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

[SCG40-P18] Towards adjoint tomography of the Nankai and Kyushu subduction zones

*Samriddhi Prakash Mishra¹, Yoshihiro Kaneko¹, Shun Adachi¹, Bryant Chow², Yusuke Yamashita³, Masanao Shinohara⁴ (1.Graduate School of Science, Kyoto University, Kyoto, Japan, 2.University of Alaska – Fairbanks, Alaska, USA, 3.Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan, 4.Earthquake Research Institute, University of Tokyo, Tokyo Japan)

Keywords:seismic tomography, full waveform inversion, adjoint methods, subduction zone

The Nankai-Kyushu subduction system is a complex seismotectonic region marked by an abrupt transition in the inter-plate coupling, with a fully coupled megathrust to the northeast. In this study, we utilize an earthquake-based, full-waveform inversion technique, termed adjoint tomography, to develop accurate and high-resolution shear-wave velocity models of the Nankai-Kyushu subduction zone. We aim to reveal the crustal structure responsible for the origin of the inter-plate coupling transition. Our target region includes ~100 regional earthquakes (4.5w<6) with ~ 400 permanent receivers, including ~ 20 temporary Ocean Bottom Seismometers (OBS) stationed in the Hyuga-nada region. We first assess the accuracy of two candidate initial velocity models (Koketsu et al., 2012; Bassett et al., 2022). For the Bassett et al. (2022) model, we use empirical relations to convert P-wave velocities to obtain the corresponding S-wave velocity model. Using a spectral element solver, we simulate the earthquakes and compare the resulting synthetics with recorded waveforms at 6 – 40 s period ranges. We quantify travel-time residuals which show that these velocity models appear to be, on average, faster than the actual velocity structure. Our results also show that seismic waves systematically amplify due to the presence of accretionary sediments. The reasonable misfit of the candidate velocity models provides confidence in moving toward a computationally intensive tomographic inversion of the region. We will present our ongoing efforts towards imaging and understanding the crustal structure of the Nankai and Kyushu subduction zones and its link to the spatial variation in the megathrust locking behaviour.