ALBERT

Description: Published

Description: Hydrographic and Oceanographic Department of Japan has conducted geophysical surveys by multi‐beam soundings, KSS - 30 type ship gravity meter, and sea‐surface proton magnetometer in the western Pacific as a part of continental shelf surveys sinc 1992. The surveyed area includes Jurassic Magnetic Quiet zone and a large number of seamounts were confirmed. Using the topographic depth soundings and potential field data (free‐air gravity and magnetic anomalies), Bouguer gravity anomalies on the assumed density values of 2300kgm - 3 and 2700kgm - 3 were compiled. The effective elastic thickness was estimated for the several seamounts. These values ranging from 5km to 30km, with the largeset occurrences in 10km are consistent with the thermal rejuvenation due to mantle plumes occurred in Middle to Late Cretaceous time. Geophysical parameters（volume, density, magnetization）of 85 seamounts were calculated. A volume histogram indicates bi‐modal pattern, one peek is around 1000～2000km 3, and the other 5000km 3. This bi‐modal pattern may be related with the different scale of magma plumes forming these seamounts. Density values of the seamounts were estimated from the correlation between calculated and observed free‐air gravity anomalies. These density values show a mono‐modal peek around a mean density of 2694±253kgm-3. Magnetizations of the seamounts are also estimated by the least square inversion method using a topographic model and a gravity basement model derived from Bouguer gravity of the assumed density of 2300kgm-3. The precision index parameters(GFR) on the topographic model give usually larger values than those on the gravity basement model. This may suggest the depressed structure observed in the gravity basement is not real but artificial resulting from filtering process. Histogram of magnetization intensities of the seamounts shows considerable wide range distribution, contrary to the density pattern. This may be ascribed to differences in magnetic field intensity at the origin or in metamorphoses of magnetic minerals composing seamounts. The Virtual geomagnetic poles (VGPs) of these seamounts generally coincide with the VGP curve estimated from DSDP and ODP results (Sager, 2006), however, Hanzawa and Katayama seamount (4,6)and seamount D4 (48) show considerable displacement from the VGP. The origin of this misfit may arise from tilting effects of the seamounts , non‐dipole components of magnetic field, or the older seamount’s formation ages in comparison with radiometric age of 80 Ma. Stage Euler poles derived from hotpot tracks(Koppers et al., 2001) failed to the reproduction of the APWP estimated by paleomagnetic method. Stage Euler poles reproducing the paleomagnetic APWP and South Pacific Isotopic and Thermal Mantle Anomaly (SOPITA) origin of West Pacific seamounts, are calculated and listed in this article for the consideration of the plate motions.

Description: Published

Description: Hydrographic Department, M. S. A. Japan conducted airborne/land magnetic survey on Osima island to investigate magnetic structure and their magnetic change accompanied by volcanic activity after eruption on November, 1986. In airborne magnetic survey, total intensity was measured by proton precession magnetometer every two seconds. Positions just over island were fixed using video image, and that over sea were determined by VLF /OMEGA positioning system. Positioning error over island was assumed less than 100 m in the usual condition. Detailed magnetic anomaly maps were made based on above survey data as shown in this report. These magnetic anomaly maps may play an important role for future study of magnetism of Osima volcano accompanied by volcanic activity. Using the above survey data, three dimensional magnetic analysis were conducted by Talwanis method. These results show that magnetization vector of Mt. Mihara become nearly half of that of Osima volcanic edifice, which may imply thermal demagnetization process of Mt. Mihara. The total intensity field derived by magnetic survey in December 1986 were also compared with that in September 1964, which were reported by Utashiro et al. (1972) to obtain magnetic change for these 22 years. This comparison shows drastic magnetic decrease amounting more than 1000 nT near Mt. Mihara, although there still remain some problems to confirm such a large amount of magnetic change. In addition, some model calculations on thermal demagnetization were shown for interpretation of magnetic change accompanied by volcanic activity. Twice of Land magnetic survey were conducted, First one was from Jan 26 to Feb 2, 1987 and second from Mar 2 to Mar 6, 1986. Vector component of magnetic field were measured by GSI first order magnetometer and proton magnetometer at three station, SO (near weather observatory), FU (near Mt. Futago), UR (eastern slope of Osima island), besides total intensity were measured at five points within caldera-rim. No distinctive magnetic change were observed between two land magnetic surveys, however comparison with magnetic contour map made by Yokoyama I. (1954) show a large amount of change at FU station.

