The Tegano fault is an active fault, which is important from the perspective of the evolutionary process of reverse faults and for earthquake disaster prevention. It is believed that the longer and older Byobusan fault, which runs side by side with the Tegano fault, affects the formation of the Tegano fault. We performed precise gravity measurements to study the origin of the fault. Then, we estimated the basement structure in conjunction with a preexisting shallow re.ection survey as the constraining condition. By adopting differential filtering, we were able to see that the dip angle of the Tegano fault is low, whereas the angle of the Byobusan fault is high. The inferred schematic pro.le of the Tegano fault is consistent with a preexisting theory of the evolution of a reverse fault; this supports the hypothesis that the Tegano fault was derived from the deep part of the Byobusan fault. Moreover, we found that most of the fault plane of the Tegano fault is shallower than 1 km below the surface. Although the tectonic landform in the Tegano fault is young (or, the Byobusan fault is old), the deep part of the Tegano fault or the Byobusan fault may be capable of producing an earthquake. When assessing an active fault, it is important to comprehensively consider not only geomorphologic and ser.cial-geologic surveys but also gravimetrical ones.
