Distribution of Holocene Marine Mud and Its Relation to Damage from the 1923 Earthquake Disaster in the Tokyo Metropolitan Area, Japan

Tokyo, which is located near the boundary between the North American and Philippine Sea plates, has been frequently struck by large earthquakes throughout the Holocene. The 1923 Taisho Kanto Earthquake is a rare historical earthquake that can be reconstructed in detail because abundant datasets were collected by investigations performed just after the earthquake. We examined 13,000 borehole logs from the Tokyo and Nakagawa lowlands to clarify the distribution and thickness of incised-valley fills and soft marine mud that had accumulated since the Last Glacial Maximum (LGM) on a grid with a resolution of 150 m x 150 m. We compared these datasets with the distribution of wooden house damage ratios caused by the Taisho Kanto Earthquake. Our results showed that the thickness of the soft mud, but not that of the incised-valley fills, was strongly correlated with the wooden house damage ratio. The mud content was >60%, water content was >30%, and S-wave velocity was ca. 100 m/s in the soft Holocene marine mud. The wooden house damage ratio was highest where the soft mud thickness was 20 m, because in those areas, both the soft mud and the wooden houses resonated with a natural period of ca. 1 s.