Nucleation process of the MJ 6.7 northern Nagano prefecture, Japan, earthquake of November 22, 2014

Abstract:

On November 22, 2014 at 10:08:18PM (JST) part of the Kamishiro fault in northern Nagano prefecture, Japan, ruptured in a reverse faulting earthquake of M_J 6.7 and M_W 6.2. A brief, but intense foreshock sequence, which included five earthquakes between M_J 2.0 and 2.8, began 4 days before the mainshock. The foreshocks concentrated in the immediate vicinity of the mainshock hypocenter. In this study we model the initiation process of the mainshock using seismograms recorded on the nearby stations of NIED Hi-net at frequencies up to 10 Hz. The initial rupture process is inferred by solving a linear system defined by the representation theorem for seismic sources. Instead of imposing a rupture velocity, the allowable slipping area on the fault is confined to lie within the radius 0.95*V_S*t. Once a node has been activated, it remains part of the solution at all later times. Thus, activated nodes may start, stop, restart, or never slip as required by the data. The system is regularized using a centered Laplacian operator to spatially smooth the model; there is no temporal smoothing. The system is solved using non-negative least-squares. We compare results obtained using synthetic seismograms for the Green’s functions (SGF) to ones using empirical Green’s functions derived from the foreshocks (M_J 2.7 - 2.8) (EGF). Even for the initial rupture which can be explained by simple models, it is found that solutions that use EGFs are more stable than those that use SGFs because some stations have strong path or site effects that are not captured by the synthetics at high frequency. The rupture initiates with a M_W 3 sub-event in the first 0.15 s, and then propagates away from the foreshock zone as a M_W 4 sub-event at 0.4 s. Our results show that the nucleation process was cascade-like.