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The Eastern Boundary Fault Zone of the Shonai Plain (EBFZSP), Northeast Japan is an active reverse fault zone, and its southern part is considered to have a high probability of earthquake occurrence. Also, the Aosawa Fault Zone (AFZ), categorised as a geological reverse fault, lies to the east of the EBFZSP by 3–10 km along strike, almost parallel to the EBFZSP. Therefore, the EBFZSP and its surrounding area are important in terms of their long-term earthquake occurrence potential and fault development history. After performing gravity measurements to compensate for the lack of coverage in the existing gravity database, a gravity gradient tensor analysis was applied around the EBFZSP to evaluate subsurface structural indices (dimensionality, dip angle, strike direction, and boundary indices). As a result, we conclude that the southern part of the EBFZSP appears to be at a relatively mature stage, which means that the part has both high two-dimensionality and boundary properties. On the other hand, the northern part of the EBFZSP appears to be relatively immature, which means that the northern part has both low two-dimensionality and boundary properties. A similar relationship was also found between the EBFZSP and the AFZ; that is, the former was immature, and the latter was mature. These features suggest that the EBFZSP developed from the south to the north, and was derived from the AFZ. We also found an area with high structural boundary indices and low two-dimensionality near some reported active folds beneath the Shonai Plain. A dip-angle index for a reverse-fault-like density structure showed results inconsistent with the dip angles observed on outcrop. To circumvent this contradiction, we propose the hypothesis that the EBFZSP, currently acting as a reverse fault, has a normal-fault-like density structure and we then attempt a phenomenological interpretation. The dip-angle index may be helpful only in simple situations.

Gravity gradient tensor analysis and its application to the Eastern Boundary Fault Zone of the Shonai Plain, Northeastern Japan