CONSTRUCTION OF GRAVITY DATA-BASE IN SOUTHWEST JAPAN

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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