CONSTRUCTION OF GRAVITY DATA-BASE IN SOUTHWEST JAPAN

本项目的目的是通过发展该地区前两个重力研究项目的成果(科学研究资助(C), No.01540331,1989-1991；科研资助项目(C)（04640400,1992-1994）。在本项目的最后三年，名古屋大学和岛根大学进行了广泛的重力测量，分别存储了13517和2795个新数据点。通过这项工作，日本西南部仍然存在的大部分空白区域消失了，这些空白区域分布在中国西部、北陆、九州中部地区，变成了精确重力数据密集填充的区域。此外，在1995年南兵库县地震震源区周边地区，东海地区从约罗山脉到Ooigawa，更多的是通过野比平原，北陆中部地区围绕富山湾等重要构造区域进行了密集的重力测量。另一方面，我们收集了来自20多个机构和组织的大量重力数据，并对所有收集到的数据进行了精度的总修正，以保持较高的精度。因此，我们构建的重力数据库现在包含了超过11000点的净数据，并且均匀且非常密集地覆盖了几乎整个日本西南部，Fossa Magna以西。所有的重力数据都在重力数据库中注册，精度达到1 magal或更高。利用这个高精度的重力数据库，我们可以随心所欲地在任何部分、任何比例尺绘制精确的重力异常图。从中可以识别出以下因素：1)沿主要地质构造线，如中位构造线、赤石构造线、阿特拉断裂等，圈定了明显的线性。2)沿主要的活动断裂和地质断裂，如六甲断裂系、约罗断裂、Isurugi断裂等，划分出非常明显的陡峭garty梯度带。3)新地质断层；从密集的重力测量数据中发现了Himi断层。根据地质调查的结果，它被认为不是一个活动断层。许多只在重力图上描绘的线性在任何地方都可以找到。到目前为止，还没有其他地球物理或地质方法知道它们。少

The purpose of present project is to create a gravity database to cover all over the southwest Japan by developping the achievement of the former two projects of gravity study in the area (Grant-in Aid for Scientific Research (C), No.01540331,1989-1991 ; and Grant-in Aid for Scientific Research (C), No.04640400,1992-1994).During the last three years of present project, Nagoya University and Shimane University made gravity measurements extensively and stored 13,517 and 2,795 points of new data, respectively. Through this work, large parts of the blank area still remaind in the southwest Japan that distributed in western Chugoku, Hokuriku, central Kyushu districts were vanished and changed to the area densely filled up with precise gravity data. Besides this, very dense gravity measurements were made at the areas tectoniccaly important such as (1) surrounding area of seismic sourse region of the 1995 southern Hyogo-Ken Earthquake, (2) Tokai district ranged from Yoro mountains to Ooigawa … More through Nobi Plain, (3) central Hokuriku district surrounding the Toyama Bay, and so on. On the other hand, we collected a large number of gravity data from more than 20 institutions and organizations, and performed total revision of accuracy on all of the collected data to keep a high precision. The gravity database thus we constructed containes now more than 11,1000 points of net data, and covers uniformly and very densely almost all southwest Japan, west of Fossa Magna. All of the gravity data are registered in the gravity database with very high precision of 1 magal or better.By utilizing this high precision gravity database, we are able to draw precise gravity anomaly maps at any portion, in any scale size, as we wish. Form those, we can recognize following factors : 1) Distinct linearments are delineated along major geological tectonic lines, such as Median tectonic line, Akaishi tectonic line, Atera fault and so on. 2) Steep garvty gradient belts are deliniated very sharply along the major active faults and geological faults such as the Rokko fault system, the Yoro fault, the Isurugi fault, and so on. 3) A new geological fault ; Himi fault was found out from the densely measured gravity data. As the result of geological survey, it is considered to be not an active fault. 4) Many linearments deliniated only on the gravity maps are found out at anywhere. They are not known so far by other geophysical or geological means. Less