Jiro Segawa

Ocean tide model obtained from TOPEX/POSEIDON altimetry data

Tidal corrections are very important for oceanographic study of altimetry because tidal signals contaminate the low-frequency part of raw altimetric signals. Accurate tidal predictions are also necessary for geophysical research, such as with the superconducting gravimeter (SCG), which needs to distinguish very weak signals from the overwhelming tidal signals. The aim of this paper is to derive accurate global ocean tide models from the TOPEX/POSEIDON sea surface height data of 5 cm accuracy, more accurate than any other previous altimeter data. TOPEX geophysical data records of cycles 9–94 were processed and analyzed. We describe response analysis of tidal waves at crossover points and application of hydrodynamical interpolation. The tidal solutions with 0.5° grid system are obtained for eight major constituents (i.e., M2, S2, N2, K2, K1, O1, P1, and Q1) and compared with the Schwiderski (1980a, b, c) model, the Cartwright and Ray (1991) model, and the eight-constituent version of the Egbert et al. (1994) model or TPXO.2. The accuracy of the models is checked using island and bottom tide gauge data in open ocean and using SCG data in Antarctica and Japan.

THE TOKYO SURFACE SHIP GRAVITY METER: RECENT DEVELOPMENTS AND RESULTS OF COMPARISON MEASUREMENTS

The Tokyo Surface Ship Gravity Meter (TSSG) employs a single vibrating string accelerometer which is inherently nonlinear. This leads to errors unless its frequency is essentially constant over the sampling intervals. A new processor incorporating pulse train logic and employing a rapid sampling rate of about 50/sec has been designed to replace the digital computer which used about 2/sec.

Deep crustal structure of the eastern Nankai Trough and Zenisu Ridge by dense airgun–OBS seismic profiling

Abstract An unprecedentedly extensive seismic refraction and wide-angle reflection survey using 65 ocean bottom seismographs revealed detailed crustal structure around the eastern Nankai Trough. A previously published crustal model shows an abrupt offset of the Moho at the south of the Zenisu Ridge, a prominent topographic high along the oceanward slope of the Nankai Trough. Our crustal model indicates that this offset of the Moho extends southwestward continuously to 138°E, decreasing its gap. The survey area experienced the last two great earthquakes in 1854 and 1944. However, the northeastern part of the survey area seems to have remained unruptured since the 1854 event. Factors controlling the size of the rupture area for great earthquakes are still a matter of debate. There are several candidates for these factors in the survey area: hypothetical tectonic boundaries that may or may not be oceanward prolongation of major on-land tectonic lines, estimated locations of slab disruption, and the extent of Moho offset along the strike of the Zenisu Ridge. The main purpose of this survey is to clarify the relation between the crustal structure and these geophysical and geological features bounding the rupture area. Our crustal model from the trough axis to the continental slope is characterized by a well-developed sedimentary wedge bounded by island arc crustal blocks, consisting of upper and lower crust, to the northwest. Furthermore, the subducting oceanic crust, which can be traced down to 25 km depth, shows that the down-dip angle steepens at 55 km landward from the trough axis. On the basis of compilation of our crustal model with previously published models around the eastern Nankai Trough, we derived an image of the entire subducting plate geometry for depths shallower than 20 km, which is still poorly constrained by the land observation of microearthquakes. Significant lateral variations of the crustal structure and the slab geometry are recognized along one prominent canyon, and the offset of the Moho at the south of the Zenisu Ridge disappears to the southwest of the canyon. Moreover, it seems that the slab disruption recognized at a depth greater than 20 km is connected to this canyon. Therefore, the lateral variation of the crustal structure along the canyon may be one of the causes to stop rupture propagation of great earthquakes. Furthermore, the crustal variation may also form a tectonic boundary that distinguishes the subduction pattern of the Philippine Sea plate, including the influence of the Izu–Ogasawara collision, in the eastern Nankai Trough from the simple subduction pattern of the western Nankai Trough.