Description: Published

Description: Published

Description: Marine meteorological data observed at mooring buoy system off Joga-shima island in Sagami Bay were analyzed by BA YTAP-G program to obtain the tidal current harmonic constants and drift current induced by wind. The data were sampled every hour for the period from Jan. 1 to Dec. 31, in 1997. The observed current vector were analyzed in combination with wind vector. Through the above analysis, it has become clear that the wind effect enhances the diurnal components as much as 2 factor for the tidal current harmonic constants, which may arise from the sea land breeze system predominant in Sagami Bay. The response coefficients of drift current correlated with discrete wind velocities were obtained for several cases by changing lag parameters. Through this analysis, it became clear that the general pattern of drift current was consistent with Ekman spiral system. The present study also made apparent the origin of the sudden enhancement of coastal current velocity (overfalls) in the Sagami Bay. One origin is the approach of Kuroshio branch into the Sagami Bay as already pointed out by several authors, and the other is the augmentation of the tidal current amplitude. The latter phenomenon is newly found and it may be triggered by the approach of the Kuroshio branch to the coastal region.

Description: Published

Description: As a part of “Basic Sea Map" project,and partly of the “Earthquake PredictionPlan" the detailed geomagnetic, hydrographic, geological, and gravitational survey, was carried out by the Hydrographic Department, Maritime Safety Agency of Japan, at sea in the southern part of Japan Trench from 1976 to 1977. Making use of some of these results, the geomagnetic anomalies of both the Daiit-Kasima and Katori seamounts were analyzed by means of the Talwanis method, with special reference to the subduction hypothesis of the Daiiti-Kasima seamount. Throughout these results, the following features were apparent. 1) The breakdown of the western half of Daiiti- Kasima seamount, which was proposed by Mogi and Nishizawa (1980a), may be considered as the depression of the magnetic body to the extent of more than 1000 m in line with the Japan Trench axis. The further consideration of its geomagnetic structure suggests us that the Daiiti-Kasima seamount has been suffering from "accretion process" during the subduction of the Pacific Plate. 2) A large amount of the uplift (3600 m to 5250 m) of the Daiiti- Kasima seamount is nonmagnetic as a whole, and the rest ( 5250 m to 7000 m) has a magnetization vector defined by dec= － 12.0°, inc= 20.1°, /J/=1072×10-2Am-1. 3) For the Katori seamount, the magnetization vector of the top portion ( 4200 m to 4750 m) is dec=-15.2°, inc= 18.3°, /J/=1390×10-2Am-1, and the rest (4750 m to 7500 m) is dec=170.0°, inc=19.5°, /J/=520×10-2Am-1. This shows that the Katori seamount has a memory of successive normal and reversal magnetic events. 4) The VGP (Virtual Geomagnetic Pole) for Daiiti-Kasima is located at N62°, E349。, and that of the top portion of Katori is at N60°, E354 °. These results are consistent with the mean VGP from other Cretaceous Pacific seamounts, and support the hypothesis of the northward drift of the Pacific Plate. Key words: Solid earth geomagnetism, Japan Trench, Seamount, Palcomagnetism.

