Nobukazu Seama

Global superconducting gravimeter observations and the search for the translational modes of the inner core

Abstract With the cooperation and collaboration of Courtier (Cantley), Ducarme (Brussels), Goodkind (Pinon Flats), Hinderer (Strasbourg), Imanishi and Seama (Kakioka), and Sun (Wuhan), a very large data set of global superconducting gravimeter observations (294,106 h) has been assembled for analysis. Except for the Brussels record, for which residuals were provided, synthetic tides for each station generated by Merriams G-wave code were used to remove tidal signals, and barometric effects were fitted and corrected off. An examination of the resulting residuals typically shows spikes and offsets (glitches) which cannot be true gravity observations and which raise the background noise level dramatically. In the case of the Cantley record, 1-min samples were available and a spline differentiator was used as an automated glitch detector. Fifteen previously undetected glitches were found and repaired by spline interpolation. Each has been associated with a real physical event by comparison with daily log files from the site and from the data acquisition system. The repaired record is very low noise, comparable to the first long record taken at Pinon Flats. The other stations are in less favourable sites but still provide long continuous records with the exception of Kakioka which has many short gaps. Both the Kakioka and Wuhan instruments have subsequently been moved to better sites. In the absence of detailed station logs as were available for Cantley, a more arbitrary record repair procedure was developed with hour-to-hour changes of more than 1 μgal being replaced by gaps of constant values, and the previous and subsequent record levels adjusted to eliminate discontinuities at the end points of all gaps. This procedure was applied to both the Strasbourg and Kakioka series. Power spectra for each residual series were then estimated using a 12,000-h Parzen window with 75% overlap. Product Spectra for data entirely outside Europe confirm the presence of the three resonances found earlier in data from Europe alone [Smylie, D.E., Hinderer, J., Richter, B., Ducarme, B., 1993. The product spectra of gravity and barometric pressure in Europe. Phys. Earth Planet. Inter., 80, 135–137.], and associated with the three translational motions of the inner core by the strict adherence of their central periods to splitting laws [Smylie, D.E., 1992. The inner core translational triplet and the density near Earths center. Science, 255, 1678–1682.]. The observed periods of the equatorial translational modes provides a very precise measure of viscosity near the ICB [Smylie, D.E., McMillan, D.G., 1998. The Inner Core as a Dynamic Viscometer. Phys. Earth Planet. Inter., this volume.]. The gravity signals from the equatorial translational modes travel in longitude at fixed rates in the opposite sense, while that from the axial mode is axisymmetric and does not move in longitude. The signals also have different dependencies on latitude. We have developed a method of exploiting these differences for simultaneous data from a distributed network of stations and tested it with synthetic data. Initial results from a test on real data using the simultaneous portions of the Brussels, Cantley, Kakioka, Strasbourg and Wuhan records are presented.

Upper mantle electrical resistivity structure beneath the central Mariana subduction system

This paper reports on a magnetotelluric (MT) survey across the central Mariana subduction system, providing a comprehensive electrical resistivity image of the upper mantle to address issues of mantle dynamics in the mantle wedge and beneath the slow back-arc spreading ridge. After calculation of MT response functions and their correction for topographic distortion, two-dimensional electrical resistivity structures were generated using an inversion algorithm with a smoothness constraint and with additional restrictions imposed by the subducting slab. The resultant isotropic electrical resistivity structure contains several key features. There is an uppermost resistive layer with a thickness of up to 150 km beneath the Pacific Ocean Basin, 80–100 km beneath the Mariana Trough, and 60 km beneath the Parece Vela Basin along with a conductive mantle beneath the resistive layer. A resistive region down to 60 km depth and a conductive region at greater depth are inferred beneath the volcanic arc in the mantle wedge. There is no evidence for a conductive feature beneath the back-arc spreading center. Sensitivity tests were applied to these features through inversion of synthetic data. The uppermost resistive layer is the cool, dry residual from the plate accretion process. Its thickness beneath the Pacific Ocean Basin is controlled mainly by temperature, whereas the roughly constant thickness beneath the Mariana Trough and beneath the Parece Vela Basin regardless of seafloor age is controlled by composition. The conductive mantle beneath the uppermost resistive layer requires hydration of olivine and/or melting of the mantle. The resistive region beneath the volcanic arc down to 60 km suggests that fluids such as melt or free water are not well connected or are highly three-dimensional and of limited size. In contrast, the conductive region beneath the volcanic arc below 60 km depth reflects melting and hydration driven by water release from the subducting slab. The resistive region beneath the back-arc spreading center can be explained by dry mantle with typical temperatures, suggesting that any melt present is either poorly connected or distributed discontinuously along the strike of the ridge. Evidence for electrical anisotropy in the central Mariana upper mantle is weak.

