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Abstract: To estimate the deep structure of the southern part of the Nojima Fault, southwest Japan without the influence of near-surface structures, we analyzed the Love-wave-type fault-zone trapped waves (LTWs) recorded by a borehole seismometer at 1800m depth. We examined the polarization, dispersion, and dominant frequency of the wavetrain following the direct S-wave in each seismogram to identify the LTW. We selected eight candidates for typical LTWs from 462 records. Because the duration of the LTW increases with hypocentral distance, we infer that the low velocity fault-zone of the Nojima Fault continues towards the seismogenic depth. In addition, since the duration of the LTW increases nonlinearly with hypocentral distance, we infer that the S-wave velocity of the fault-zone increases with depth. The location of events showing the LTW indicates that the fault-zone dips to the southeast at 75° and continues to a depth of approximately 10km. We assumed a uniform low-velocity waveguide to estimate the average structure of the fault-zone. We estimated the average width, S-wave velocity, and Q s of the fault-zone by comparing an analytical solution of the LTW with measured data. The average width, S-wave velocity, and Q s of the fault-zone are 150 to 290m, 2.5 to 3.2km/s, and 40 to 90, respectively. Hence the fault-zone structure with a larger width and smaller velocity reduction than the fault-zone model estimated by previous surface observation is more suitable to represent the average fault-zone structure of the Nojima fault. The present study also indicated that the shallow layers and/or a shallow fault-zone structure drastically changes the characteristics of the LTW recorded at the surface, and therefore cause a discrepancy in the fault-zone model between the borehole observation and surface observation. [Copyright &y& Elsevier]

In order to estimate the averaged fault zone structure, i.e. the width, shear wave velocity andQs 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 andQs 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. [ABSTRACT FROM AUTHOR]