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

[SCG45-03] Do small earthquakes exhibit predominantly pulse-like ruptures?

*金子 善宏¹、Kano Yuto¹、Shearer Peter² (1.京都大学 理学研究科、2.University of California, San Diego)

キーワード：Earthquake rupture dynamics、Pulse-like rupture、Stress drop、Double corner frequency spectra

We investigate the spectral characteristics and source properties of earthquake rupture models characterized by self-similar, self-healing slip pulses. We compare pulse-like and crack-like rupture models across various idealized and heterogeneous source scenarios to identify conditions under which far-field displacement spectra characterized by two corner frequencies (referred to as double-corner frequency spectra) emerge. Five primary mechanisms are identified as the causes of second high-frequency corners: (i) slip-pulse width, (ii) rupture directivity, (iii) gradual rupture arrest, (iv) characteristic length of slip heterogeneity, and (v) Mach waves from supershear rupture. Spectra associated with slip-pulse width are most evident at small take-off angles, while those linked to rupture directivity and the characteristic length of slip heterogeneity appear at large take-off angles. Estimated stress drops depend strongly on rupture mode and speed. Pulse-like models exhibit greater variability in estimated stress drops due to unknown rupture speeds than crack models do, with pulse-like ruptures systematically underestimating moment-based stress drops by up to 40%. Observational data indicate a relationship between normalized corner frequency and scaled energy that fits better with pulse-like rupture models than with crack models. These findings imply that small- to moderate-sized earthquakes predominantly exhibit pulse-like rupture behavior, similar to larger earthquakes. While the gradual rupture arrest mechanism can produce double-corner spectra, they are unlikely to represent realistic earthquake sources. Our results highlight the importance of incorporating pulse-like rupture dynamics into earthquake source models and point out challenges in interpreting seismic source spectra and stress drop estimates, particularly when rupture characteristics are uncertain.