Seismological Characterization of the 2021 Yangbi Foreshock-Mainshock Sequence, Yunnan, China: More than a Triggered Cascade

The 2021 M-w 6.1 Yangbi earthquake in southwest China is preceded by three major foreshocks: 05/18 M-w 4.3, 05/19 M-w 4.6, and 05/21 M-w 5.2. It provides a valuable chance to revisit two end-member foreshock models: cascade-up and pre-slip models. We first determine the fault structure with relocated aftershocks and the focal mechanisms obtained from a multipoint-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 northeast. Second, for each major foreshock, we estimate and delineate the rupture area based on its aftershock location and a spectral ratio analysis. Based on the inferred rupture model, we finally calculate the evolution of Coulomb stress and examine potential interactions between 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, three repeater sequences are detected in the foreshock period, and the aftershock zones expand logarithmically with time. The Yangbi mainshock is probably triggered by multiple major foreshocks through both seismic and aseismic processes. This detailed seismological characterization of Yangbi sequence lends supports for an improved understanding on the foreshock mechanism: (a) the controlling mechanisms are not limited to cascade-up and pre-slip. Instead, multiple mechanisms can operate together; and (b) aseismic slip does not always provide more deterministic information on the mainshock.