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

2014; International Union of Geological Sciences; Volume: 37; Issue: 4 Linguagem: Inglês

10.18814/epiiugs/2014/v37i4/004

2586-1298

Masanobu Shishikura,

Methane Hydrates and Related Phenomena

North American Plate (Fig. 1).While historical records show that two great earthquakes, the 1703 Genroku Kanto Earthquake (M 8.2) and the 1923 Taisho Kanto Earthquake (M 7.9) (hereafter, the 1703 Genroku Earthquake and the 1923 Taisho Earthquake, respectively), occurred along this trough (Usami et al., 2013), records of earthquakes occurring before the 1703 Genroku Earthquake are ambiguous.However, instead of concentrating on the few remaining historical records of past eras, tectonic geomorphological studies have aimed at recovering information on past earthquakes from marine terraces.Such studies have been conducted since the coseismic uplift associated with the 1923 Taisho Earthquake was discovered along the coasts facing the Sagami Trough (Imamura, 1925;Watanabe, 1929).Although the progress of paleoseismological records of the past 40 years in this area has helped elucidate the history and cycle of interplate earthquakes along the Sagami Trough (e.g.Shishikura, 2003), the observed results do not fully agree with more recently obtained geodetical observation data.The resulting contradictions are very important, not only in terms of scientific understanding, but also because of their relevance when formulating disaster prevention plans for metropolitan areas.In fact, Japanese government offices tasked with disaster prevention measures have recently announced seismic hazard breakdowns for Japan's metropolitan areas (Cabinet Office, 2013; Earthquake Research Committee, 2014), but those efforts did not consider this problem in detail.Therefore, in this paper, I will provide an overview of current research into paleoearthquakes and associated tsunami events in this region, discuss the distribution of the region's seismically active faults, and then examine their possible slip recurrence, terrace uplift magnitudes, and tsunami deposit records. Historical earthquakes and tsunamis along the Sagami Trough 1923 Taisho Kanto EarthquakeThe 1923 Taisho Kanto Earthquake, which killed more than 105,000 people, was one of the worst natural disasters in recorded Japanese history (Moroi and Takemura, 2004).Significant crustal movements and tsunamis accompanied this earthquake.According to tide gauge data and the re-leveling of benchmarks and triangular points undertaken by the Land Survey Department (1926), the Boso Peninsula, Miura Peninsula, and Oiso coastal area 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.