[S-CG45] Science of slow-to-fast earthquakes

[SCG45-P43] Hypocenter depth variation on a vertical strike-slip fault in a damage zone governed by rate and state friction

*Deng shuai¹、Chao Liang¹、Jean Paul Ampuero²、Kaoshan Dai¹ (1.Institute for Disaster Management and Reconstruction ,Sichuan University、2. Université Côted'Azur,CNRS ,Observatoire de la Côte d'Azur,IRD,Géoazur,Seismology)

キーワード：hypocenter depth variation, strike-slip fault, damage zone

The depth distribution of earthquake hypocenters is nonuniform and reflects the complex interaction of the fault’s stress state, friction parameters, geometry, tectonic loading history, and depth-dependent bulk rock rheology. In a simple earthquake cycle model with a single asperity, nucleation is mainly clustered at the transition zone between the velocity-strengthening (stable sliding) and velocity-weakening (stick-slip) regions, where the stress concentration is the highest. Thakur et al. (2020) found that a near-fault rock damage zone (modelled as a low rigidity layer) can cause variation of earthquake nucleation site and even lead to a diffused bimodal distribution of hypocenter depth. However, this work was limited to a narrow parameter range, with a/b of about 0.79 and a ratio of fault length to nucleation size (Rice and Ruina,1983), Ru of about 25, and it is unclear whether this phenomenon persists for a wider range of friction parameters. Using a fully dynamic earthquake cycle model, we systematically explore the hypocenter depth distribution over a wider range of parameters a/b (from 0.2 to 0.8) and Ru (from 10 to 100). When a/b is less than 0.5, earthquakes always nucleate near the frictional transition zone and the hypocenter depth does not vary significantly within a same sequence. When a/b is greater than 0.5 and Ru less than 20, episodic aseismic slip (failed nucleation) is the dominant factor causing variation in the hypocenter depth, which is enhanced by the near-fault damage zone compared to the model with a homogeneous undamaged solid. When Ru exceeds 20, partial rupture occurs frequently, and some earthquakes occasionally nucleate at the stress concentration left by previous partial ruptures, resulting in hypocenters closer to the middle of the seismogenic zone. On the other hand, earthquakes with hypocenter depths shallower than 5 km and the resulting bimodal distribution observed in Thakur et al. (2020) are caused by loading from the shallow velocity-strengthening zone, and ceases to exist after removing the shallow velocity-strengthening zone. Our work improves the understanding of the factors controlling the variation in earthquake nucleation and the resulting depth distribution of earthquake hypocenters.