Abstract
This paper describes Holocene marine deposits around Sagami Bay and discusses Holocene crustal movement in relation to crustal deformation associated with great earthquakes in the Sagami Trough. Flights of Holocene marine terraces are distributed on the coasts along the Oiso Hills and Miura Peninsula, which are uplifted regions located on the northeastern side of the Sagami Trough. Marine terraces located in the southern part of Miura Peninsula indicate that the emergences occurred around 6, 000yrs BP, 4, 000yrs BP, and 3, 000yrs BP on the basis of the ages of sea caves and archaeological data.
Remarkable uplifted features in the Holocene are observed along the northeastern part of Sagami Bay; to the north of this uplifted zone a belt of subsidence in the Holocene is located on the line connecting Hadano to Kanazawa-Hakkei through Ofuna.
The emergence of the southwestern part of the Oiso Hills around 6, 500yrs BP is inferred from the fossil molluscan assemblage, the K-Ah (Kikai-Akahoya) volcanic ash, and related carbon-14 dates. The Kozu-Matsuda active fault which delineates the western margin of the Oiso Hills did not move at the time of the emergence around 6, 500yrs BP. The Oiso Hills to the east of the Kozu-Matsuda fault have uplifted and the Morito River Lowland to the west of the fault has subsided along with the slip of the Kozu-Matsuda fault since 6, 500yrs BP.
Marine sediments that emerged around 4, 500yrs BP are present over 5m above the present sea level in Odawara city, which is located in the southwestern margin of the Ashigara Lowland. The upper limit of Holocene marine deposits descends eastward to the Morito River Lowland.
No geological evidence of emergence around 6, 000yrs BP is found in Izu Peninsula to the southwestern side of the Sagami Trough. The post-glacial Jomon tranagression continued until 4, 000yrs BP in the northern part of Izu Peninsula and until 3, 000 to 2, 000yrs BP in the southern part of the peninsula. Izu Peninsula became a site of emergence after these periods in its various region.
Tsunami deposits have been discovered in marine sediments located in submerged drowned lowlands around Sagami Bay. The ages of emergence of marine terraces have been inferred from the ages of these tsunami deposits. The history of coseismic vertical movements at Sagami Bay may be clarified by further study of tsunami deposits.
