Deep structure of the Mozumi-Sukenobu fault, central Japan, estimated from the subsurface array observation of fault zone trapped waves

In order to estimate the averaged fault zone structure, i.e. the width, shear wave velocity and Qs of the Mozumi-Sukenobu fault, central Japan, we analysed the Love-wave-type fault zone trapped waves recorded by a seismic array deployed in a subsurface survey gallery. We selected the typical Love-wave-type trapped waves based on the characteristics of dispersion, amplitude, apparent velocity and the polarization of wave trains following the direct shear wave. We selected nine events showing typical Love-wave-type trapped waves from 154 earthquakes. Epicentres of the events were located along the surface trace of the Mozumi-Sukenobu fault. The nine events showing trapped waves were found to be located in a plane dipping to the south at an angle of approximately 75° to 85°. The duration of the fault zone trapped waves increase non-linearly with the increase of hypocentral distance. This result implies an increase in shear wave velocity of the fault zone with depth and/or horizontal distance from the array. In order to obtain averaged fault zone structure from hypocentre to receiver, we assumed fault zone structure to be a 2-D uniform low-velocity waveguide, and modelled the dispersion curve and the waveform of trapped waves using an analytical solution. The width, shear wave velocity and Qs of the fault zone were estimated to be 160 to 400 m, 2.9 to 3.1 km s−1 and 60 to 90, respectively. The width of the fault zone estimated in this analysis is consistent with the width of the extent of fault zone at the survey gallery. We also conducted 3-D finite difference simulation (3-D FDM) on fault zone trapped waves using depth-dependent structure models referred to the 2-D averaged structure, indicating the possible depth-dependent structures. The 2-D modelling is useful as an initial guide to the 3-D imaging of a fault zone.