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

S U M M A R YThe 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.

Imaging the subsurface using induced seismicity and ambient noise: 3-D tomographic Monte Carlo joint inversion of earthquake body wave traveltimes and surface wave dispersion

“…In the above results there is a high velocity anomaly at the location of the southern earthquake cluster (Figure 6,8,9): in the results from joint inversion the magnitude of the velocity anomaly is slightly lower, but is nevertheless clearly identifiable. Similar features have been observed previously and are generally interpreted as earthquake asperities that concentrate stress (Lees 1990;Eberhart-Phillips & Michael 1998;Chiarabba & Amato 2003;Tajima et al 2009;Li et al 2013;Zhang et al 2013). However it is also possible that this correlation is caused by the trade off between source parameters and velocity values.…”

“…In the real data results, the high velocity anomaly at the location of the southern cluster therefore may reflect the true structure of the subsurface, e.g., earthquake asperities following previous interpretations (Lees 1990;Eberhart-Phillips & Michael 1998;Chiarabba & Amato 2003;Tajima et al 2009;Li et al 2013;Zhang et al 2013). However, since we still observe subtle multimodilities in the joint inversion results, and the real Earth may have a more complex structure, there is still the possibility that the details of the recovered model are obscured by the trade off between source parameters and velocity models.…”