関東山地「飯能-御坂測線」における稠密自然地震観測(<特集>首都直下地震防災・減災特別プロジェクトPart 2)

概要

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In central Japan, the Philippine Sea plate (PHS) subducts beneath the Tokyo Metropolitan area in the Kanto region. In the western Kanto region, the Izu-Bonin arc within the PHS has been colliding from the south with the Honshu arc, forming a complex structure called the Izu collision zone (ICZ). The geometry of the subducting PHS and the overlying crustal structure of the ICZ are important to constrain the process of earthquake occurrence. To obtain detailed images of PHS beneath the northern part of the ICZ, dense seismic array observations were carried out along a 60-km-long line between Hanno and Fuefuki trending ENE-WSW. Seventy-six three-component portable seismographs were deployed on the survey line at 500 to 750m intervals, and waveforms were recorded during a four-month period from October 2008. High-quality data were obtained for the analysis using receiver function and seismic interferometry to obtain images of PHS. We relocated 263 local earthquakes with arrival time corrections at each station. The relocated hypocenter distribution shows that a double seismic zone associated with the westward subducting Pacific plate is located at a depth of 100－200km beneath the survey line. The seismicity associated with the northwestward subducting PHS can be traced to a depth of about 30km beneath the Tanzawa Mountains.
2010-02-26