S U M M A R YThe distortion properties of the apparent S-wave radiation pattern in the high-frequency seismic wavefield of over f > 2 Hz is investigated using a large number of waveform records of the main shock and 29 aftershocks of the Tottori-Ken Seibu, Japan, magnitude (M w ) 6.6 earthquake in 2000. The dense seismic records from the KiK-net strong motion network show a clear four-lobe pattern in the apparent S-wave radiation pattern in the low-frequency wavefield of f < 2 Hz, and shows an almost isotropic distribution in all directions as the frequency increases above 5 Hz. The distortion of the apparent S-wave radiation pattern in the high-frequency wavefield increases as travel distance increases. Therefore, the path effect caused by the scattering of seismic waves due to small-scale heterogeneities in the crust is a major cause of distortion of the radiation pattern. This hypothesis is examined by a 2-D finite-difference method simulation of seismic waves in heterogeneous structure models. The results of simulations clearly demonstrate the collapse of the S-wave front due to seismic wave scattering in the heterogeneous structure. By comparing the observed wavefield and the results of simulations using different sorts of stochastic heterogeneous models, the most preferable model that can explain the observation is described by a von Karman autocorrelation function with correlation distance of a = 3-5 km, order of κ = 0.5 and rms value of ε = 0.07. However, our simple stochastic random heterogeneity model proposed, herein, somewhat overestimates the scattering of low-frequency signals below 2 Hz.
We have developed an open-source software package, Open-source Seismic Wave Propagation Code (OpenSWPC), for parallel numerical simulations of seismic wave propagation in 3D and 2D (P-SV and SH) viscoelastic media based on the finite difference method in local-to-regional scales. This code is equipped with a frequency-independent attenuation model based on the generalized Zener body and an efficient perfectly matched layer for absorbing boundary condition. A hybrid-style programming using OpenMP and the Message Passing Interface (MPI) is adopted for efficient parallel computation. OpenSWPC has wide applicability for seismological studies and great portability to allowing excellent performance from PC clusters to supercomputers. Without modifying the code, users can conduct seismic wave propagation simulations using their own velocity structure models and the necessary source representations by specifying them in an input parameter file. The code has various modes for different types of velocity structure model input and different source representations such as single force, moment tensor and plane-wave incidence, which can easily be selected via the input parameters. Widely used binary data formats, the Network Common Data Form (NetCDF) and the Seismic Analysis Code (SAC) are adopted for the input of the heterogeneous structure model and the outputs of the simulation results, so users can easily handle the input/output datasets. All codes are written in Fortran 2003 and are available with detailed documents in a public repository.
The spatial distributions of shallow slow earthquakes are related to stress accumulation and structural characteristics of the shallow plate boundary, which are important for understanding megathrust earthquakes. To investigate spatiotemporal variation of shallow very low frequency earthquake (SVLFE) activity along the Nankai Trough, we conducted centroid moment tensor inversion method incorporating effects of offshore heterogeneous structures. By applying centroid moment tensor method into long‐term onshore seismograms, we obtained spatiotemporal variation of SVLFE activity occurred from June 2003 to May 2018. We find that SVLFE activities are related to the spatial variations of the slip‐deficit rate and pore fluid around the Philippine Sea plate boundary. SVLFEs are effectively activated by mechanical weakening due to rich pore fluid in areas surrounding strongly locked zones. Our results imply that spatial variations of long‐term SVLFE activity provide information on tectonic environments, which could constrain the rupture behavior of shallow plate boundaries during future megathrust earthquakes.
Cross‐correlation analysis was applied to long‐term onshore broadband records from April 2004 to March 2021 to detect and relocate shallow very low frequency earthquakes (VLFEs) southeast off the Kii Peninsula, along the Nankai Trough, Japan. We then determined the moment rate functions of detected shallow VLFEs using the Monte Carlo‐based simulated annealing method. According to this new comprehensive catalog, shallow VLFEs are widespread beneath the accretionary prism toe, but shallow VLFEs with large cumulative moments are localized around the western edge of the paleo‐Zenisu Ridge, which is subducted beneath southeast off the Kii Peninsula. Our results from the long‐term shallow VLFE catalog are well consistent with previous studies in this region, suggesting that heterogeneous structures and stress conditions due to the subducted paleo‐Zenisu Ridge promote the occurrence of shallow slow earthquakes. The relocated shallow VLFE epicenters illustrated three major episodes characterized by a similar activity area and five minor episodes characterized by different areas. The three major episodes exhibited slow frontal migration with different initiation locations, directions, and speeds, as well as several rapid reverse migrations. Episodes of minor activity were distributed in different locations within part of the area of major activity. Different patterns of shallow VLFE migration could reflect temporal changes in the pore‐fluid distribution or stress conditions of the plate boundary.
The spatiotemporal characteristics of shallow slow earthquake activity are linked to the tectonic environments of shallow plate boundaries. In this work, the spatiotemporal variations of shallow very low frequency earthquake (SVLFE) activity along the Nankai Trough were investigated using a cross‐correlation analysis. The SVLFEs migrated or spread eastward along the strike direction of the trench during large SVLFE episodes. Migrations and clusters of SVLFEs suggest the occurrence of shallow slow slip events. The observed lateral variations in SVLFE activity patterns reflect the heterogeneous distributions of effective strengths at the shallow plate boundary along the Nankai Trough. Migrations and clusters of SVLFEs tended to be concentrated in the regions surrounding the stress accumulation peaks on the Philippine Sea Plate boundary. The stress accumulated in the transitional regions between high‐strength and low‐strength zones can be released by shallow slow earthquakes.
Due to complex three-dimensional (3D) heterogeneous structures, conventional one-dimensional (1D) 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 3D 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 3D velocity structure model. Significant differences of focal mechanisms and centroid depths between previous 1D and our 3D catalogues were found in the solutions of offshore earthquakes. By introducing the 3D 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 3D CMT could provide better constraints of dip angles. Our 3D 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.
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