1976 年 29 巻 1 号 p. 55-70
Seven steps of marine terraces are well developed on the Ogi Peninsula, Sado Island. The seventh (lowest) one is a raised abrasion bench less than 2m high, emerged at the time of the destructive earthquake of 1802, hence it is named the 1802 terrace. The sixth terrace is about 2-4m high, and probably was formed at the time of Holocene transgression. Higher five terraces (Pleistocene terraces V-I) are well preserved over the most part of the peninsula and have the height of 32-40m, 70-55m, 94-118m, 123-137m and more than 165m, respectively.
The height of former shorelines represented by shoreline angle of each terrace shows that all the terraces tilt northward. Generally, the higher the terrace, the larger the amount of tilting. However, the tilt of the lowest two terraces (the 1802 and Holocene terraces) is almost the same, ca. 1.5′. This indicates that the 1802 tilting caused by the earthquake was a first event after the formation of the Holocene terrace of ca, 6, 000 years old and the recurrence interval of the events was more than 6, 000 years. A uniform regional difference of 2m in height between these two terraces is probably interpreted as a superposed result of the eustatic lowering of sea level and a regional uplift during last 6, 000 years.
It is possible to estimate the average intervals of earthquakes after the terrace formation by comparing the tilting rate of all Pleistocene terraces with that of the 1802's. Thus, the average recurrence intervals are estimated at about 8, 600 years since Terrace III was formed and about 5, 000 years since Terrace IV was formed. These values are consistent with the interval of more than 6, 000 years which is estimated from tilt of the 1802 and the Holocene terraces. It is concluded, therefore, that the earthquake has taken place repeatedly in a similar manner with a recurrence interval of about 5, 000-9, 000 years during at least last 105 years.
Uplift and northward tilting of the Ogi Peninsula at the time of the 1802 earthquake (Magnitude 6.6) is significantly larger than those of Awa-shims at the Niigata earthquake of 1964 (M 7.4), though its magnitude was smaller.
This fact and the limited areal deformation by the 1802 earthquake imply that the epicenter of this earthquake was located very close to the coast of the Ogi Peninsula, probably within a few kilometers off Shukunegi. A reverse faulting is inferred to have occurred along a northward-dipping fault plane at the 1802 earthquake.