Deep crustal structure around the Atotsugawa fault system, Central Japan: A weak zone below the seismogenic zone and its role in earthquake generation

We carried out travel-time tomography around the Atotsugawa fault in central Japan, using arrival-time data obtained from a dense temporary seismograph network in joint observations by the Japanese University group. The observed velocities beneath the fault are 10-13% lower than those of the assumed host rocks (pyroxene amphibolite and hornblende-pyroxene gabbro) in the lower crust. Because the seismogenic layer is thickest in the central part of the fault, reaching a depth of ∼15 km, we infer that the low-velocity anomaly is caused by aqueous fluids. Fluid fractions in the lower crust are estimated to be 2-3% and ∼10%, assuming pyroxene amphibolite and hornblende-pyroxene gabbro, respectively. A distinct low-velocity anomaly is imaged in the central part of the Atotsugawa fault at a depth of 10 km, where seismic activity is very low at the upper 7 km and creeplike movement is observed at the surface. This anomaly is horizontally isolated but vertically connected to the lowvelocity anomaly in the lower crust. We interpreted that abundant aqueous fluids supplied from the lower crust are responsible for this anomaly. High pore fluid pressure may enhance the stability of frictional slip, resulting in aseismic or episodic slip along the fault.