Analyses of Pre-, Co-, and Post-seismic Vertical Crustal Movement on the Southeastern Akita Earthquake of 1970

An Evidence of Pre-slip at the Base of the Seismogenic Layer

Abstract

The 1970 Southeastern Akita Earthquake occurred just under a precise leveling route, and its source fault was located at the center of dense seismic observation network. Detailed focal mechanism parameters and crustal movements of pre-, co-, and post earthquake were reported. Seismic reflection experiments show that horizontal detachment exists in the deeper extension of the earthquake source fault at the base of the seismogenic layer in a depth of about 13km. Authors reconstruct a series of faulting processes on the above mentioned fault plane, due to vertical crustal movement and observed fault figures. Simple dislocation model assuming uniform slippage shows following faulting processes. 1. Vertical crustal movement in 4 to 14 years before the 1970 earthquake (1966-56) are interpreted as a result of aseismic pre-slip on the horizontal detachment at the base of the seismogenic layer (13km in depth). 2. The aftershock distribution shows coseismic fault slip occurred on the reverse fault and western margin of detachment. The vertical crustal movement from 4 years before to 4 years after the 1970 earthquake were caused of slip on the this fault and detachment which exist its extension, and surface disturbances. 3. The vertical crustal movement from 4 to 10 years after the earthquake (1980-74) is caused of aseismic slip on the horizontal detachment. 4. Net of the pre-slip is comparative to that of the main shock (ca. 25cm). 5. Direction of Pre-slip on the detachment is same to general direction of P-axis in the Northeastern Japan. 6. Length of Pre-slip region is longer than that of earthquake source fault.