History of the great Kanto earthquakes inferred from the ages of Holocene marine terraces revealed by a comprehensive drilling survey

We measured the emergence ages of four marine terraces in the Chikura lowland, which lies to the southeast of the Boso Peninsula, in eastern Japan, to reevaluate the history of the great earthquake occurrences along the Sagami Trough over the past 10,000 years. The dates of the marine terraces are measured via radiocarbon dating of shell fossils obtained from the marine deposits. The sampling method employed in this study collects core samples using a dense and systematic drilling survey, which increased the reliability when correlating shell fossils with marine terraces. In addition, radiocarbon dating was performed with accelerator mass spectrometry, which produces more highly accurate measurements than those measured in previous studies. Moreover, we explored the surface profiles of the terraces with detailed digital elevation model (DEM) data obtained using LiDAR. The maximum emergence ages of the marine terraces were dated at 6300 cal yBP, 3000 cal yBP, and 2200 cal yBP from the top terrace excepting the lowest terrace (which was estimated at AD1703). In addition, another previously unrecognized terrace was detected between the highest and the second terrace in both the dating and the geomorphological analyses and was dated at 5800 cal yBP. The newly obtained ages are nearly a thousand of years younger than previously estimated ages; consequently, the intervals of the great earthquakes that occurred along the Sagami Trough are estimated to be much shorter and more varied than those of previous estimations. This result revises the data used in the current assessment of the probabilities of earthquakes along the Sagami Trough, which could devastate the Tokyo metropolitan area. Furthermore, it demonstrates that the current approach could be a powerful tool to increase the accuracy of assessments of the other areas with depositional marine terraces.