Marine Gravity Measurements over the Seto Inland Sea, Western Japan.

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Surface ship gravity measurements on board the R/V Tansei-maru, Ocean Research Institute, University of Tokyo, were carried out over the Seto Inland Sea, western Japan. This region is one of the widest vacant area of gravity data in and around the Japanese Islands. During the cruise for full 4 days, gravity data were obtained at more than 6000 points. The ship was forced to change courses frequently due to steering clear of many islands and ships, so that we examined the gravity data carefully. Based on the analyses of variation of measured gravity values due to changes of ship's heading and those of crossover errors at intersections of the ship's track, it was confirmed that the most of error data were generated when the ship changed her course. Most of error data were detected by assuming an upper limit of horizontal gravity gradient to be 10 mgal/km, which value is appropriate for land gravimetry. We omitted about 350 data from measured one after these analyses . A drift effect was also corrected based on gravity values at ports of calls. After these processings, most of the gravity data are estimated to be accurate to 3-4 mgals. Obtained gravity anomalies in and around the Seto Inland Sea revealed a narrow(about 30 km width) and low (several tens mgal) gravity anomaly zone along the north of the Median Tectonic Line. This zone continues at least from Beppu Bay in the west to Osaka Bay in the east.
日本測地学会の論文