Invisible Faults, revealed from Gravity Anomalies.

详细的重力数据现在覆盖了日本各地，足以讨论重力异常的特征空间变化。重力异常的分布表明重力异常具有陡峭的水平梯度带。将重力异常水平陡坡带定义为重力异常水平梯度大于或等于16 MGals的一阶水平梯度带持续至少10公里的区域。基于我们的重力数据库，利用各种图像处理技术分析了该区域的分布。因此，许多SHGZ在日本各地都能被认出，但在中部地区相对较少。尽管许多SHGZ的分布与主要地质构造边界不一致，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是位于上地壳更深约10公里处的隐伏断层型地质构造。在被厚厚的软沉积物覆盖的第四系平面上，探测基底地层中的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.