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

Abstract

SUMMARY 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.

Long-period ground motion simulation using centroid moment tensor inversion solutions based on the regional three-dimensional model in the Kanto region, Japan

“…Although the optimal depth was very close to the upper surface of the Paci c Plate, high (> 80 %) VR solutions appeared at a wider depth range (36-56 km). These features are similar to CMT solutions in the Hyuga-nada region, southwest Japan (Figure 4 of Takemura et al 2020). The centroid depth constraint is also not high for shallow crustal earthquake ( Figure S2).…”

“…Additionally, the VR of the 3D CMT solution was not high (~57 %). With the exception of this event, the differences in focal mechanisms and centroid depths were not signi cant compared to the results for offshore earthquakes along the Nankai Trough (Figure 8 of Takemura et al 2020).…”

“…Other technical details of CMT inversions using a 3D model and the evaluation of Green's functions are described in Takemura et al (2020).…”

“…The Mw values are very important for ground motion simulations because seismic moments are directly related to the amplitude of the ground motion. We also evaluated the differences in Mw between 3D CMT and F-net MT solutions along the Nankai Trough using the results from (Takemura et al 2020; https://doi.org/10.5281/zenodo.3674161). We found both larger and smaller Mw values, compared to the F-net catalog, in the Nankai region ( Figure 5).…”

“…However, in regions with strong heterogeneities, such as thick sediments and subducting oceanic plates, focal mechanisms could be incorrectly estimated using conventional 1D CMT methods. To address this issue, the CMT inversion based on Green's functions using the local/regional 3D model has been developed in such regions (e.g., Lee et al 2013;Hejrani et al 2017;Okamoto et al 2018;Takemura et al 2018bTakemura et al , a, 2019bTakemura et al , 2020Wang and Zhan 2020;Hejrani and Tkalčić 2020). By using the 3D CMT results of moderate earthquakes along the Nankai Trough, Takemura et al (2020) demonstrated that the differences in centroid depths and focal mechanisms between 1D and their 3D…”