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

Abstract:

It is widely identified that aftershock zones often expand along strike and dip-directions with time following a mainshock dynamic rupture. Notable aftershock expansions are especially observed in subduction zone earthquake sequences at time scales of days to years. Here, we examine the aftershock sequence recorded during the first 32 days following the 2007 Mw 6.7 Noto-Hanto earthquake, Japan, which occurred at shallow crustal depths in an active compressional zone along the eastern margin of the Japan Sea. To obtain the precise information about aftershock activity, we used a searching technique for events that have similar waveforms to those of template event from continuous seismograms recorded by a dense seismic network installed near the source region. 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 mainshock. During the month after the mainshock, the aftershock area expanded along-strike to approximately four times the size of the mainshock rupture area. Aftershock expansion toward the SW was especially extensive, and developed more rapidly than toward the NE. A close-up view of the along-strike evolution of the aftershocks shows that the speeds of the migrating seismicity fronts were not constant, but evolved in several repeated step-like acceleration–deceleration phases, by the activation of a series of spatially clustered seismic bursts. Spatial correlation between the aftershock extension and afterslip distribution suggests that 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 mainshock (Kato and Obara, 2014).