(2)The Genroku Earthquake (December 31, 1703, M 7.9-8.2)

Genroku Earthquake is thought to have been an interplate earthquake whose focal region extended from Sagami Bay to the tip of the Boso Peninsula as well as the area along the Sagami Trough in the open sea southeast of the Boso Peninsula. It generated strong ground motion over a wide area centered in the southern Kanto region. Reports on the damage theorized that ground motion corresponding to a seismic intensity 6 in JMA scale occurred over a wide area in the southern Kanto area, and that corresponding to a seismic intensity 7 in JMA scale occurred in the coastal area of Sagami Bay and the southern tip of the Boso Peninsula (Fig.5-16). Damage was particularly heavy in the domain of Odawara, and almost all Shukuba (postal relay stations) were destroyed from Kawasaki to Odawara. The death toll inside the domain was about 2,300. A tsunami hit the coastal areas of the Boso Peninsula and Sagami Bay, causing more than 6,500 fatalities, particularly on the Boso Peninsula. The Habu Pond on Izu Oshima Island was collapsed, rushing into the sea. In all, more than 10,000 fatalities are estimated to have occurred due to the ground motion, tsunami, and other factors.

An upheaval of the ground occurred with this earthquake from the Boso Peninsula to the coast of Sagami Bay. This upheaval is thought to have reached about 5 m at its highest point. The volume of this upheaval was the greatest on the Boso Peninsula (Fig.5-17). There is a coastal terrace on this peninsula that is thought to have been created by the upheaval of the seashore with this earthquake. Four levels of coastal terraces have been created in about 6,000 years, including this terrace. Other earthquakes that caused an uplift of the seashore in the same manner as the Genroku Earthquake are thought to have occurred in the past (Fig.5-18, Fig.5-19).

Both the Genroku Earthquake and the 1923 Kanto Earthquake were great earthquakes of M 8 that occurred along the Sagami Trough. These earthquakes resemble each other in the extent of the damage caused and the conditions of the crustal deformation. Both earthquakes are thought to have occurred in the same location or very near to each other. The extent of the damage and the crustal deformation on the Boso Peninsula was greater during the Genroku Earthquake, however, and the tsunami during that earthquake reached as far as the Sotobo area. Therefore, the source region of the Genroku earthquake is thought to have extended closer to the Boso Peninsula than that of the Kanto earthquake.