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The northeastern margin of the Tibetan Plateau (NE Tibetan Plateau) exhibits active geological structures and has experienced multiple strong earthquakes, with M ≥ 7, throughout history. Particularly noteworthy is the 1920 M81/2 earthquake in the Haiyuan region that occurred a century ago and is documented as one of the deadliest earthquakes. Consequently, analyzing seismic risks in the northeastern margin of the Tibetan Plateau holds significant importance. The b value, a crucial parameter for seismic activity, plays a pivotal role in seismic hazard analyses. This study calculates the spatial b values in this region based on earthquake catalogs since 1970. The study area encompasses several major active faults, and due to variations in b values across different fault types, traditional grid-search methods may introduce significant errors in calculating the spatial b value within complex fault systems. To address this, we employed the hierarchical space–time point–process (HIST-PPM) method proposed by Ogata. This method avoids partitioning earthquake samples, optimizes parameters using Akaike's Bayesian Information Criterion (ABIC) with entropy maximization, and theoretically allows for a higher spatial resolution and more accurate b value calculations. The results indicate a high spatial heterogeneity in b values within the study area. The northwestern and southeastern regions exhibit higher b values. Along the Haiyuan fault zone, the central rupture zone of the Haiyuan earthquake has relatively higher b values than other regions of this fault zone, which is possibly related to the sufficient release of stress during the main rupture of the Haiyuan earthquake. The b values vary from high in the west to low in the east along the Zhongwei fault. On the West Qinling fault zone, the epicenter of the recent Minxian–Zhangxian earthquake is associated with a low b value. In general, regions with low b values correspond well to areas with moderate–strong seismic events in the past 50 years. The spatial differences in b values may reflect variances in seismic hazards among fault zones and regions within the same fault zone. [ABSTRACT FROM AUTHOR]

Systematic measurements of water temperature are lacking but useful in understanding the relationship between water temperature and earthquakes. Based on the water temperature data, geological structure, borehole structure, and temperature gradient in the Jinjia well, Southwest China, we systematically analysed the water temperature changes related to earthquakes. The water temperature of the Jinjia well recorded the coseismic changes caused by the Wenchuan M7.9 and Panzhihua M6.1 earthquakes in 2008. We also found abnormal changes in the water temperature, after which moderate to strong earthquakes occurred in the surrounding region. The preseismic abnormal changes of the Jinjia well were rising-recovery (rising to a high value and continuing for a period of time before decreasing or quickly recovering), with the range of 0.007–0.07 °C. The maximum change (0.07 °C) occurred before the M7.9 Wenchuan earthquake in 2008. According to the Molchan error diagram, the most likely time for an earthquake to occur is within approximately 4 months after the water temperature exceeds the threshold temperature. In the Jinjia well, the installation depth of the temperature sensor affected the correlation between the temperature changes and earthquakes with a seismic energy density above 10−3 J·m−3. The shorter the distance between the sensor and the fault, the higher the probability of water temperature changes related to earthquakes. [ABSTRACT FROM AUTHOR]

The Selaha segment of the Xianshuihe fault zone was the seismogenic fault of the 2014 Kangding Ms6.3 earthquake. This segment has been considered as one of the most dangerous areas for surface rupture-induced earthquakes with a magnitude of M≥7 due to the long elapsed time of the latest surface rupture event (the 1725 Kangding Ms7.0 earthquake). Obtaining the spatial distribution of the latest seismic surface rupture along the Selaha fault is significant for determining the seismic activity history, assessing the seismic potential, and preventing and mitigating disasters. However, there is still considerable controversy about the extension northwestern of the latest surface rupture. In order to solve this problem, we excavated two large paleoseismic trenches in the Zhonggu village where the previous data believed that there was no coseismic surface rupture, and obtained the rupture history of this region. The latest event (E6) occurred after A. D. 746±51. Based on the trench profile evidence, geomorphological features, and historical earthquake records, the latest event E6 is associated with the 1725 Kangding Ms7.0 earthquake, indicating that the surface rupture of the event E6 has extended to at least the Zhonggu village. [ABSTRACT FROM AUTHOR]

As revealed by field investigations, the co-seismic surface rupture zone of the 2010 MS7.1 Yushu earthquake, Qinghai is a characteristic sinistral strike-slip feature consisting of three distinct sinistral primary ruptures, with an overall strike of 310°--320° and a total length of 31 km. In addition, an approximately 2-km-long en-echelon tensile fissure zone was found east of Longbao Town; if this site is taken as the north end of the rupture zone, then the rupture had a total length of ~51 km. The surface rupture zone is composed of a series of fissures arranged in an en-echelon or alternating relationship between compressive bulges and tensile fissures, with a measured maximum horizontal displacement of 1.8 m. The surface rupture zone extends along the mapped Garzê-Yushu Fault, which implicates it as the seismogenic fault for this earthquake. Historically, a few earthquakes with a magnitude of about 7 have occurred along the fault, and additionally traces of paleoearthquakes are evident that characterize the short-period recurrence interval of large earthquakes here. Similar to the seismogenic process of the 2008 Wenchuan earthquake, the Yushu earthquake is also due to the stress accumulation and release on the block boundaries resulting from the eastward expansion of Qinghai-Tibet Plateau. However, in contrast with the Wenchuan earthquake, the Yushu earthquake had a sinistral strike-slip mechanism resulting from the uneven eastward extrusion of the Baryan Har and Sichuan-Yunnan fault blocks. [ABSTRACT FROM AUTHOR]

