Near-fault deformation and Dc′′ during the 2016 Mw7.1 Kumamoto earthquake

An Mw7.1 Kumamoto earthquake occurred at 01:25:05 on April 16, 2016 (JST). The earthquake involved a rupture at a shallow depth along a strike-slip fault with surface breaks. Near-fault ground motion records, especially those of a strike-slip earthquake, can provide us with direct information on the earthquake source process. During the earthquake, near-fault seismograms were obtained at KMMH16 station located about 500 m off the fault. The ground displacements were well recovered from the double numerical integration of accelerograms at KMMH16 both on the surface and at the bottom of the 252-m-deep borehole. Fault-parallel static displacement was estimated to be about 1.1 m from the acceleration waveforms. The Dc′′ value, which is defined as double the fault-parallel displacement at peak velocity time, was proposed as a proxy of the slip-weakening distance. Using both the velocity and displacement fault-parallel waveforms, the Dc′′ value was estimated at about 1 m. This value was between 30 and 50% of the total slip on the fault, which is consistent with previous observations.