Spatial variation in coda Q in the northeastern part of Niigata\u2013Kobe Tectonic Zone, central Japan: implication of the cause of a high strain rate zone

Abstract

We have analyzed the spatial variation in coda Q in the northeastern part of a high strain rate zone, the Niigata–Kobe Tectonic Zone (NKTZ), to investigate the cause of the high strain rate by correlating coda Q with the differential strain rate and the S-wave velocity. The spatial distributions of coda Q in the 2–4 and 4–8 Hz frequency bands are found to be negatively correlated with the differential strain rate. Coda Q in the 2–4 Hz frequency band is correlated spatially with the S-wave velocity at a 25 km depth, as has been reported previously. We also find a positive correlation between the perturbation of the S-wave velocity at a 10 km depth and coda Q in the 4–8 Hz frequency band, implying that the spatial distribution of coda Q in this frequency band is mainly attributed to the heterogeneity of the upper crust. This feature is different from that of the central part of the NKTZ reported previously, which indicates a difference in the cause of the high strain rate. Therefore, we suggest that the high deformation rate in the upper crust, which is characterized by a thick sediment basin, as well as that in the ductile lower crust, contributes to the generation process of the high strain rate on the surface in the northeastern part of the NKTZ.Graphical abstract.