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The Izu-Bonin (Or Izu-Ogasawara) and Mariana Arcs Are Major Topo- Graphic Features in the Circum-Pacific Belt, Extending Curvili

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The Izu-Bonin (Or Izu-Ogasawara) and Mariana Arcs Are Major Topo- Graphic Features in the Circum-Pacific Belt, Extending Curvili J. Phys. Earth, 24, 275-311, 1976 GRAVITY IN THE JUNCTION BETWEEN THE JAPANESE AND THE IZU-BONIN ISLANDS Jiro SEGAWA* and Carl BOWIN** *Ocean Research Institute, Universityof Tokyo, Tokyo, Japan **WoodsHole OceanographicInstitution , WoodsHole, Mass., U.S.A. (Received July 22, 1975;Revised June 21, 1976) Gravity values from the central Honshu to the northern part of the Izu-Bonin (Izu-Ogasawara) Arc have been compiled, and the free air and Bouguer gravity anomaly maps as well as gravity tables have been given. The broad gravity low over the triple junction between the Pacific plate, the Eurasia plate and the Philippine-Sea plate has been explained by consider- ing the possible behavior of the Pacific plate descending from the junction between the Japan and the Izu-Bonin Trenches: It may be that the slab descending at the Japan Trench and the slab descending at the Izu-Bonin Trench geometrically have converging velocity components under the junc- tion area to cause subduction of the Philippine-Seaplate beneath the Eurasia plate, and this subduction gives rise to the negative gravity anomaly. Ex- planation of some other gravity lows observed over the Japanese Islands has also been made tentatively. 1. Introduction The Izu-Bonin (or Izu-Ogasawara) and Mariana Arcs are major topo- graphic features in the circum-Pacific belt, extending curvilinearly through about 3,000km, and separating the Philippine Basin from the western Pacific Basin, This paper discusses the gravity in and around the northern portion of the Izu-Bonin Arc north of 30°N. This region constitutes a T-T-T triple junction between the Pacific plate, the Eurasia plate and the Philippine-Sea plate. These plates are bordered by the Japan Trench (Pacific vs. Eurasia), the Izu-Bonin Trench (Pacific vs. Philippine) and the Nankai Trough plus the Sagami Trough (SUGIMURA,1972) (Eurasia vs. Philippine). Gravity anomaly maps in and near the Japanese Islands were published by TOMODA(1973), who combined the land and marine data. Although the maps cover the northern end of the Izu-Bonin Arc, the data are not sufficient. In this paper, more data have been added, so that the gravity field can be bet- ter understood. The authors are of the opinion that a reduced contoured map of gravity anomalies would not always be satisfactory if one wants to use the gravity 275 276 J. SEGAWA and C. BOWIN anomalies for his own purposes. This is the reason why this paper includes gravity tables, although it might appear to some as a mere waste of paper. There are three major junctions between oceanic trenches near Japan: They are, from northeast to southwest, the junction of the Kurile Trench with the Japan Trench (Kurile-Japan junction), the junction of the Japan Trench with the Izu-Bonin Trench (Japan-Bonin junction), and the junction of the Nankai Trough with the Ryukyu Trench (Nankai-Ryukyu junction). The regions at these three junctions are associated with broad gravity lows, where active seismicity and crustal movements have been observed (MIYA- MURA,1972; TSUBOKAWA,1972). In this paper the authors first discuss the relationship between the gravity low and the tectonic movement in the area of the Japan-Bonin junction, and then similar discussions are expanded to the other junctions between two adjoining trenches. 2. Gravity Data Gravity data from fourteen sources have been used; thirteen for the sea region and one for the Japanese Islands. In the following descriptions the LaCoste and Romberg land gravity meter, the LaCoste and Romberg air-sea gravity meter and the Tokyo surface ship gravity meter are denoted by L & R G-meter, L & R S-meter and TSSG, respectively. 1) L & R G-meter data covering the Japanese Islands obtained by the GEOGRAPHICALSURVEY INSTITUTE of JAPAN(1955, 1957, 1964, 1965, 1966 and 1969). 2) Data from the Vening Meinesz pendulum (KUMAGAI,1953; WORZEL, 1965). 3) L & R S-meter measurements of the Hakuho-maru cruise KH72-1 of the Ocean Research Institute between May and August 1972 (unpublished). 4) L & R S-meter measurements of the Hakuho-maru cruise KH72-2 between October and December 1972 (unpublished). 5) TSSG measurements of the Takuyo cruise of the Hydrographic Department, Maritime Safety Agency, Japan, in April 1966 (unpublished). 6) TSSG measurements of the Meiyo cruise of the Hydrographic Department in June 1968 (HYDROGRAPHICDEPARTMENT, 1969). 7) TSSG measurements of the Umitaka-maru cruise of the Tokyo Uni- versity of Fisheries from July to August 1966 (TOMODAand SEGAWA,1967). 