近畿における広帯域強震動アレイ観測

Abstract

We are observing seismic waves from small to large earthquakes at a seismic array network deployed in the Kinki district to elucidate and predict vibrational characteristics for strong ground motions. The network is composed of four stations on rock sites and nine stations on ground sites. At each station, we install a three-component velocity seismometer with a wide dynamic range of 78dB over a broad frequency band from 0.025 to 30 Hz. From comparisons among the seismograms from an earthquake of M=5.2 occurring in the aftershock region of the 1946 Nankai Earthquake of M=8.1, we obtained the following results.(1) The seismic motions at a hard ground site (OSA) on a diluvium in the central Osaka Plain are characterized by a distinctive surface-wave group around 20 to 30 seconds after S arrivals. The amplitude of the wave group is comparable to that of the principal part of S waves. The wave group vibrating in the north-south direction is composed of Love wavesthat propagate not from the epicenter in the south direction but from the east mountainous region. (2) The seismic motions at a soft soil site (WOS) in the Osaka Bay areas are characterized by S waves strongly reflected from the interface between a sedimentary layer and a basement rock around 4 to 5 seconds after S arrivals.Their amplitudes reach 60 to 130% of S-wave amplitudes. This can not be explained by a horizontally layered model of the underground structure.