Abstract
We derived a three-dimensional S-wave velocity model for the San-in area of southwest Japan to examine heterogeneous structures such as tectonic faults. Many earthquakes occur in this area, but much of the activity has been relatively recent, so the fault distribution has yet to be fully clarified. Here, we used continuous ambient noise data from a dense seismic network, deployed from November 2009 to extract Rayleigh and Love wave dispersion data between station pairs, and then applied a direct surface wave inversion to the phase velocities of each station pair to determine a three-dimensional S-wave velocity model. In the resulting model, faults and a previously unrecognized tectonic boundary appeared as low-velocity anomalies or velocity boundaries, and the velocity anomalies were also associated with many past earthquake hypocenters. These results contribute to our understanding of heterogeneous structures caused by recent tectonic motion and of possible future tectonic activity, such as intraplate earthquakes. Surface wave tomography using ambient noise recorded in dense seismic networks could also be applied in other parts of the world to reveal new heterogeneous geological structures (i.e., unrevealed tectonic faults) and could contribute to disaster mitigation.
Highlights
In a convergent plate margin, subsurface structures are complex, and relatively new faults may become activated
Where tiobs f is the observed surface wave travel time, ti f is the travel time calculated from a reference model that can be updated in the inversion, K is the number of grid points in the horizontal direction, νik is bilinear interpolation coefficients along the ray path associated with the i th travel-time data, Ck f is the phase velocity, where θk represents the 1D reference model at the k th surface grid point, and αk zj, βk zj, and ρk zj are the P-wave velocity, S-wave velocity, and mass density, respectively
The low-velocity zone observed in the northern part of the study area along the coast (Shimane peninsula) was observed in the S-wave velocity structure estimated by S-wave tomography using data of natural earthquakes by Zhao et al (2004), as well as in the S-wave velocity structure constructed from ambient noise by Nishida et al (2008)
Summary
In a convergent plate margin, subsurface structures are complex, and relatively new faults may become activated. To reveal heterogeneities related to seismic activity, in this study, we estimated high-resolution subsurface structures in the San-in area.
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