Step-like migration of early aftershocks following the 2007 Mw 6.7 Noto-Hanto earthquake, Japan

Seismic data reveal that the aftershock zone expands with time after a main shock event. Here we examine the aftershock sequence recorded during the first 32 days following the 2007 Noto-Hanto earthquake, Japan. By applying a matched-filter technique to the data, we detected about 10 times more events than those listed in the routinely constructed earthquake catalog. The aftershock area expanded along the fault strike as a logarithmic function of time, beginning immediately after the main shock. Aftershock expansion toward the SW was especially extensive and developed more rapidly than toward the NE. Interestingly, the aftershock area expanded in a step-like manner by the activation of a series of spatially clustered seismic bursts. Spatial correlation between the aftershock extension and the afterslip distribution suggests that the expansion of the aftershock area could be mainly driven by aseismic afterslip. The aftershock area continues to expand, even though 7 years have now passed since the main shock.