Tectonic context of fluid venting at the toe of the eastern Nankai accretionary prism: Evidence for a shallow detachment fault

During the Kaiko-Nankai diving cruise the peak of the venting activity was located near the top of the very first anticline. The most prominent morphological feature between the mid-slope (3870 m) and the apex of the fold (3770 m) is a 20 m high cliff cutting through subhorizontal massive mudstones affected by numerous joints. The trend of this scarp is oblique to the fold axis and structurally controlled along two sharply defined NNE-SSE and E-W directions. Fresh talus and blocks found locally suggest active tectonics and recent erosion. Intense deformation is evident from strongly tilted strata restricted to the base of the cliff that we interpret as an upslope thrust. At the scale of Seabeam mapping, this thrust can be followed eastward for more than 5 km along the 3820 m isobath. Two seismic lines recorded during one of the pre-site surveys show deformation at shallow depth, including small-scale folding and thrusting affecting only the wedge-shaped top sequence. Deeper layers can be traced continuously below this sequence. We conclude that the boundary between the “piggy-back” basin and the frontal fold turbidites acts as a shallow detachment fault, and interpret the base of the cliff as the outcrop of the fault. Dense colonies ofCalyptogena clams and strongly nonlinear thermal gradients locate the major peak of fluid activity at the edge of the plateau above the main cliff. Scattered biological colonies as well as white bacterial mats and cemented chimneys were also found in a narrow belt along the base of the cliff. Fluid activity is thus closely related to the shallow detachment fault, fluid being expelled both at the outcrop of the fault and above it through the overlying strata, possibly using the very dense joint network.

Geophysical study of the mafic belts along the margins of the japanese islands

Abstract Along the margins of the Japanese Islands there are large belts of mafic rocks that are now deeply buried. The origin of the belts is unknown: they could be either remnants of oceanic crust now trapped within the continental area, or they could be igneous bodies of continental origin. In this paper a discussion of this problem is given and new results presented.

EMRIDGE: The electromagnetic investigation of the Juan de Fuca Ridge

From July to November 1988, a major electromagnetic (EM) experiment, known as EMRIDGE, took place over the southern end of the Juan de Fuca Ridge in the northeast Pacific. It was designed to complement the previous EMSLAB experiment which covered the entire Juan de Fuca Plate, from the spreading ridge to subduction zone. The principal objective of EMRIDGE was to use natural sources of EM induction to investigate the processes of ridge accretion. Magnetotelluric (MT) sounding and Geomagnetic Depth Sounding (GDS) are well suited to the study of the migration and accumulation of melt, hydrothermal circulation, and the thermal evolution of dry lithosphere. Eleven magnetometers and two electrometers were deployed on the seafloor for a period of three months. Simultaneous land-based data were made available from the Victoria Magnetic Observatory, B.C., Canada and from a magnetometer sited in Oregon, U.S.A.Changes in seafloor bathymetry have a major influence on seafloor EM observations as shown by the orientation of the real GDS induction arrows away from the ridge axis and towards the deep ocean. Three-dimensional (3D) modelling, using a thin-sheet algorithm, shows that the observed EM signature of the Juan de Fuca Ridge and Blanco Fracture Zone is primarily due to nonuniform EM induction within the ocean, associated with changes in ocean depth. Furthermore, if the influence of the bathymetry is removed from the observations, then no significant conductivity anomaly is required at the ridge axis. The lack of a major anomaly is significant in the light of evidence for almost continuous hydrothermal venting along the neo-volcanic zone of the southern Juan de Fuca Ridge: such magmatic activity may be expected to have a distinct electrical conductivity signature, from high temperatures, hydrothermal fluids and possible melt accumulation in the crust.Estimates of seafloor electrical conductivity are made by the MT method, using electric field records at a site 35 km east of the ridge axis, on lithosphere of age 1.2 Ma, and magnetic field records at other seafloor sites. On rotating the MT impedance tensor to the principal axis orientation, significant anisotropy between the major (TE) and minor (TM) apparent resistivities is evident. Phase angles also differ between the principal axis polarisations, and TM phase are greater than 90° at short periods. Thin-sheet modelling suggests that bathymetric changes accounts for some of the observed 3D induction, but two-dimensional (2D) electrical conductivity structure in the crust and upper mantle, aligned with the ridge axis, may also be present. A one-dimensional (1D) inversion of the MT data suggests that the top 50 km of Earth is electrically resistive, and that there is a rise in conductivity at approximately 300 km. A high conductivity layer at 100 km depth is also a feature of the 1D inversion, but its presence is less well constrained.

