Poster #226, Seismology

Foreshock Sequence of the 2021 Mw6.1 YangBi Earthquake Sequence, Yunnan, China: Not a Triggered Cascade

Yijian ZHOU, Abhijit Ghosh, Lihua FANG, Han YUE, & Shiyong ZHOU
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Poster Presentation

2021 SCEC Annual Meeting, Poster #226, SCEC Contribution #11508 VIEW PDF
The 2021 Mw6.1 YangBi earthquake occurs near the southwest boundary of Chuandian block, Yunnan, China. The mainshock is preceded by three major foreshocks: 05-18 Mw4.3 (f1), 05-19 Mw4.6 (f2), and 05-21 Mw5.2 (F). This foreshock sequence provides a valuable chance to deepen the insights towards two end-member models of earthquake interaction: cascade-up and slow-slip triggering. We first construct the fault model with a high-resolution matched filter seismic catalog. Results show that the foreshock f1 and f2 occur on the same fault segment with the mainshock (Fault_M), and the largest foreshock initiates on another subparallel sub-fault (Fault_F). We estimate the source parameters of the majo...r foreshocks with spectral ratio analysis. The stress drop of f1 and f2 is significantly lower than that of F, indicating a weaker fault strength of Fault_F. We determine a rectangular rupture surface based on the rupture length delineated by relocated aftershocks and the rupture area estimated from spectral ratio method. Based on the fault model, we calculate the Coulomb failure stress (CFS) change from each foreshock. We find that: (1) f1 triggers f2 with static stress change, (2) f2 does not cause large CFS change on the hypocenter of F and M, thus afterslip may be a possible triggering mechanism, (3) F can not trigger M by static stress change, and the interaction can only be realized through dynamic stress. Thus, the YangBi sequence is not a simple triggered cascade, but combines multiple mechanisms.