Tono Research Institute of Earthquake Science (TRIES) carried out a seismic reflection survey passing through an “over-step” part of the Enasan fault, Tono area, central Japan. The fault geometry in the over step part was, however, not clear, because the survey was done along a single line on a granite exposure zone.
Then, we performed a precise gravity survey to delineate the shape of basement. The feature of basement topography suggests that the Enasan fault is not continuous under the over step part. This result agrees with the recent geomorphologic/geologic investigations that the activities of crustal blocks at the east side and the west side of step-over are different in age (the east block acted lately). Furthermore, we found the trend of a low gravity anomaly belt, or a characteristic concave feature of the basement locating north of the fault, to be deviated from the fault strike. If it is a remnant of the past faulting, then it can be suggested that the regional stress field was NE-SW compression, when the concave feature was formed. Also, it is consistent with the stress field when the Ena cauldron was formed.
