[S-CG40] Science of slow-to-fast earthquakes

[SCG40-18] Insights into Back-Propagating Rupture from Numerical Simulations, Laboratory Experiments, and Natural Observations

*Xiaotian Ding¹, Shiqing Xu¹, Eiichi Fukuyama², Futoshi Yamashita³ (1.Department of Earth and Space Sciences, Southern University of Science and Technology, Shenzhen, China., 2.Department of Civil and Earth Resources Engineering, Kyoto University, Kyoto, Japan., 3.National Research Institute for Earth Science and Disaster Resilience, Tsukuba, Japan.)

In recent years, an intriguing feature of back-propagating rupture (BPR) has been reported during both slow and fast earthquakes, where earthquake rupture can reverse its propagation direction with respect to the original one (Ide et al., 2011; Houston et al., 2011; Hicks et al., 2020; Okuwaki et al., 2023; Vallée et al., 2023). The existence of BPR challenges the traditional view of rupture propagation as a consistent “forward” problem, while remaining less understood by the earthquake science community. In this study, we combine numerical simulations, laboratory experiments, and observations of one recent natural earthquake to elucidate the nature and excitation mechanisms of BPR. First, we use fracture mechanics and thought experiments to argue that BPR is an intrinsic feature of dynamic ruptures; however, its observability is usually masked by the superposition effect of interfering waves behind the primary rupture front. Based on this argument, we propose that perturbation to an otherwise smooth rupture process, such as lateral variation in elastic moduli, stress conditions, fault frictional or geometric properties, can highlight the individual phase of BPR and hence can make it observable. Then, we conduct numerical simulations of dynamic ruptures associated with a variety of perturbations and confirm our above idea. We further cross-validate the above idea by meter-scale rock friction experiments, where sudden breakage of an asperity, rupture reflection from a free surface, and coalescence of two rupture fronts are found to be capable of producing a clear phase of BPR. Finally, we analyze the strong motion data for the 2023 Mw 7.8 Kahramanmaraş (Türkiye) earthquake and identify a possible phase of BPR along the Amanos segment, due to rupture propagation encountering a restraining bend. Our study provides important insights into BPR, which can help improve the understanding of earthquake physics and the assessment of seismic hazards.