. The effectiveness of our new approach was validated through inverse numerical tests. We apply the new approach to the 2015 M_w7.8 Gorkha earthquake and obtain a dip-varying rupture model by jointly inverting the teleseismic, near-fault high-rate GPS, static co-seismic GPS and InSAR data. Our results show a ramp–flat décollement-ramp fault geometry of the earthquake. The shallow ramp may have prevented the rupture from breaking through to the surface. The variation of dip angle changing with depth leads to significantly different rupture velocities and rupture lengths at shallow and deep fault portions. Particularly, the northeastern downdip ramp behaves as a geometric barrier and rapidly slows down the rupture propagation in 35–45 s after the rupture initiation. In contrast, the rupture duration and fault length in the updip portion are relatively long since there is no significant lateral dip change. Furthermore, the approach can improve our understanding of the relationship between rupture behaviour and fault geometry for other thrust low-dip-angle (dip < 45°) earthquakes.

The Xiaojiang Fault (XJF) Zone locates in the southeastern of Tibetan Plateau and defines the boundary between the South China and Sichuan-Yunnan blocks. Historical large earthquakes were hosted on the XJF, though its seismic hazard in the near future is under debate. In this study, we utilize portable broad-band seismic network to unravel the microseismic activities along XJF, and to further investigate the fault structures and their properties. Adopting PALM, a newly developed earthquake detection algorithm, we obtained ~13,000 relocated events. The micro-seismicity reveals widespread off-fault structures showing conjugate geometry, while the major faults present low seismicity. The fault branches conjugate to the main-fault present intensive microseismicity, which hosts repeating events and presents high b-value. Regional GPS stations reflect slips are mostly concentrated along the XJF, while the slip rate on off-fault branches correlates with seismic activities on these structures. Combining with other recent seismological and magnetotellurics evidences, we suggest a low strength on these off-fault structures, which may partially release tectonic stress loading and serve as a barrier for future big earthquakes. On the XJF, the microseismic events are clustered on the fault junctions with low b-value. A special set of clusters between 25°N to 25.5°N show an along-strike variation of depth from 10 to 25-km, imaging the boundary between creeping and locked fault portions. We revisit the seismic hazard problem of XJF, and conclude that XJF is at the late stage of inter-seismic period.

A moment magnitude (M_w) 5.0 earthquake hit Qiaojia, Yunnan, China on 18 May 2020. Its hypocentre is only approximately 20 km away from the Baihetan reservoir, the second largest hydropower station in China. The Baihetan Reservoir is located at the junction of multiple fault zones on the eastern boundary of the Sichuan-Yunnan rhombic block, an area with high background seismic activity. The Baihetan Reservoir was planned to be impounded in April 2021 and the M_w 5.0 earthquake occurred during its water-retaining. Thus, it is critical to investigate the seismogenesis of the Qiaojia M_w 5.0 main shock and evaluate the risk of inducing earthquakes near the Baihetan Reservoir after impoundment. In this study, we built a complete and accurate earthquake catalogue to analyse seismicity in the reservoir area before and after the M_w 5.0 Qiaojia earthquake. We adopted a machine learning-based seismic phase picker, PhaseNet, to automatically detect seismic picks from continuous raw seismic data. Seismic phase picks were associated and located using sequential earthquake association and location methods, including REAL, VELEST and hypoDD. We eventually obtained high-precision locations of 1640 earthquakes by the hypoDD. The distribution of earthquake locations indicates that a concealed fault nearly vertical to the surface accommodated the M_w 5.0 Qiaojia main shock. The majority of its aftershocks is located within a narrow depth range of 8–13 km, indicating that the stresses in the hypocentral area were concentrated near the hypocentre of the M_w 5.0 earthquake. Along with focal mechanism solutions, we suggested that the M_w 5.0 Qiaojia earthquake is more likely a tectonic earthquake. However, we cannot exclude the possibility that earthquakes could be induced after the impoundment of Baihetan Reservoir, because the identified concealed fault is located in the middle of many large fault zones and only 20 km away from the Baihetan Reservoir.