This study conducted a field survey and multiproxy analyses on sediment cores retrieved from the Kobatake-ike pond in Choshi City, Chiba Prefecture, Japan. Kobatake-ike pond is located at a high elevation (i.e., 11 m above present-day sea level) and faces the southern part of the Japan Trench. Three event sand layers were detected within the continuous mud and peat sequences of 3000 years. Based on the multi-proxy analyses, including mineralogical composition, diatom assemblages, and geochemical markers, these sedimentological events were associated with past tsunamis. The most recent event was a sandy layer and is attributed to the AD 1677 Enpo tsunami, which was reported by an earlier study conducted in the pond. Our results demonstrated that two older sand layers are associated with large tsunamis that struck the Choshi area in AD 896–1445 and in BC 488–AD 215. In addition, the age ranges of these events seem to overlap that of large earthquakes and tsunamis known from the central part of the Japan Trench. This implies a possible spatiotemporal relation of earthquake generations between the central and southern parts of the Japan Trench. However, since the error ranges of the ages of tsunami deposits at the southern and central parts of the Japan Trench are still large, further investigation is required to clarify the relations of large earthquakes in both areas.
This study conducted a field survey and multiproxy analyses on sediment cores retrieved from the Kobatake-ike pond in Choshi City, Chiba prefecture, Japan. Kobatake-ike pond is located at a high elevation (i.e., 11 m above present-day sea level) and faces the southern part of the Japan Trench. Three sedimentological events were detected within the continuous mud and peat sequences of three thousand years. Based on the multi-proxy analyses, including mineralogical composition, diatom assemblages, and geochemical markers, these sedimentological events were associated with past tsunamis. The most recent event was a sandy layer from the AD 1677 Enpo tsunami, reported by an earlier study conducted in the pond. The estimated recurrence interval for tsunamis was approximately 700 years, comparable to that of the central part of the Japan Trench. The timing of at least two of three sedimentological events seems to have been close (i.e., intervals of few tens of years) before or after the occurrence of large earthquakes and tsunamis along the central part of the Japan Trench. This implies a possible spatiotemporal relation of earthquake generations between the central and southern parts of the Japan Trench, which can be associated with drastic changes in stress fields in the neighborhood of the rupture area. The risk of future large earthquakes and tsunamis along the southern part of the Japan Trench seems exceptionally high, considering the stress change caused by Tohoku-oki earthquake with 9.0 Mw in 2011. This study necessitates the need to investigate the spatiotemporal relationship of large earthquakes from the perspectives of geology, seismology, and history.
Paleotsunami studies along the southern part of Japan Trench are scarce. Additional geological evidence must be collected. This study conducted field survey and multi-proxy analysis for drilling cores taken from a pond of Choshi City, Chiba prefecture, Japan, where the 1677 Enpo tsunami deposit was reported earlier. The pond is suitable for detection of sedimentary evidence of low-frequency but large tsunamis only because it is located at a high elevation (11 m). Three event deposits are intercalated in the three thousand year long continuous mud and peat sequences. Based on multi-proxy analyses conducted of grain size distributions, diatom assemblages, and geochemical markers, these event deposits were identified as tsunami deposits; the most recent was the AD 1677 Enpo tsunami. The estimated recurrence interval of the tsunami is approximately 700 years, which is comparable to those of the central part of the Japan Trench. It is noteworthy that the timing of these tsunami events along the southern part of the Japan Trench seems to have been close (few tens of years intervals) before or after the occurrence of large earthquakes and tsunamis along the central part of the Japan Trench. Therefore, a spatial-temporal relation of earthquake and tsunami generations might exist between central and southern parts of the Japan Trench, which might be explained by drastic changes of stress fields surrounding the rupture area of a huge earthquake. Considering the current situation, by which the Mw=9.0 2011 Tohoku-oki earthquake had occurred at central part of the Japan Trench, this possibility should be investigated carefully from perspectives of seismology and history because risks of future occurrence of large earthquake and tsunami events along the southern part of Japan Trench might be extremely high.
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