跡津川断層系付近における詳細な震源分布と地下構造

Abstract

中部地方北部の跡津川断層系において，地震活動の調査を実施してきた．また，人工地震による地下構造調査も行われた．これらのデータを解析して，断層系を構成する跡津川断層と茂住-祐延断層では，それぞれ独立して垂直な地震活動が深さ15km程度まで見られ，別の断層面が活動していることがわかった．また，人工地震探査によって，深さ約12および20kmに反射面が検出された．浅い方の反射面は地震発生層の下限付近に対応し，大地震の発生機構に関連する可能性を示唆している．

Detailed seismic activity in the Atotsugwa fault area was studied by a dense seismic networkconsisting of stations of routine and temporary observations operated by universities and otherinstitutes, including the Hi-net. As a result, seismic activities occur on different planes along theAtotsugawa and Mozumi-Sukenobu faults, which are active faults, belonging to the Atotsugawa faultsystem. Besides focal depth distributions along both of the faults have nearly the same concaved shapewith shallower events in the both ends the faults. Seismic refraction and wide-angle reflection surveysrevealed two clear seismic reflectors at about 12 and 20km deep. The former is located at the base ofthe seismogenic layer. This suggests that the reflector plays an important role of initial breakage oflarge inland earthquakes. The latter is the same reflector known as a S-wave reflector in the middlecrust, which may be related to the fluid in the deep crust.