History of the paleo-earthquakes along the Sagami Trough, central Japan: Review of coastal paleoseismological studies in the Kanto region

Abstract

Two subduction zone interplate earthquakes have been recorded along the Sagami Trough, the first in AD 1703 (Genroku Earthquake) and the second in AD 1923 (Taisho Earthquake). While the source areas of these two events overlapped within and around the Sagami Bay, the 1703 Genroku Earthquake had a larger rupture area, which propagated to off the Boso Peninsula. Currently, our understanding of prehistorical earthquakes has been facilitated by Holocene marine terraces and tsunami deposits, through which we have come to the understanding that past Kanto earthquakes can be divided into two types - the Taisho-type and the Genrokutype. Taisho-type earthquakes are thought to be more common, occurring approximately every 400 years on average, but after several occurrences (at 2,000-2,700 year intervals) a Genroku-type earthquake would take place and propagate to off the Boso Peninsula. These less frequent coseismic Genroku-type ruptures are not consistent with global navigation satellite system (GNSS) geodetic data, which show strong plate interface coupling in this area due to a high slip-deficit rate. To resolve this discrepancy, it is necessary to consider hypotheses such as changes to the long-term slip-deficit rate, the occurrence of aseismic fault slips, or even the existence of another earthquake type. Furthermore, recent marine terrace survey reports in the Boso Peninsula have shown that the emergent ages between the eastern coast and western coast do not necessarily correlate, which lends credence to the possibility that another earthquake type occasionally occurs in the region off the Boso Peninsula.