The Mikawa Bay Region, central Japan, is characterized by many active faults recording Quaternary activities. It is, however, difficult to understand the overall tectonic character of the region due to a thick sedimentary cover. We report the first finding of Neogene basin inversion in southwest Japan by estimating the depth and structure of the basement surface in the Mikawa Bay Region by analyzing gravity data. Our gravity basement map and two-dimensional density-structure model automatically determined using the genetic algorithm revealed a half-graben bounded on the south by the north-dipping Utsumi Fault. The motion of the Utsumi Fault, which inverted from normal faulting during the Miocene to recent reverse faulting, indicated the inversion of the half-graben. The timing of the inversion of the fault motion, i.e., the reverse faulting of the Miocene normal fault, can be compared with an episode of basin inversion observed at the eastern margin of the Japan Sea, northeastern Japan. The Takahama Fault in the southwestern part of the Nishi–Mikawa Plain is considered to have formed as a result of the backthrust of the Utsumi Fault under inversion tectonics. If the Takahama Fault is indeed the backthrust fault of the Utsumi Fault, the root of the Takahama Fault may be deep such that the Takahama Fault is seismogenic and linked to the 1945 Mikawa earthquake.

日本中部的三川湾地区的特征是记录第四纪活动的许多活动断层。但是，由于沉积层厚，很难理解该地区的整体构造特征。我们通过分析重力数据估算三川湾地区基底表面的深度和结构，从而报告了日本西南部新近纪盆地倒转的首次发现。利用遗传算法自动确定的重力基底图和二维密度结构模型显示，南侧有一个北岩层，北侧是内海断层。Utsumi断层的运动从中新世期间的正常断层转变为最近的反向断层，表明该半断层的倒转。断层运动反转的时间，即 中新世正断层的逆断层可以与日本东北部日本海东缘的盆地倒转事件相比较。西三河平原西南部的高ham断裂被认为是由于反转构造下的内海断裂的逆推作用而形成的。如果Takahama断层确实是Utsumi断层的逆推断层，则Takahama断层的根可能很深，以至于Takahama断层具有地震成因并与1945年三川地震有关。西三河平原西南部的高aka断裂被认为是由于反转构造下的内海断裂的逆冲作用而形成的。如果Takahama断层确实是Utsumi断层的逆推断层，则Takahama断层的根可能很深，以至于Takahama断层具有地震成因并与1945年三川地震有关。西三河平原西南部的高ham断裂被认为是由于反转构造下的内海断裂的逆推作用而形成的。如果Takahama断层确实是Utsumi断层的逆冲断层，则Takahama断层的根可能很深，以至于Takahama断层具有地震成因并与1945年三川地震有关。