Abstract
A fault movement of the 1918 Omachi earthquake is estimated based on the vertical movement along the levelling route in Omachi and its vicinities. The vertical movement during the period of 1891-1920 is analyzed by Miyabe's method. In the 1918 Omachi earthquake, the land block of the northwestern part of Omachi was upheaved and tilted down 6.0×10-5 rad to N75°W. Changes in the heights of the bench marks projected onto the N75°W-575°E direction suggest that the land block was thrust up eastward by a high angle reverse fault dipping to the west. This fault coincides with that of the 1964 Niigata earthquake and is similar to the topographical feature along the Shinano River Seismic Zone. It is presumed that the plate boundary between the Okhotsk Plate and the Eurasian Plate is a collision type, not a simple eastward subduction type as is accepted so far.
