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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]

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]

The thermal diffusivity is the key parameter that controls near‐surface temperature where periodic temperature variation is progressively attenuated and delayed with depth. This article presents the results of apparent thermal diffusivity using temperatures recorded by a bedrock temperature measurement network in the fault zones of western Sichuan. High sensitivity temperature sensors (10−4 K) were installed at a maximum depth reaching 30 m. The apparent thermal diffusivities were deduced from both amplitude damping and phase shifting of annual temperature variations between two different depths. Under pure conduction, the thermal diffusivity determined through the phase method (αΦ) should be equivalent to that determined through the amplitude method (αA), whereas effects of the upward (downward) water flow are evidently reflected in the amplitude decay to make αΦ larger (lesser) than αA. The discrepancy between αΦ and αA can thus be a tracer of water movement or convective heat transfer. The calculated αΦ of the measurement stations varies from 1.22 × 10−6 to 3.00 × 10−6 m2/s, and the estimated αA ranges from 0.93 × 10−6 to 2.41 × 10−6 m2/s. Two regimes of heat transfer underground were suggested from the results. Conductive heat transport prevails over the nonconductive processes at five stations, which is characterized by αΦ coincident with αA for the same depth pair. On the contrary, the values of αΦ differ from αA at six stations in the intersection area of the Y‐shaped fault system, implying that convective heat transfer also plays a comparably important role. This finding is consistent with the hot springs distribution of the area. The results also indicate that water moves upward with an average Darcy velocity of approximately −1 × 10−7 m/s in this region. Our research provides new evidence for the hydrothermal activity in the fault zones at the eastern margin of the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Following the Lushan M7.0 earthquake on 20 April 2013, a topic of much concern is whether events of M7 or greater could occur again on the southern segment of the Longmenshan fault zone. In providing evidence to answer this question, this work analyzes the tectonic relationship between the Lushan event and the 2008 Wenchuan earthquake and the rupture history of the southern segment of the Longmenshan fault zone, through field investigations of active tectonics and paleoearthquake research, and our preliminary conclusions are as follows. The activity of the southern segment of the Longmenshan fault zone is much different to that of its central section, and the late Quaternary activity has propagated forward to the basin in the east. The seismogenic structure of the 2008 Wenchuan earthquake is the central-fore-range fault system, whereas that of the 2013 Lushan event is attributed to the fore-range-range-front fault system, rather than the central fault. The southern segment of the Longmenshan fault zone becomes wider towards the south with an increasing number of secondary faults, of which the individual faults exhibit much weaker surface activity. Therefore, this section is not as capable of generating a major earthquake as is the central segment. It is most likely that the 2013 earthquake fills the seismic gap around Lushan on the southern segment of the Longmenshan fault zone. [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]

• We estimate fluid fluxes using 5-year multi-depth temperature data at 6 locations. • Flux changes after two earthquakes in near and intermediate fields are identified. • Fluid flow changes mimic distribution of coseismic volume-strain in the near-field. Heat as a groundwater tracer has potential application in identifying earthquake-induced changes in fluid flow. Since convective heat transport modifies the phase and amplitude of periodic oscillations under pure heat conduction condition, temporal variation of groundwater flux can thus be derived from multi-depth periodic temperature signals. Here we present groundwater flux changes after two large earthquakes nearby, determined from in situ bedrock temperature measurements in the Xianshuihe fault zone, at the eastern margin of the Tibetan Plateau. Five-year temperature time series at six different locations are recorded by high sensitivity temperature sensors (10−4 K) installed at various depths ranging from 0 to 20 m. Groundwater fluxes are estimated through time-varying amplitudes and phases of the annual oscillations, extracted from the Dynamic Harmonic Regression technique. Results show different hydrological responses to these two earthquakes. After the 2013 M s 7.0 Lushan earthquake, sustained rise in upward flux was revealed at all locations in the intermediate-field, contradicting to the modeled coseismic static dilatational strains. However, groundwater flux patterns after the 2014 M s 6.3 Kangding earthquake are consistent with the coseismic volumetric strain distribution of a quadrantal pattern of this event. We propose that enhanced hydraulic permeability by dynamic stress might cause the increased upward fluid flux after the Lushan earthquake in the intermediate-field, while coseismic static volumetric strain may account for the hydrological changes after the Kangding earthquake in the near-field. This is the first time that temperature–time series are used to quantify continuous earthquake-related changes in groundwater fluxes. In situ temperature monitoring can provide insights into earthquake-driven hydrologic responses, and also offer additional information to identify temporal changes in crustal strain or hydraulic properties. [ABSTRACT FROM AUTHOR]

Permeability, which is an important property that controls the transportation processes in the shallow crust, is known to change during large earthquakes. Several studies have independently documented changes in permeability or groundwater motion through water level, streamflow, water temperature, and geochemical observations. In this study, we first compared water level data from three wells located in the Longmenshan Fault zone, where the 2008 M w 7.9 Wenchuan earthquake occurred. We then analyzed the changes in aquifer permeability associated with the 2021 M w 7.4 Maduo earthquake, which occurred 640 km away from the study area, using water-level tidal response and multi-depth temperature observations from one of the three wells. Tidal response analysis of water-level data indicated a great increase in the phase shift of the M 2 wave from negative to positive, reflecting an enhancement in vertical permeability of the aquifer due to the Maduo earthquake. Multi-depth temperature observations indicated that the co-seismic water temperature increased successively from deeper to shallower depths, indicating the upward migration of the water (at a transient rate of 1.5–2.3 × 10−2 m/s) in observation section. The results indicated that our method is effective for studying changes in the permeability of groundwater motion in an aquifer related to an earthquake. • We combined water-level tidal response and multi-depth temperature observations. • The studied well was located on Longmenshan Fault zone affected by Maduo Earthquake. • Water-level tidal analysis indicated enhanced vertical permeability. • Multi-depth temperature observations indicated upward migration of the water. • Our method identifies permeability changes/groundwater motion during earthquakes. [ABSTRACT FROM AUTHOR]