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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. • We obtained a high-resolution interseismic catalog (2016–2019) for the Xiaojiang Fault Zone. • Widespread microseismicity are detected on off-fault structures, but the major fault shows low seismicity. • The major Xiaojiang fault is strongly locked with low b-value, and could be at a late stage of interseismic loading. • The off-fault structures generate intense microseismicity, repeaters, and high b-values, indicating a weak crustal strength. [ABSTRACT FROM AUTHOR]

Seismic risk in China is high. In particular Sichuan province has been hit hard in recent years with the successive occurrence of the 2008 MW 7.9 Wenchuan, 2013 MW 6.6 Lushan, and 2017 MW 6.5 Jiuzhaigou earthquakes with more than 87,000 fatalities and economic losses of about 1.5 trillion US dollars. In the scope of the National System for Fast Report of Intensities and Earthquake Early Warning project, the China Earthquake Administration (CEA) is building a nationwide earthquake early warning (EEW) system with about 3,200 strong-motion, ~2,000 broadband and ~10,200 low-cost MEMS sensors. The MW 7.9 Wenchuan mainshock and other large earthquakes are challenging for EEW, because traditional seismic network stations tend to saturate and source dimensions need to be known to predict seismic ground-motion at high accuracy. In this study, we run playbacks of local and regional on-scale strong-motion waveforms recorded by the China Strong Motion Networks Center (CSMNC) during the MW 7.9 Wenchuan, MW 6.6 Lushan, and MW 6.5 Jiuzhaigou earthquakes to study the possible performance of the Finite-Fault Rupture Detector (FinDer) algorithm (Böse et al., 2018). We show that the FinDer line-source models agree very well with the later observed aftershock distributions and finite-fault models determined from waveform inversion. If the available set of strong motion data had been operated in real-time and available to such an EEW system, more than 70% of sites experiencing shaking intensities of MMI IV-VI could receive 5 or more seconds of warning, and more than 30% of sites experiencing shaking intensities of MMI VI-IX could receive 10 or more seconds of warning. FinDer finite-fault models could also help providing faster and more accurate loss estimates in the aftermath of devastating earthquakes. [ABSTRACT FROM AUTHOR]