Abstract
The Mid-Niigata Prefecture Earthquake (M6.8) occurred along Higashiyama anticline on October 23, 2004, accompanying anticlinal deformation attaining maximum upheaval of 60cm or more (Fig2). Isoseismals based on the scale of JMA (Japan Meteorological Agency) were shown in Fig. 1, and the damaged areas were observed in several places in the south part of Northeast Honshu. The seismicity in the area had been activated since the North-Miyagi Earthquake (M6.4) occurred on July 26, 2003 (Fig5 and 6) and shallow earthquakes including the Mid-Niigata Prefecture earthquake took place on the environs of the backbone range, where intermediate earthquakes occurred, suggesting vertical deep faults surrounding the backbone range. Seismicity has been also active in Kanto Plain and the sea off those areas. Historical record from the 1800's to 1950 showed the simultaneous and intermittent activity in the south part of Northeast Honshu for some tens to 20 years or so. The strain analysis deduced from displacements of triangulation stations indicates the areas of large maximum shear greater than 10_-5 in which not only shallow destructive earthquakes but intermediate ones have occurred, so the sheared areas must spread vertically their roots down to the areas where the intermediate earthquakes take place. Judging from those data, the earthquake prediction must be based on the measurement of crustal deformation and the observation of seismicity in those wide area.
