The Spatio-temporal Change in the Activity of the Intermediate-depth Earthquakes in the Pacific Plate beneath the Northeastern Japan Arc

Abstract

In order to interpret various seismic activities in relation to the subduction process of the Pacific plate beneath the northeastern Japan arc, information about the spatio-temporal stress change in the slab is indispensable. In this study, we estimated the stress change by analyzing the activity and focal mechanisms of the intermediate-depth earthquakes in detail. Predominant focal mechanism is of down-dip compression type in the upper plane, while down-dip extension type dominates in the lower plane of the double-planed deep seismic zone. If the overall down-dip compressional stress in the plate increases, it is expected that the upper plane seismicity becomes active while the lower plane activity is suppressed. Thus, we can qualitatively estimate the stress change in the plate from the seismicity. For this purpose, we analyzed Z-value which indicated the spatio-temporal change in the seismicity, and R-value which indicated the ratio of earthquake occurrence frequencies between upper plane and lower plane. Both analyses revealed that the relative frequency of earthquakes in the upper plarle to that in the lower plane increased after large earthquakes that occurred on the plate boundary. Z-value distribution indicates that the upper plane seismicity had been active in the westward extension of the after-slip region of the 1994 Far-off Sanriku earthquake during the period of about 1.5 years after the event. Moreover, normal fault type earthquakes near the slab surface disappeared in the same period. These phenomena suggest that down-dip compressional stress in the plate increased by the large earthquake on the plate boundary.