Abstract
The new first order levelling along the coast of Izu Peninsula was carried out by the Geographical Survey Institute during the period from 1967 to 1968. This survey was done after 30 years since the former similar survey had been finished. The author tried to summarize the studies of the vertical movement of crust in this peninsula on the occasion of the new levelling work. It is well known fact that some peninsulas toward the Pacific Ocean are gradually inclined to the ocean. T. Terada and N. Miyabe stated that this trend can be found also in Izu Peninsula. But, the comparison of the results of several repeated levellings can not prove this trend so cleary. We can observe this trend only for the period from 19301931 to 19671968 at the most southern part of peninsula, and the rate of this inclination is so small that we can not subject this trend to be closely associated to occurence of great earthquake. In nothern part of peninsula, we can notice the uplift zone that runs from northwest to south-east. Mean rate of uplift during the period from 19301931 to 19674968 is approximately +2.2 mm/year. Along this uplift zone several earthquake swarms were observed during the seismic activities of 1930 in this peninsula. At the region of Ito earthquake swarm of 1930, upheaved bench marks show only a small subsidence after earthquake swarm. We tried to apply subsidence effect corrections to the displacement of bench marks in order to get the vertical movement closely associ ated only to the activities of the earthquake swarm. This can be possible, because subsidence seems to be a secular change. By this analysis, it is proved that the crust responses as a plastic substance rather than elastic to the earthquake-generating forces. Relative height change of bench marks on both side of Tanna Fault seems to be very stable at present. But, at the western region of Tanna Fault, a remarkable, local upheval can be observed. This part is quite the same as one of fore-shocks areas of Kita-Izu earthquake of 26 Nov. 1930. The rate of upheval had became very large before the occurence of Kita-Izu earthquake and then small. Furthermore, at this region, change of gravity of +0.14 mgal was observed by comparison of the gravity values observed in 1955 and 1968 by the Geographical Survey Institute. Formerly, Prof. N. Yamazaki explained the topography near Tanna Fault by two tilted blocks, one of which is inclined to southward and other is northward. Vertical movement of triangulation points before and after the Kita-Izu earthquake and the distri bution of Bouguer anomalies around Tanna Fault show that Prof. N, yamazaki's consideration seems to be appropriate.