8) TSSG measurements of the Umitaka-maru cruise from July to August 1967 (SEGAWA,1968). 9) TSSG measurements of the Hakuho-maru cruise from July to August 1967 (SEGAWA,1970). 10) TSSG measurements of the Umitaka-maru cruise from November Gravity in the Junction between the Japanese and the Izu-Bonin Islands 277 1967 to February 1968 (unpublished). 11) TSSG measurements of the Hakuho-maru cruise KH68-3 from July to August 1968 (SEGAWA, 1970). 12) TSSG measurements of the Hakuho-maru cruise KH69-1 in April 1969 (unpublished). 13) TSSG measurements of the Hakuho-maru cruise KH69-2 from April to June 1969 (unpublished). 14) Graf-Askania Gss2 meter measurements of the Lamont-Doherty Geological Observatory (Only plotted sheets of free air anomalies (WATTS and TALWANI, 1974) are available to the authors). The data sources Nos. 1, 2, 6, 7, 9 and 11 are documented in detail in the references where gravity tables are also given. Therefore, only the data sources Nos. 3, 4, 5, 8, 10, 12 and 13 are handled here, and the gravity logs during the measurements will be summarized as follows. The gravity data from these measurements are given in Tables 1 to 9. 2.1 Data source No. 3 KH72-1 cruise (OCEANRESEARCH INSTITUTE, 1975). Data numbers 1 to 99 in Table 1. The L & R S-meter (S-32) used for this measurement was operated on a gyro-stabilized platform. The meter was calibrated at each port of call by means of gravity ties with the known gravity stations on land. Unnegligible amount of haphazard drift of the meter was observed during the survey, as follows: Tables 1-9. Gravity tables. NO: Data number. JST: Japanese Standard Time. GMT: Greenwich Mean Time. LAT: Latitude. LON: Longitude. Depth: Water depth in meter. R: Normal gravity in gal, according to the International Gravity Formula of 1930. G: Observed gravity in gal. dGf: Free air gravity anomaly in mgal. dGb: Simple Bouguer gravity anomaly in mgal. Note: Employment of a new absolute gravity at Potsdam and a new gravity formula (1967) would give rise to the change of gravity anomalies of the areas con- cerned by the amount of less than 1.5 mgal. 278 J. SEGAWA and C. BOWIN Tab le 1 Gravity in the Junction between the Japanese and the Izu-Bonin Islands 279 280 J. SEGAWA and C. BOWIN Tab l e 2 Gravity in the Junction between the Japanese and the Izu-Bonin Islands 281 282 J. SEGAWA and C. BOWIN Table 3 Gravity in the Junction between the Japanese and the Izu-Bonin Islands 283 284 J. SEGAWA and C. BOWIN 4 Table Gravity in the Junction between the Japanese and the Izu-Bonin Islands 285 286 J. SEGAWA and C. BOWIN Table 5 Gravity in the Junction between the Japanese and the Izu-Bonin Islands 287 288 J. SEGAWA and C. BOWIN Table 6 Gravity in the Junction between the Japanese and the Izu-Bonin Islands 289 290 J. SEGAWA and C. BOWIN l e Tab7 Gravity in the Junction between the Japanese and the Izu-Bonin Islands 291 292 J. SEGAWA and C. BOWIN Table 8 Gravity in the Junction between the Japanese and the Izu-Bonin Islands 293 294 J. SEGAWA and C. BOWIN Table 9 Gravity in the Junction between the Japanese and the Izu-Bonin Islands 295 where Gg and Gs denote the gravity values measured at ports by the land meter and the sea meter respectively. The differences, Gs-Gg, indicate the sea meter drifts in the interval between two successive ports of call. The drift of the sea meter used was positive, with the magnitude of 3.5 mgal or less during the period of 10 to 20 days. These drifts have been corrected by assuming that the amount of the drift was proportional to the time lapse. Because of the malfunctioning of the sea meter, the authors could not make the calibra- tion at the last station (Tokyo) in this case. Ship's positions and speeds were measured by using not only the celestial and radio navigation systems but also continuous records of the ship's speed and heading. Although the satellite navigation equipment was not used in this case, the continuous records of the ship's speed and heading were very useful for the estimation of the Eotvos corrections. So far as the data used for this study are concerned, the errors of measurement resulted from the meter drift may be less than 1 mgal, whereas those resulted from inaccurate ship's speeds may be a few milligals. The sys- tematic errors of positioning may be usually one nautical mile or less, but in some adverse case it is suspected to be more. 2.2 Data source No. 4 KH72-2 cruise (OCEAN RESEARCH INSTITUTE, 1975). Data numbers 100 to 676 in Tables 1 to 3. The same L & R S-meter was used as during the cruise KH72-1. A Mag- navox satellite navigation equipment was installed on the Hakuho-maru. Cal- ibration of the sea meter by comparing with the land meter carried out at each port of call has resulted as follows: The differences, Gs-Gg, are not systematic.
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