Vertical Crustal Movements of Okusiri Island Associated with the 1993 Hokkaido-Nansei-Oki Earthquake.

Abstract

We estimated the coseismic vertical crustal movements of Okusiri island associated with the 1993 Hokkaido-Nansei-Oki earthquake by comparing the measurement clata of the heights of offshore rocks around Okusiri island before and after the earth quake. The results show us the subsidence all over the island; the least subsidence is about 25 cm at the northeast tip, while the largest subsidence is about 100 cm at the south part. From the scattering of the multi-measurement data, we guess that the standard error of measurement is about 3 cm, which is enough for the detection of the coseismic crustal movements. A fault model explains the observed trend of subsidence which is just west of Okusiri island with the dislocation of about 5 m, dip angle of 50 degrees descending westward.