Abstract
This paper discusses a coseismic uplift which occurred around 8,200 cal BP along the southeast Boso Peninsula on the Pacific coast of central Japan facing the Japan Trench and Sagami Trough. Estimated uplift is based on eleven 14C ages and analyses of sedimentary facies, fossil assemblages of foraminifera and mollusks from the outcrops along the Onjaku River. The deposit is represented by alternation of mud, sand, and gravel beds deposited in a drowned valley system.
The results suggest that the relative sea-level rapidly fell around 8,200 cal BP. This phenomenon is discordant with the relative sea-level curve obtained from the Tokyo Bay area of small tectonic movements, which indicates a continuous relative sea-level rise toward a peak around 7,000 cal BP. Relative sea-level fall estimated from the study area probably reflected a local tectonic uplift.
Sand and gravel beds alternating with mud beds might be deposited by storm surges, river floods, and/or tsunamis that intruded into the muddy drowned valley. The conglomerate bed deposited just before the relative sea-level fall around 8,200 cal BP is interpreted as a tsunami deposit caused by the same earthquake that uplifted the study area.
