Abstract
Paleo-seismic events that occurred around the Sagami Trough during the past 10, 000 years were recorded by sandy beds in Holocene bay-floor mud beneath four alluvial plains on the Boso Peninsula and Miura Peninsula of central Japan. Facies analysis indicates that normal deposition of bioturbated mud was interrupted by more than ten thin sand and sandy gravel beds, each of which fines upward from an erosional base. Some sandy beds contain cross-lamination indicative of both landward and seaward currents. These coarse deposits in the bay-floor mud are called “event deposits”.
Quantitative and qualitative analyses of fossil assemblages from the view point of taphonomy revealed the source and depositional processes of the event deposits. The event deposits include transported and mixed molluscan assemblages such as muddy bay floor and rocky coasts. This suggests the existence of reworking events around the bay area. Invasions of open marine water into the embayments account for fully marine ostracodes being found mixed with inner-bay assemblages in two of the sandy beds.
Radiocarbon ages from 137 samples support correlation of seven of the sandy beds beneath four of the plains. They designate the repeated occurrences of reworking events near Sagami Bay. Five of the beds gave ages similar to those previously obtained for emergent processes of Holocene terraces in the region. Age agreement strongly suggests that these event deposits originated from earthquake-induced tsunamis.
Rapid lowering of sea-level was shown in two event horizons with sand sheets by analyzing ostracode and molluscan assemblages. These events were probably coseismic uplifts accompanied by tsunamis.
Detection of the coseismic events from bay-floor deposits supplies a powerful information to paleo-earthquake research and also contributes to seismo-tectonic studies as described in this paper.