Description: Published

Description: The Hydrographic and Oceanographic Department of Japan have been conducted marine gravity surveys since 1965. Based on theses survey results, precise Bouguer gravity anomaly maps of the Japanese Island arcs and its adjacent seas were compiled by a newly developed terrain correction method (Ueda, 2003). Topographic mesh data with the size of 10” x 10” are used as basic mesh data for the terrain corrections of marine gravity survey data with line spacing about 2 mile or less, and 30” x 30” size mesh data for the marine gravity survey data at a scale of 1/500,000. For interpolations of the depths within a rectangular area defined by the 4 basic mesh data points, a weighted gradient interpolation method was used. Japanese island arcs and its adjacent continental shelf regions are covered by 6 sheets of Lambert’s conformal conic projection maps. Besides, the small scale gravity anomaly maps covering whole Japanese islands and its adjacent seas including the Shikoku basin, west Philippine sea, and the northwestern part of the Pacific ocean are also compiled in two sheets. These gravity maps recover the characteristic features of the gravity anomalies as described hereafter. (1) In the Tohoku Japan arc, a relative positive Bouguer anomaly zone runs in N-S in parallel to the Japan trench. This positive zone is separated into two zones at about N38˚00’, E141˚40’, incorespondig to the seaward extension of the Nanbu-Kitagami geological complex. (2) In the Suruga bay, contour lines of Bouguer anomalies are deflected northward sharply, which suggests the uplift of the lower crust beneath the Izu peninsula region. The local Bouguer anomaly highs are recognized in the inner region of the Nankai trough, such as in the vicinity of Shiono-Misaki Cape, MurotoMisaki and Ashizuri-Misaki Cape, and in the southwest of the Tosa Bay. These anomalies may be related with the zonation of the subducting Philippine plate. (3) Bouguer anomalies over the Okinawa trough show a convex feature reflecting the uplift of the lower crust, however, the maximum Bouguer anomaly value (~160mGal) itself is much less than that of the usual oceanic crust, which indicates the existence of the continental crust beneath the Okinawa trough. (4) In the Izu-Ogasawara arc, a high Bouguer anomaly zone is recognized in the fore-arc region, which continues southward to the Bouguer anomaly high associated with the Ogasawara ridge. In the northern arc, Bouguer anomalies decrease step-wisely toward the inner arc, showing two sharp gradient zones to the east of the volcanic front. The across-arc boundaries separating the Izu-Ogasawara arc also confirmed, one is the “Torishima-seiho tectonic line” connecting the places around N30˚30’, E140˚30’ and N29˚40’, E139˚00’ and another is the “Nishinoshima tectonic line” connecting N27˚50’, E141˚00’ and N26˚, E140˚00’, respectively. (5) Bouguer anomalies in the northern part of the Shikoku basin decrease eastward with gentle gradient, and, Free-air anomalies, oppositely, increase eastward. The relative Bouguer anomaly lows are recognized over the Amami Plateau, Daito Ridge, Oki-daito Ridge as well as Kyusyu Palau ridge, which may suggest the existence of the continental crusts beneath these ridges. The Bouguer anomalies associated with basins are as follows; Kikai basin is +320mGal, Kita-Daito basin is 300~370mGal, Minami-Daito basin is 270~330mGal and that of the Philippine basin in the south of the Oki-Daito Ridge is 370~380mGal respectively. These variations reveal the different thickness of the oceanic crusts. In the Philippine basin, The gravity features change drastically at the line connecting the places around N24˚, E131˚ and N21˚, E129˚, which may imply the tectonic line separating the Philippine basin in eastern and western part. Key wards: Bouguer gravity anomaly, Free-air gravity anomaly, Japan trench, Japan arc, Suruga trough, Nankai trough, Izu-Ogasawara arc, Okinawa trough, Shikoku basin, Amami Plateau, Daito ridge, Oki-Daito ridge, Kyusyu-Palau ridge, Philippine basin.

Description: Published

Description: In response to the eruption of Miyake-jima volcano starting in June 27, 2000 and the associated swarm earthquakes in the west of lvliyake-jima island, the comprehensive surveys and ocean bottom earthquake observation were conducted by the Japanese Hydrographic Department. Some of the survey results and the derived sub-bottom structure were presented with consideration of teetonics as to the northern part of the Zenisu ridge inferred from geophysical data of topography, magnetic and gravity anomalies covering the northern part of the Izu-shoto region The narrow multi-beam soundings in the west of the Miyake-jima island show the small volcanic cones in the southeast offing of the Kozu sluma island The seismic records proved the existence of many normal faults segmenting the sedimentary layers, which may imply the compressive stress field in NW-SE direction. It is also confirmed that the swarm earthquake region coincides well with the northeastern edge of magnetic basement high. Magnetic basement highs implying the dyke intru sion were also recognized between the Nii-jima and Kozu-shima islands. These features may suggest upwelling of magma through NW SE trending deep fractures caused by regional stress field. The general distribution of magnetic and gravity anomalies covering the northern part of the Izu shoto region, show the good correspondence with the tectonic lines mferred from seismic activity The northern part of the Zenisu ridge has been segmented into three part, Omurodashi ,Ni i jima, and Kozu-shima blocks, and in between there exist magnetic basement highs implying the dyke mtrusion thiough the deep fractures in the crust. The deflection of the major axis of Nii-jima islands is ascribed to the rotational movement caused by the huge dyke intrusions occurred in the east of the island.

