Abstract
The southeast offshore Mie earthquake occurred on April 1, 2016 near the rupture area of the 1944 Tonankai earthquake, where seismicity around the interface of the Philippine Sea plate had been very low until this earthquake. Since this earthquake occurred outside of seismic arrays, the focal mechanism and depth were not precisely constrained using a one-dimensional velocity model, as in a conventional approach. We conducted a moment tensor inversion of this earthquake by using a three-dimensional velocity structure model. Before the analysis of observed data, we investigated the effects of offshore heterogeneous structures such as the seawater, accretionary prism, and subducting oceanic plate by using synthetic seismograms in a full three-dimensional model and simpler models. The accretionary prism and subducting oceanic plate play important roles in the moment tensor inversion for offshore earthquakes in the subduction zone. Particularly, the accretionary prism, which controls the excitation and propagation of long-period surface waves around the offshore region, provides better estimations of the centroid depths and focal mechanisms of earthquakes around the Nankai subduction zone. The result of moment tensor inversion for the 2016 southeast offshore Mie earthquake revealed low-angle thrust faulting with a moment magnitude of 5.6. According to geophysical surveys in the Nankai Trough, our results suggest that the rupture of this earthquake occurred on the interface of the Philippine Sea plate, rather than on a mega-splay fault. Detailed comparisons of first-motion polarizations provided additional constraints of the rupture that occurred on the interface of the Philippine Sea plate.
Highlights
Along the Nankai Trough, Japan (Fig. 1), the Philippine Sea plate (PHS) is subducting beneath southwestern Japan at a rate of 2–6 cm per year (e.g., Heki and Miyazaki 2001; Seno et al 1993)
We conducted a 3D moment tensor (MT) inversion of the 2016 SE offshore Mie earthquake, which occurred near the source region of the 1944 Tonankai earthquake (M7.9), by using long-period (30–100 s) displacement seismograms of regional seismic networks
By using synthetic seismograms in a full 3D model, we demonstrated the effects of offshore heterogeneities, such as the subducting oceanic plate and low-velocity layers, on the MT inversion of offshore earthquakes in subduction zones
Summary
Along the Nankai Trough, Japan (Fig. 1), the Philippine Sea plate (PHS) is subducting beneath southwestern Japan at a rate of 2–6 cm per year (e.g., Heki and Miyazaki 2001; Seno et al 1993). Before MT inversion with observed data, by using synthetic long-period seismograms of an assumed low-angle thrust earthquake at a depth near the PHS interface, we investigate the effects of 3D heterogeneous structures on the MT inversion of shallow offshore earthquake We demonstrate that both the accretionary prism and subducting PHS provide better estimations of centroid depths and focal mechanisms of offshore earthquakes in the subduction zone. Displacement spectra for the 2016 SE offshore Mie earthquake (upper panel) show a clear spectral peak around the periods of 10–20 s, which correspond to the dominant period range of ground motions propagating through the accretionary prism in the Nankai subduction zone (e.g., Hayakawa et al 2005; Furumura et al 2008; Nakamura et al 2015) Such differences between crustal and offshore earthquakes were found in displacement waveforms at N.ABUF.
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