Foreshocks and Nucleation Process of the 2018 Ms 4.8 and 5.2 Chengduo Doublet, China

Abstract

Foreshocks may offer critical insights into the nucleation process of an upcoming mainshock, yet their underlying physical mechanisms remain a topic of ongoing debate. On 5 and 6 May 2018, two significant earthquakes with magnitudes of Ms 4.8 and 5.2, respectively, struck Chengduo, China. Notably, a sequence of foreshocks began three days prior to the Ms 4.8 earthquake. In this study, we conduct a comprehensive analysis of the earthquake sequence, including earthquakes detection and relocation, focal mechanisms inversion of two mainshocks, rupture dimension estimation and stress perturbation analysis of the large foreshocks, and repeating foreshocks identification. A total of 638 earthquakes were detected, which is about 3.4 times that of the catalog. The spatial-temporal evolution of earthquakes shows the foreshocks and the Ms 4.8 mainshock occurred on the same unmapped northeast-east–southwest-west-trending, near-vertical fault. The foreshock activity process can be divided into two stages. The rupture patches of foreshocks within the same stage exhibit significant overlap, and two groups repeating earthquakes identified in foreshock sequence, indicate aseismic slip occurred during the period of foreshock activity. The epicenters of two stages are concentrated in two adjacent regions, and some large foreshocks occurred in the stress increased region, suggest that stress transfer may also play an important role in the triggering process. Our results support a combination effort of aseismic preslip and cascade stress triggering in the nucleation process of the 2018 Chengduo doublet.