Centroid moment tensor inversions of offshore earthquakes using a three-dimensional velocity structure model: slip distributions on the plate boundary along the Nankai Trough

S. Takemura1, R. Okuwaki2, T. Kubota3, K. Shiomi3, T. Kimura3 and A. Noda3 (2020). Geophysical Journal International, 222 (2), 1109-1121, doi:10.1093/gji/ggaa238

1ERI Univ. Tokyo, 2Univ. Tsukuba, 3NIED

Due to complex 3-D heterogeneous structures, conventional 1-D analysis techniques using onshore seismograms can yield incorrect estimation of earthquake source parameters, especially dip angles and centroid depths of offshore earthquakes. Combining long-term onshore seismic observations and numerical simulations of seismic wave propagation in a 3-D model, we conducted centroid moment tensor (CMT) inversions of earthquakes along the Nankai Trough between April 2004 and August 2019 to evaluate decade-scale seismicity. Green’s functions for CMT inversions of earthquakes with moment magnitudes of 4.3–6.5 were evaluated using finite-difference method simulations of seismic wave propagation in the regional 3-D velocity structure model. Significant differences of focal mechanisms and centroid depths between previous 1-D and our 3-D catalogues were found in the solutions of offshore earthquakes. By introducing the 3-D structures of the low-velocity accretionary prism and the Philippine Sea Plate, dip angles and centroid depths for offshore earthquakes were well-constrained. Teleseismic CMT also provides robust solutions, but our regional 3-D CMT could provide better constraints of dip angles. Our 3-D CMT catalogue and published slow earthquake catalogues depicted spatial distributions of slip behaviours on the plate boundary along the Nankai Trough. The regular and slow interplate earthquakes were separately distributed, with these distributions reflecting the heterogeneous distribution of effective strengths along the Nankai Trough plate boundary. By comparing the spatial distribution of seismic slip on the plate boundary with the slip-deficit rate distribution, regions with strong coupling were clearly identified.

The CMT catalog in this study is available from https://doi.org/10.5281/zenodo.3674161.