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Seismological Characterization of the 2021 Yangbi Foreshock-Mainshock Sequence, Yunnan, China: More than a Triggered Cascade
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  • Yijian Zhou,
  • Chunmei Ren,
  • Abhijit Ghosh,
  • Haoran Meng,
  • Lihua Fang,
  • Han Yue,
  • Shiyong Zhou,
  • Youjin Su
Yijian Zhou
Department of Earth and Planetary Sciences, University of California Riverside

Corresponding Author:yijian.zhou@email.ucr.edu

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Chunmei Ren
Institute of Theoretical and Applied Geophysics
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Abhijit Ghosh
University of California, Riverside
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Haoran Meng
Southern University of Science and Technology
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Lihua Fang
Institute of Geophysics, China Earthquake Administration
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Han Yue
Peking University
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Shiyong Zhou
Peking University
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Youjin Su
Yunnan Earthquake Agency
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The 2021 Mw 6.1 Yangbi earthquake in southwest China is preceded by three major foreshocks: 05/18 Mw 4.3, 05/19 Mw 4.6, and 05/21 Mw 5.2. It provides a valuable chance to revisit two end-member models describing earthquake interaction: cascade-up and pre-slip model. We first determine the associated fault structure with relocated aftershocks and focal mechanisms obtained from multi-point-source inversion. We find that the mainshock and two smaller foreshocks occur on an unmapped near-vertical fault, and the largest foreshock occurs on a mapped stepover fault that dips to NE. Secondly, for each major foreshock, we estimate and delineate their rupture area based on aftershocks and spectral ratio analysis. Based on the rupture model, we finally calculate the evolution of Coulomb stress, with which to interpret the causality of each major event. Results show that the Yangbi sequence can be explained by the cascade triggering mechanism, while we also find evidence for aseismic slip that contributes to the triggering process: the first foreshock is preceded by a short-term localized cluster, and the aftershock zone of the second foreshock extents through time. The nucleation of mainshock is probably contributed by multiple major foreshocks through both seismic and aseismic processes. This detailed seismological characterization of Yangbi sequence lend supports for a deeper understanding on the foreshock mechanism: (1) the controlling mechanisms are not limited to cascade-up & pre-slip, multiple mechanisms can operate together; and (2) aseismic slip does not always provide more predictability on the mainshock.