Seismicity Rate Changes in the Inferred Locked Zone of the Tokai Region in the Latter Half of the 1990's

Abstract

Recently, the microearthquake seismicity observed around the Tokai region has shown several anomalous changes. It is considered that the conditions in this area have been steadily progressing toward the occurrence of the next Tokai earthquake, due to locked subduction of the Philippine Sea plate. The clearest change was recognized in August 1999, when the seismicity rate inside the subducted slab in and around the locked zone decreased. This quiescence continued for more than one year. After October 2000, the seismicity turned active. On the other hand, the seismicity rate inside the crust around the same zone was also found to have been gradually decreasing since the end of 1996. Two moderate-sized earthquakes, the central Shizuoka prefecture earthquakes happened concurrently with these anomalous phenomena, in October 1996 (M4.3), and in April 2001 (M5.1). It seems probable that these events influenced the state of the locked subduction, and caused the seismicity rate change. Spatial pattern analyses on these anomalies revealed that the decrease in the seismicity rate does not always extend over the entire locked zone, but forms a patchwork pattern. As a result, the zone is partitioned into two parts: one is quiescent, and the other is rather activated. Such results resemble previous cases reported by Wyss's group, who suggested that a characteristic seismicity rate change precedes the occurrence of a main shock. Typically this involves a quiescent zone appearing to surround a non-quiescent zone. Wyss's group regard the non-quiescent part as an asperity. Based on their reports, I examined the relationship between the duration of the quiescence (Tq: year) and the area of the asperity (Anq: kmkm²), and derived an equation of Tq=0.27√Anq. This relationship seems reasonable, as it shows the proportionality of Tq to the size of the asperity estimated in linear dimension. If this is applicable to the present case in the Tokai region, we can obtain an evaluation of about 7 years of Tq for the current anomalous stage.