糸魚川ー静岡構造線断層帯における観測網整備によって明らかになつた地震波速度構造と地震活動

摘要

We have investigated the 3-D seismic velocity structure and seismicity along and around the Itoigawa-Shizuoka tectonic line active fault system. In order to observe earthquakes at densely distributed stations, 15 borehole-type seismometers were deployed along and around the fault system and five stations in the area were transferred from the Japan Meteorological Agency (JMA) to the National Research Institute for Earth Science and Disaster Prevention (NIED). In tms study we analyze seismograms observed from 2003 to 2009 at the NIED Hi-net stations and other permanent stations, operated by JMA and Japanese universities, located m the region. The obtained results reveal low velocity zones (LVZs) distributed in the upper crust, along the fault system. Predominant LVZs are located in the northern and southern edges of the fault system. We interpreted the northern LVZ to correspond to the partially melted body beneath the Hida Mountains weak shear strength may prevent faulting and form the edges of the fault system. We have also studied the detailed hypocenter distribution by using the double-difference relocation technique. The relocated hypocenters are widely distributed along the fault system and the relationship between the relocated earthquakes and the geologically mapped faults is not so clear. Only two earthquake clusters are consistent with the geometry of the fault system. A predominant cluster consisting of strike-slip type earthquakes is located just beneath the Gogukuji fault, which belongs to the fault system. The clustered seismicity is distributed along the fault.%糸魚川ー静岡構造線断層帯において簡易高感度地震観測点の整備を行い,そのデータによる地震波速度構造解析と高精度の震源再決定を行った.推定された速度構造によると,大局的には断層帯に沿う低速度領域が分布しているが,その低速度の度合いには地域性が認められることが分かった.また,断層帯の端部には顕著な低速度領域が存在し,いずれも脆性破壊としての地震を起こしにくい領域と対応している可能性があることが分かった.このような構造が,断層の端部を規定している可能性がある.高精度の震源再決定の結果からは,牛伏寺断層の直下と断層帯南部の一部について,断層帯との関係が疑われる震源クラスターが存在することが分かった.特に,牛伏寺断層の傾斜が地表から深部にいたるまでほぼ鉑直であるとするならば,検出された震源クラスタ一は牛伏寺断層そのものにおける地震活動と解釈される.