Relationship between geologic structure and the source fault of the 2004 Mid-Niigata Prefecture Earthquake, central Japan

The geologic structure of the Uonuma Hills, the site of the 2004 Mid-Niigata Prefecture Earthquake (MJMA 6.8), Japan, is examined based on published geologic maps, drill hole data and seismic reflection profiles. The Uonuma Hills are located in an active folding area on the eastern margin of the Niigata sedimentary basin, and are divided into three discrete regions from north to south based on the geologic structure. The southern extent is marked by a west-dipping homoclinal structure produced by reverse faulting of the west-dipping Muikamachi active fault. The central section exhibits a regional anticlinorium produced by west-dipping reverse faulting with a steep angle at depth (> 5 km) and a more shallow angle (30°) near the surface. This fault can be correlated with the source fault responsible for the main shock. The Muikamachi fault consists of two fault segments and the source faults of the northern and the southern segments show discrete geometry forming an step over. Thus, judging from the geologic structure, the 2004 Mid-Niigata Prefecture Earthquake ruptured to the maximum possible extent in this fault segment in the central part. Furthermore, this study reveals that the activity on the Muikamachi fault, which has an apparently continuous surface trace, is produced by two different source faults at depth, with important implications for seismic risk assessment.