Abstract

Hachijo Island, part of the Izu-Ogasawara Islands, is situated near major plate boundaries, including the Nankai Trough, Sagami Trough, and Izu-Ogasawara Trench, making it highly susceptible to tsunamis triggered by earthquakes in these regions. Despite this vulnerability, limited tsunami research has been conducted in the area. This study aimed to uncover the tsunami history of Hachijo Island through geological surveys and numerical modeling, providing insights into potential tsunami sources. The field investigation at west coast revealed an event layer characterized by rounded gravels which is comparable to previously recognized tsunami deposits in this area. Radiocarbon dating results indicate that these deposits formed after the seventeenth century. Based on the dating results together with sedimentary features such as cracks, the 1703 Genroku earthquake and tsunami is more likely because geological evidence fits the historical descriptions. Numerical simulations for the 1703 Genroku tsunami were conducted to evaluate the fault source responsible for the tsunami deposits. The runup height observed at west coast could not be reproduced using a previously proposed fault model. This research developed several revised fault models by changing parameters to reflect their influence on Hachijo Island. Among these, one model successfully reproduced the impact of tsunamis on both Hachijo Island and surrounding regions. The geological records of Hachijo Island provide key constraints for tsunami source modeling and highlight the importance of remote island data in tsunami hazard assessments. This study improves our understanding of tsunami risks in regions and provides a refined model to guide future research and disaster mitigation strategies.