Reactivation of minor faults in a blind active fault area: A case study of the aftershock area of the 2000 Western Tottori Earthquake, Japan

Abstract

The 2000 Western Tottori Earthquake (Mw 6.6, Mjma 7.3) in Japan occurred in an area where no active faults are known to exist. We examined the distribution, occurrence, paleostress field, and K–Ar ages of minor faults in the aftershock region of this earthquake to understand the tectonics and deformation process in a blind active fault area. Five stress stages were identified: NW–SE compressional strike-slip and NE–SW extensional normal faulting stress field in Stage 1; clockwise rotation of ~ 40° between 18 and 16 Ma in Stage 2; N–S compressional strike-slip and N–S extensional normal faulting stress field in Stage 3; stress state with σHmin (σ3) oriented NE–SW in Stage 4; and E–W compressional strike-slip faulting stress field in Stage 5. The results from the relationship between stress fields and K–Ar ages of fault indicate that most minor faults in the epicentral area of the 2000 Western Tottori Earthquake were formed along pre-existing joints during rifting and opening of the Japan Sea during the late Oligocene. Some minor faults show signs of reactivation under the present stress field. However, a large active fault with a clear topographic expression has not yet been developed due to the small displacement of minor faults. These findings indicate that detailed structural analysis of minor faults and reconstruction of stress history are important for seismic hazard assessment in areas where fault activities are initial stages.

Keywords

• 2000 Western Tottori Earthquake
• Paleostress field
• K–Ar dating
• Fault gouge
• Blind active fault
• Fault development