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

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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.