Invisible Faults, revealed from Gravity Anomalies.

详细的重力资料覆盖了日本全境，足以讨论重力异常的空间特征变化。重力异常分布为陡水平梯度重力异常带。我们将重力异常的陡峭水平梯度带定义为水平梯度带的第一级在16 mg或以上的水平梯度持续至少10 km的区域。在重力数据库的基础上，采用不同的图像处理技术，分析了SHGZ的分布。因此，在日本各地都能看到许多SHGZ，但在Chugoku地区相对较少。虽然许多SHGZ的分布并不与主要的地质构造边界重合，但它们在距离主要地质构造边界几十公里的范围内平行出现，表明它们之间存在内在联系。地质和地震证据表明，SHGZ是上地壳深处约10 km深处的隐伏断裂型地质构造。其中一些可能是活动断层。在被厚软沉积物覆盖的第四纪面，探测基底层活动断层的工具有限。然而，重力异常有可能通过详细的重力测量来揭示它们。

Detailed gravity data now cover all over Japan for sufficient enough amount to discuss characteristic spatial variation of gravity anomalies. Distribution of gravity anomalies indicates steep horizontal gradient zones of gravity anomalies. We defined the steep horizontal gradient zone of gravity anomaly (SHGZ of GA) is as of the zone in which the horizontal gradient of 16 mgals or larger sustains for at least 10 km for the first ordered of the horizontal gradient zone.We analyzed distributions of the SHGZ based on our gravity database by using various image processing techniques. As a result, many SHGZ can be recognized all over Japan, but relatively rare over the Chugoku district. Even though the distribution of the many SHGZ do not coincide with the major geological tectonic boundaries, they appear in parallel within several tens km from them, suggesting that internal relationship between them.Geological and seismological evidences suggest the SHGZ are hidden fault-type geologic structures in a deeper part of the upper crust up to about 10 km deep. Some of them may active faults.In the Quaternary planes, where are covered by thick soft sediments, there are only limited tools to detect active faults in basement stratums. Gravity anomalies, however have potential to reveal them through detailed gravity measurements.