地震動の長周期成分を用いた首都圏地震観測網(MeSO-net)の設置方位推定

概要

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A remarkable high-density seismograph network, called MeSO-net, is under construction in the Tokyo metropolitan region, central Japan. At the MeSO-net station, a three-component accelerograph is installed at the bottom of a 20-m deep borehole. To analyze horizontal component waveforms observed at a borehole-type seismic station, we first have to know the azimuth of the seismometer. In this study, based on cross-correlation analyses of long wavelength seismograms with a high S/N, we estimate sensor azimuths at MeSO-net stations. As reference stations, we use seismograms observed by NIED F-net broad-band seismographs and Hi-net tiltmeters. At more than 80% of stations, N-components of seismographs are within ±10° from magnetic north. We confirm that the sensors at three stations are rotated more than 90°. Although our results agree well with results estimated using P-wave first motion of a local earthquake, there are systematical azimuthal differences between two results at stations distributed in the northern Kanto region. Because the subducting Pacific slab lies beneath the Kanto region, this feature may indicate that the diffracted wave passing through the high-velocity slab was observed as a first pulse at stations located in the northeastern Kanto region.
2010-01-28