Research on the Quaternary Tectonics in the surrounding area of the Epicenter of 2000 Tottoriken-seibu Earthquake

2000年托托里肯-西武地震震中周围广泛分布着花岗岩类岩石，部分地区被晚上新世至早更新世玄武岩熔岩不整合覆盖。利用大节理、节理带和剪切带对花岗岩中的断裂系统进行了分析，并根据横切关系估算了各系统的活动序列。结果表明，该区发育13个断裂系统(A~L)，3个系统(A~C)在末期恢复活动，玄武岩熔岩喷发中心线性排列识别的深部断裂和切割熔岩的断裂属于A-D系统，推测早更新世已有活动。在震中东北约10公里处的Koshikibara玄武岩中新发现的一条活动断裂属于B系，与地震的震源断裂平行，并表现出相同的位移样式。在两条断裂之间，与之平行发育的是玄武岩喷发中心的线性排列所指示的深裂缝。这些相似性表明，在同一应力场下可能形成三条断裂，可能发生在早更新世。近期该区地震活动活跃，余震带与B系重合。2000年Tottoriken-Seibu地震的余震带最早出现与B系平行，随即转向与C系平行。可以认为，这种变化是震中B系统和C系统持续重新激活的结果，并瞬间触发了附近较大的C系统断裂的重新激活。

A surrounding area of the epicenter of the 2000 Tottoriken-seibu Earthquake is widely occupied by granitic rocks, which is unconformably covered by the late Pliocene to the early Pleistocene basalt lavas in places. Analysis of fracture systems in granitic rocks are carried out by using large joints, joint zones and shear zones, and a sequence of activity of each system is estimated on the basis of cross-cutting relationships. The results show that 13 fracture systems (A〜L) develop in this area and 3 systems (A〜C) reactivated at the last stage.Deep fractures which are recognized by linear arrangements of eruption centers of basalt lavas and faults cutting the lavas belong to A to D systems, which are estimated to have acted already in the early Pleistocene. An active fault which newly was found in the Koshikibara Basalt about 10km northeast of the epicenter belongs to B system and is parallel with and shows the same style of displacement with an earthquake source fault of the earthquake. Between both faults a deep fracture indicated by a linear arrangement of eruption centers of basalt is developed parallel with them. These similalities imply that three fractures may be formed under the same stress fields, probably in the early Pleistocene.Recently seismicity in this area have been active and their after shock zones coincide with B system. An after shck zone of the 2000 Tottoriken-seibu Earthquake appeared parallel with B system at the earliest stage, immediately it changed its direction to be parallel with C system. It may be concluded that the change was resulted from continuous reactivation of B and C systems at an epicenter of the earthquake, and instantly triggered to reactivate a nearby larger fracture of C system.