Numerical estimation of the sea effect on the distribution of induction arrows in the Japanese island arc

Abstract Anomalous distribution of induction arrows in the Japanese island arc has been supposed to be due, at least partly, to induced electric currents flowing in the surrounding seas. None the less, no reliable estimation has been made of this regional problem. Here we attempt to solve the problem, using the algorithm of bimodal electromagnetic induction in a thin-sheet conductor. The non-uniform conductance of the thin sheet corresponds to seawater depths in the surrounding seas. It is found out that induction arrows are controlled strongly by the open seas such as the Pacific Ocean. The effect of the semi-closed Japan Sea is somewhat weaker, and is dependent on period. A detailed comparison between the calculated and the observed induction arrows over the Japanese island arc indicates, however, that the sea effects alone cannot fully account for the observations. The differences between the observed and the calculated responses delineate lateral inhomogeneities in the subsurface conductivity structure which are not included in the present model.

Detecting fluid circulation by electric field variations at the Nankai Trough

An ocean bottom electrometerobel was installed at the end of August 1989 on the landward slope of the Nankai Trough (33°38.56′N, 137°54.54′E, 3790 m deep). This electrometer has 40 m long electrodes, which were extended with the aid of the manipulators of the submersibleNautile. The electrometer was successfully retrieved at the end of November 1989, having provided electric field variations comprising both north-south and east-west components for as long as 80 days. The data obtained were analyzed by applying thebaytap-g analysis software, which was developed originally for the analysis of the Earth Tide. The electric field variations were divided into geomagnetic induction, oceanic tide and long-period trend components. The residual trend component was obtained by subtracting the oceanic dynamo effect caused by the strong western boundary ocean current around the Kaiko-Nankai area from the trend component. An order estimation of streaming potential variations has proved that the residual component thus obtained may be a direct monitor of the subsurface fluid venting through faults. The numerous colonies of clam shells that have been found in this area also indicate sub-surface fluid venting through faults.

OFFSHORE EMSLAB: objectives, experimental phase and early results

Abstract A large electromagnetic (EM) experiment dedicated to the exploration of lithosphere and asthenosphere associated with a spreading oceanic plate over its various tectonic regimes, from ridge accretion to subduction, was carried out during the second half of 1985 over an area extending from the Juan de Fuca Ridge, eastward across the coastal zone, the Cascade area and beyond. Referred to by the acronym ‘EMSLAB’, for ‘EM Sound of Lithosphere and Asthenosphere Beneath’ the Juan de Fuca Plate, the experiment involved two principal arrays, one on land, the other on the adjacent seafloor. We report here on the outcome of the oceanic portion of EMSLAB, named OFFSHORE EMSLAB for convenience. The OFFSHORE EMSLAB array of seafloor instrumentation included 40 self-contained and free recording units including magnetometers, electrometers and other oceanographic devices. The latter were intended to provide information on the EM fields generated by the interaction of oceanic motions with the main Earths field, so as (1) to decontaminate ionospheric signals prior to magnetotelluric interpretation, and (2) to illustrate the beneficial contributions that EM observations may provide to ocean studies. The collected database is described and assessed, and an early illustration of its information content is given.

Density structure of the Mariana Arc and its vicinity obtained from successive inversion of the gravity anomaly

Abstract Free-air anomalies measured by surface ships and those derived from GEOS-3/SEASAT-1 satellite altimetry were both used to determine the density structure in the upper mantle of the Mariana Arc and its vicinity. Compilation of the updated free-air anomaly data was made by minimizing crossover errors in the ship measurements and adjusting regional discrepancies of altimetry-derived free-air anomalies to fit the ship measurements. Referring to the assumed density model for the flat basin of the Western Pacific, a two-dimensional density structure across the Mariana Arc was investigated on the basis of the residual gravity anomaly (RGA), to which a successive inversion method was applied. The structure estimated from the first RGA shows that, in the regional sense, the lithosphere of the Western Philippine Basin is 50–60 km thick and that of the Parece Vela Basin is 20–40 km thick, while the result also shows the existence of a locally anomalous structure from the West Mariana Ridge over to the East Mariana Ridge. Leaving the lithosphere determined from the first RGA as it was, the second RGA, which is regarded as being caused by an asthenospheric anomaly, was derived and used to estimate the locally low-density body. The estimation shows that an abnormally low-density body lies between the Mariana Trough and the East Mariana Ridge. If the low-density body is assumed to be confined from the Moho to the asthenospheric surface at a depth of 80 km, it can be concluded that the mean density of the body is 3.18 g/cm3 and that it is 65 km thick and 500 km wide. The location of the low-density body is near the apparent volcanoes of the Mariana Arc.