Source Model of the AD 1611 Keicho Tsunami Estimated from Numerical Simulation with Geological and Historical Evidence

概要

Destructive tsunamigenic earthquakes have occurred along the Kuril and Japan trenches repeatedly. Historical documents indicate that the AD 1611 Keicho tsunami struck the Pacific coast of Tohoku, northern Japan. Although historians and geologists have investigated the event extensively, the source of the tsunami is still controversial (Kuril and Japan trenches). The source and magnitude of the AD 1611 earthquake are particularly important in understanding of the recurrence interval of unusually large earthquakes along the two trenches. Thus, I explored a source model that can satisfy the historical and geological records from Hokkaido and Tohoku in the 17th century consistently. The historical, geological, and tsunami engineering works related to the AD 1611 earthquake and tsunami were comprehensively reviewed and the reliability of those was reevaluated by this study. The careful review shows that the geological evidence of the AD 1611 tsunami at the Ishinomaki plain is critically important to reinforce the reliability of historical information of Miyagi prefecture. Therefore, geological fieldwork and multi-proxy analyses for the tsunami deposits on the Ishinomaki Plain were carried out. The combination of the reconstruction of the paleoenvironment using pollen analysis and high-resolution radiocarbon dating with Bayesian analysis suggested that the youngest tsunami deposit from the Ishinomaki Plain should be ascribed to the AD 1611 tsunami. Based on the reevaluated historical records together with previously identified and newly added geological evidence and tsunami inundation modeling, I found that the fault model along the northern Japan and geological records from western Hokkaido and Tohoku in the 17th century. However, the model has an unusually large slip amount compared to findings from the seismological study. This indicates that tsunami deposits at western Hokkaido may have been formed by a local tsunami triggered by, for example, submarine landslide. If this is the case, the AD 1611 Keicho tsunami source model should be assumed as an extraordinary large fault along the middle Japan Trench. Future investigation on the seafloor topography is needed to evaluate the possibility of the submarine landslide at western Hokkaido and to confirm the AD 1611 Keicho tsunami source model.