Abstract: In historical records, no earthquake of magnitude comparable with that of the Wenchuan Mw 7.9 earthquake has ever been reported in Chengdu and Longmenshan regions. It is important, therefore, to assess if there is any evidence of prior large earthquakes in the recent geological record. In order to explore this problem, 3 trenches were excavated separately at 3 sites along the surface rupture zones in the middle segment of the Beichuan–Yingxiu Fault and Jiangyou–Guanxian Fault of the Longmenshan fault zone, and the offsets of geomorphic surfaces were measured in the trenches. The basic facts of recurrence of large earthquake on the seismogenic fault of the Wenchuan earthquake are demonstrated in this paper on the basis of the accumulated deformation amounts of old and young geomorphic surfaces as well as the paleoearthquake events revealed in the trenches. The document that at Leigu along the Beichuan–Yingxiu Fault, at Bailu along the Jiangyou–Guanxian Fault, and Xiaoyudong on a NW-trending fault, the height of fault scarp on the second terrace has a multiple relationship to that on the first terrace after the Wenchuan earthquake of 12, May. The elevation difference of the marker horizon (the yellow sandy clay bed) of the second terrace revealed in the trenches on both sides of the fault is nearly twice (or three times larger in Bailu) of that produced by the Wenchuan earthquake. These observations suggest the occurrences of paleoearthquake events with surface deformation comparable to that produced by the Wenchuan Mw 7.9 earthquake. According to the dating data and the fact that there is no such slip of comparable paleoearthquakes in the 2300-year-long historic literature in Longmenshan region, we suggest that the penultimate event occurred between 2300aBP and 3300aBP. [ABSTRACT FROM AUTHOR]

Fault bends can serve as fault segment boundaries and are used in seismic hazard assessment. Recent studies addressing whether rupture propagation would be arrested at such structural complexities have commonly focused on computational modeling. However, multi-cycle paleoseismic rupture observations through fault bends have seldom been reported. In this study, we used trenching and radiocarbon dating to reveal paleoseismic rupture histories on the southern segment of the Anninghe fault (ANHF) along the southeastern margin of the Tibetan Plateau to explore multi-cycle surface rupture behavior at an extensional fault bend (with an angle of about 30°) at Xichang between the ANHF and Zemuhe fault (ZMHF). Specifically, nine trenches were opened in a fault depression at Maoheshan site and five paleoseismic events were identified. These have been named E1 through E5 respectively corresponding to events at 1400–935 BCE, 420–875 CE, 830–1360 CE, 1295–1715 CE, and 1790 CE–Present. After comparison with the historical records of earthquakes around Xichang and previous paleoseismic results, we suggest that the five seismic events are constrained at: 1365 BCE–935 BCE, 814 CE, 950 CE- 1145 CE, 1536 CE and 1850 CE, respectively. The average recurrence interval of earthquakes along the southern segment of the ANHF is about 700–800 yr. Furthermore, the evidence indicates that surface-faulting events along the southern segment of the ANHF appear to be unevenly spaced in time. Moreover, based on comparisons of seismic events along the ANHF and ZMHF, we find that two fault segments simultaneous ruptured during the 814 CE and 1850 CE earthquakes, event E3 and the 1536 CE earthquake ruptured the ANHF but not rupture the ZMHF. We suggest that the Xichang fault bend is not a persistent fault boundary, indicating that extensional fault bends with an angle of about 30° may not effectively terminate seismic ruptures on strike-slip faults. [ABSTRACT FROM AUTHOR]

Abstract: The 2008 Wenchuan earthquake (M W 7.9) ruptured two large thrust faults within the Longmenshan fault zone (along the eastern margin of the Tibetan Plateau): the Beichuan-Yingxiu Fault and the Guanxian-Jiangyou Fault. Previously, coseismic displacements, including vertical and horizontal slip along the surface rupture, have been investigated in detail, along with far-field horizontal shortening, estimated using GPS measurements. However, the distribution of near-field horizontal shortening across the surface rupture is relatively ambiguous because there are few markers in the earthquake zone and very few appropriate landforms for surveying. Palinspastic restoration of paleoseismic trench logs, as an alternative approach, may compensate for these deficiencies and reflect true deformation types and shortening across the surface rupture. We present several methods for estimating shortening based on the genesis of earthquake scarps. The results demonstrate that the shortening ranges from 2.5m to 0.6m for localities along the Guanxian-Jiangyou Fault with the highest values at the southwestern end. Along the Beichuan-Yingxiu Fault, near-field shortening ranges from 1.3m to 3.4m with two maxima toward the NE and SW ends of the fault, near the towns of Shenxigou and Leigu — these locations correlate with the two most heavily damaged regions. Horizontal shortening on the Guanxian-Jiangyou Fault is generally less than that across the Beichuan-Yingxiu Fault. Based on the distribution of near-field horizontal shortening from trenching across surface rupturing, the maximum near-field horizontal shortening across the surface rupture for the Wenchuan earthquake is ∼5.3m. [ABSTRACT FROM AUTHOR]