Seismic structure in and around the source area of the 2004 mid-Niigata, Japan, earthquake: 3-D waveform modelling based on local tomography images

Abstract

SUMMARY The seismic velocity structure in and around the source area of the 2004 mid-Niigata earthquake, which featured complicated heterogeneities, was investigated by combining waveform modelling and traveltime tomography inversion using low-frequency (0.05 ≤ f ≤ 0.2 Hz) and high-frequency (f ≥∼ 1 Hz) data, respectively. On the footwall of the main shock that includes multiplanar faults, 3-D finite-difference waveform modelling using only a previously proposed tomography model was not sufficient to synthesize the observed waveforms in 0.05 ≤ f ≤ 0.2 Hz at most stations. Thus, we derived a final 3-D model 3DM-28, examining body wave amplitudes, phases and traveltimes. The image produced by model 3DM-28 shows a clearer contrast between low- and high-velocities than that seen in the original tomography models due to the higher velocity on the footwall. This increase in velocity, particularly in the seismogenic zone, also indicates that short-wavelength low-velocity anomalies revealed in the revised tomography image may be more localized in the vicinity of the multiplanar faults than that shown in the original one. Moreover, the low-velocity anomaly zone within a depth range of 15–20 km beneath the seismogenic zone (lower crust) appears to be associated with the short-wavelength low velocity anomalies at shallower depths (upper crust). These characteristics may support the hypothesis of infiltration of pressurized fluids from the lower crust into the multiplanar fault system. The volumes or distributions of such fluids may be clarified by further examination of ‘unsatisfactory fit waveforms’ recorded at stations along the strikes of the major faults. We suggest that the clear velocity contrasts between the hangingwall and footwall and the upper crust and lower crust, including the effects of fluids, all seem to be essential characteristics of the seismogenic conditions in this earthquake sequence.