Relative quiescence in the stress-shadow regions prior to the large earthquakes off the east coast of Miyagi Prefecture, northern Japan, and its implication for the intermediate-term prediction

This paper is concerned with the intermediate-term prediction of the forthcoming M7 ~ 8 class earthquake on the plate boundary, off the east coast of Miyagi Prefecture, northern Japan, which is the highest probability among the long-term forecast announced to the public. Regional seismicity in the regions of stress-shadow preceding each of the previous ruptures in 1936 and 1978 shows quiescence relative to the predicted rate by the ETAS model (the relative quiescence), whereas the seismicity is well predicted in the regions of neutral or increasing Coulomb failure stress (CFS), which leads to the effect of possible precursory slip within or near the source. Anticipating similar scenarios, a number of sequences of earthquakes or aftershocks from the activities during 1979-2004 in northern Japan are analyzed by fitting the ETAS model to examine the relation to the CFS increments in the considered regions using the source model of the 1793, 1936 and 1978 ruptures. Surmising the precursory slips, it is likely that the results of the normal activity and relative quiescence in respective activities are due to those of the 2003 Miyagi-Ken-Oki intra-slub earthquake of M7.0 rather than those of the expected rupture on the plate boundary. Thus, it is unlikely that the predicted rupture will imminently occur within a couple of years from the analyzed time of 2004, but it is recommended to monitor the future activity to detect the anticipated relative quiescence in the stress shadow areas due to the interplate rupture models. Key words; Aftershock sequences, Coulomb failure stress, ETAS model, Seismicity rate change, Precursory slips