[S-SS07] 地震発生の物理・断層のレオロジー

[SSS07-P19] Investigation of slip diversity containing slip direction using slip inversion even with empirical Green’s functions

*Ritsuya Shibata¹、Naofumi Aso¹ (1.Department of Earth and Planetary Sciences, School of Science, Tokyo Institute of Technology)

キーワード：empirical Green's function, waveform inversion, source process

Estimating spatio-temporal slip distributions during earthquakes is useful for understanding those dynamic friction parameters or background stress fields. To achieve this, many studies using the slip inversion such as the multi-time-window method have been implemented (Olson and Apsel, 1982; Hartzell and Heaton, 1983), which reproduces observed waveforms by convolving model parameters of source time functions and Green’s functions at each subfault. In the theoretical methods, although we can give two or five degrees of freedom to a focal mechanism using theoretical waveforms (e.g. Ide and Takeo, 1997; Shimizu et al., 2020), a detailed velocity structure such as a 3-D structure is required (e.g. Bouchon, 1981; Takeo, 1985). In contrast, empirical Green’s function (EGF), which is obtained by utilizing seismograms of nearby small earthquakes with similar focal mechanisms, requires the occurrence of such appropriate events (e.g. Hartzell, 1978). To reduce these strong constraints, we propose a new slip inversion method using the radiation-corrected EGF (Shibata et al., 2022), which assumes two degrees of freedom on fault slips by rotating radiation patterns of EGF. Specifically, based on the ray theory, we can calculate the theoretical radiation patterns of the target event and the EGF event assuming a simple structure, and we correct the amplitude of the EGF seismogram for each of the P and S waves using the radiation patterns in each component at each station. Following this concept, we applied this new method to real earthquakes and discussed its applicability by comparing it with the conventional method.
As a practice, we investigated the spatio-temporal slip distribution, moment rate function, and reproductivity of the synthetic waveforms of the 2014 M_w 6.3 Iyonada earthquake at a depth of 78 km. We used the underground KiK-net stations of the National Research Institute for Earth Science and Disaster Resilience (NIED) within the epicentral distance of 100 km from the target event. Considering the centroid moment tensor of the target event (strike/dip/rake = 245º/29º/157º) estimated by the F-net (NIED), we allow slip direction (rake) between 112º and 202º, which is within 45º from the centroid rake, by correcting the EGF focal mechanism. To calculate the radiation pattern, we estimated the take-off and incident angles by ray tracing in a horizontally-layered structure of the JMA2001 (Ueno et al., 2002) using the TauP package (Crotwell et al., 1999). As a result of the real data application, we obtained better moment estimations with better waveform fittings by the radiation-corrected EGF compared to the conventional method. In addition to the waveform reproductivity, we could estimate the plausible various slip directions at each subfault even with the EGF method.