The subsurface observation of fault-zone trapped waves : applications to investigations of the deep structure of active faults

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Analyzing fault-zone trappedwaves is an effective approach to imaging the deep structure of fault zones. In this paper, we apply a fault-zone trappedwave analysis to investigate the deep structure of the Nojima andthe Mozumi-Sukenobu faults in Japan. We observedfault-zone trapped waves at subsurface seismic stations. We obtained typical candidates of fault-zone trapped waves at subsurface stations. The duration of fault-zone trapped waves becomes larger with hypocenter distance. This result indicates that trapped waves were generated at the hypocenter and traveled along the fault zone. We then estimatedthe averagedfault zone structure from hypocenter to receiver. The thickness andthe shear wave velocity of the fault zone were estimatedby fitting synthetic dispersion curves and waveforms to the observed ones. The thickness of the fault zone is estimatedto be 150-290m for the Nojima fault and 130-400m for the Mozumi-Sukenobu Fault. For both of these fault-zones, the shear-wave velocity is reduced by 10 to 20% of the surrounding velocity. The low-velocity fault zones of the Nojima andthe Mozumi-Sukenobu faults continue to a depth of about 10km from the locations of the earthquakes showing trappedwaves. For the Nojima fault, the thickness andthe shear-wave velocity of the fault zone are not comparable with those estimated from previous surface observations. This discrepancy might be due to distortions of the seismograms causedby surface observations andlateral variations of fault-zone structure. The trapped waves tend to be degraded by thick sediments near the surface. Borehole observations will be requiredto detect typical fault-zone trappedwaves, andenable us to analyze fault-zone property with high accuracy.

The subsurface observation of fault-zone trapped waves : applications to investigations of the deep structure of active faults

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