Description: Published

Description: Since 1967 the Hydrographic Department has been carrying out the marine magnetic surveys on the continental shelves around Japan as a part of the project of the Basic Maρ of the Sea. Some analyses using these data have been already published by Oshima et al. (1975) and Segawa et al. (1976). Among various interesting features found in the magnetic survey data, the present paper deals with analytical studies on the anomaly caused by Seamount Zenisu Oki, located in the northern part of the Izu-Bonin Island Arc, and on the remarkable negative anomalies in the southern part of Okhotsk Sea (Oshima et al., 1975〕． Talwani’s (1965) method of the arbitrary shaped model is applied to evaluate the magnetization vector J together with the regional bias B. Test calculation for this method is made for Seamount Ryohu to yield satisfactory agreement with the result by Uyeda and Richard (1965), (Table 1). Then, analysis is made for Seamount Zenisu Oki, which is approximated by a polygon of five layers (Fig. 2), taking the depth of the seamount top to be 2200, 2700，……， 5200 m, successively (Table 2, Fig. 3). Tendency of G-value obtained from the analysis suggests that the anomaly source locates at the depth of 3300 m, i.e. about 1000 m below the actual top of the seamount. Comparing the inclination and declination derived from the most probable value of J with those at present, it may be considered that the seamount has drifted for about 15°～20° northward from its birth place to the present location. However, it remains another possibility that the seamount was formed in the west, because the position of the paleo-magnetic pole derived from the value of J deviates considerably from that by Nagata (1961). The negative anomaly in the southern part of Okhotsk Sea (Fig. 5) can be divided into regional and local properties. The regional anomaly is the one distributing from Japan Sea to Okhotsk Sea (Ueda, 1976). Model calculation is made to interpretate the local anomaly. It is found that a prism shaped body. having the magnetization vector of declination=0° and inclination= -45° or -90°, shows similar feature to the actual local anomaly. This fact suggests that the basic rocks in this area were magnetized in a reversal epoch during Neogene Tertiary.

Description: Published

Description: Geomagnetic total intensity anomalies at the sea around Nansei-Syoto（Ryukyu Islands) and Senkaku Islands were compiled into three sheets of geomagnetic anomaly maps based on the geomagnetic surveys made by the Hydrographic Department from 1975 to 1982. The survey area is divided into several zones according to the feature of the geomagnetic anomalies considering the geological/topographical characteristics of the region concerned, and geotectonic implication of the geomagnetic anomalies for each zone is investigated. A complicated anomaly zone exists along the Paleo-Ryukyu Volcanic Belt and the Ryukyu Volcanic Belt (Konishi, 1965) and a magnetic smooth zone is recognized over the area of the Ryukyu Geoanticline and the continental slope located in the east of the volcanic zones. The southern end of the Quaternary volcanic front located along the Tokara volcanic chain has been recognized near Io Tori Sima up to the present (Matsumoto, 1977), and it is newly recognized that the front extends along the east boundary of the Okinawa Trough and the south boundary of the West Ryukyu knolls group from the results of geomagnetic and geological/topographical surveys (Kato, et al., 1982). The fact that the short wavelength magnetic anomalies dominate along the volcanic front in contrast to a rather long wavelength feature of the topographic undulations may indicate that the basic rocks intruding into the non magnetic Pre-Miocene basement complex form the volcanic islands and knolls of the Tokara volcanic chain. There are no magnetic anomalies over the uplift of knolls and bank inside the Tokara volcanic chain. The positive magnetic lineated anomaly zone is seen from Kume Sima to Igyo-Sone bank to the west of Okinawa Sima. The fact that there was a green tuft volcanism in the Miocene at Aguni Sima may indicate a volcanic activity in the Miocene in the area of positive magnetic anomaly. The Tokara channel and Kerama Gap have been considered to be left-lateral faults which cut the zonal structure of the geological and topographical features of this area, but this consideration is not positively supported by the magnetic anomalies.

Description: Published