LOCAL CHARACTERISTICS OF THE FMAX OF BEDROCK MOTION IN THE TOKYO METROPOLITAN-AREA, JAPAN

On the basis of S-wave acceleration data observed at the bottom of a 2.3 km borehole drilled at the Shimohsa Crustal Activity Observatory (SHM), we studied the source region dependence of the f(max) of bedrock motion in the Tokyo metropolitan area. In order to study a variety of f(max) in this area, we divided the earthquakes that occurred beneath the Kanto district into six source groups. This division of earthquake sources leads us to the conclusion that the f(max) of events that occurred in the boundary zone where the Philippine Sea plate is colliding with the Pacific and Eurasian plates is lower than 10 Hz and that of interplate events is higher than 20 Hz. This conclusion is certified using data recorded at another deep borehole site at Iwatsuki. We cannot deny the existence of source-controlled f(max), on the assumption that the path attenuation effect is independent of source region. Using estimates of frequency-dependent quality factor for S-waves obtained by Aki (1980), we find that the f(max) of the source spectrum from events that occurred in the boundary zone where the Philippine Sea plate is colliding with the Pacific plate is about 10 Hz and that from events that occurred in other source regions which do not include the active seismic region in eastern Yamanashi Prefecture, is higher than 25 Hz.