Distinct regional differences in crustal thickness along the axis of the Mariana Trough, inferred from gravity anomalies

We have compiled extensive gravity and bathymetry data for the whole Mariana Trough, which were collected during several Japanese scientific cruises over the last few years. This study aims to clarify the lateral distribution of the local differences in geochemical signatures, which have been observed locally in the Mariana Trough. Shipboard free-air gravity anomaly data from eight Japan Agency for Marine-Earth Science and Technology (JAMSTEC) cruises were compiled with those crossover errors of 2.85 mgal. Mantle Bouguer anomalies (MBA) were calculated by subtracting the predictable gravity signal due to the seawater/crust and crust/mantle density boundaries. The crustal thickness variation along the spreading axis was estimated from the MBA. Different features in crustal thickness, its variation, and segment length for each segment, allow us to identify four distinct regional differences in magmatic activity along the spreading axis of the Mariana Trough. Segment in region A (to the north of 20°35′N) shows the largest sectional dimensions of crust along the axis and it is probably affected by an additional supply from island arc magma sources. A variety of crustal thickness values and of along-axis crustal thickness variations in region B (between 15°38′N and 20°35′N) suggests two types of segments. One is similar to a slow spreading ridge segment that has a plume-like mantle upwelling under the spreading axis, and the other is a magma-starved segment. Region C (between 14°22′N and 15°38′N) is a less magmatic region (individual crustal thickness averages of 3.4–4.1 km). Region D (to the south of 14°22′N) has higher individual crustal thickness averages of 5.9–6.9 km, suggesting higher magmatic activity with a sheet-like mantle upwelling under the spreading axis. Different features in the MBA for off-axis areas suggest that these four regions have existed since the Mariana Trough started spreading. Moreover, comparison between our results of crustal thickness and previous geochemical results indicates that less magmatic spreading segments with thin crust, which are locally distributed in both regions B and C, probably result from mantle source depleted of water and incompatible elements. This suggests that lateral compositional variation of water and incompatible elements exists on a segment scale in the mantle source beneath the spreading axis of the Mariana Trough.

A marine deep-towed DC resistivity survey in a methane hydrate area, Japan Sea

Abstract We have developed a new deep-towed marine DC resistivity survey system. It was designed to detect the top boundary of the methane hydrate zone, which is not imaged well by seismic reflection surveys. Our system, with a transmitter and a 160-m-long tail with eight source electrodes and a receiver dipole, is towed from a research vessel near the seafloor. Numerical calculations show that our marine DC resistivity survey system can effectively image the top surface of the methane hydrate layer. A survey was carried out off Joetsu, in the Japan Sea, where outcrops of methane hydrate are observed. We successfully obtained DC resistivity data along a profile ~3.5 km long, and detected relatively high apparent resistivity values. Particularly in areas with methane hydrate exposure, anomalously high apparent resistivity was observed, and we interpret these high apparent resistivities to be due to the methane hydrate zone below the seafloor. Marine DC resistivity surveys will be a new tool to image sub-seafloor structures within methane hydrate zones.

Sea-floor magnetization in the eastern part of the Japan Basin and its tectonic implications

Abstract The tectonic history of the Japan Basin is revealed by the distribution of magnetization and the topography of basement in the eastern part of the Japan Basin: 40°30–43°00′N, 136°30–139°00′W. The vector and total intensity of the geomagnetic field were measured on the R/V “Hakuho-maru” expedition in 1986 and the R/V “Tansei-maru” expedition in 1987 with a track spacing of about 4 nautical miles. Geomagnetic anomalies were obtained from these data and from other cruises using IGRF/DGRF. Furthermore, data on sediment thickness were compiled with all the available seismic profiles from the area. A depth to basement map was prepared using a regression equation of velocity in the sediments and one-way reflection time. Inversions were performed using both the gridded geomagnetic anomaly field and the depth to basement data to calculate a gridded distribution of magnetization. The key results of the distribution of magnetization are summarized in the following three categories: 1. (1) Magnetic lineations exist only in the northern part of the area studied where the basement is deepest and fairly flat. The lineations have two trends: N40°E in the north and N60°E in the south. 2. (2) No magnetic lineations were observed in the morphologically high relief area in the southernmost part of the area studied. (3) Some round-shaped distributions of magnetization exist. The three categories correspond to oceanic, continental and volcanic areas, respectively. They are probably characteristic features of rifting along the continental margin.

Gravity anomalies of the Mid-Atlantic Ridge north of the Kane Fracture Zone

High-resolution gravity and multibeam bathymetric data were newly obtained from the Mid-Atlantic Ridge (MAR) between latitudes 23°30′N and 28°N. Mantle Bouguer anomalies (MBA) show magmatic accretion focused at discrete centers along the spreading axis except near the oblique portion of the MAR between 24°30′–24°50′N. The MBA also indicates that crustal production near the TAG hydrothermal field has been relatively low during the last 2–3 My. The MBA show 120°-striking off-axis traces, indicating northward migration of the oblique portion and northward extension of the segments south of the oblique zone despite a long-wavelength southward deepening of the axial topography. Observed gravity anomalies support a tectonic interpretation proposed from morphology and geomagnetic anomalies that the segment south of the oblique zone has extended northward by repeatedly adding short segments which migrated westward through asymmetric spreading in the oblique zone.

A study on correction equations for the effect of seafloor topography on ocean bottom magnetotelluric data

Consideration of the effect of seafloor topography on ocean bottom magnetotelluric data is needed to estimate a reliable resistivity structure. Selection of the correction equation which precisely takes account of distortions is important when we correct the effect of seafloor topography with forward modeling. Corrections of synthetic data based on three different correction equations are carried out, and the corrected responses and the true response are compared to investigate which correction equation is the best in application to distorted magnetotelluric data on the seafloor. Differences in the corrected responses are remarkable at periods shorter than several thousands seconds. These differences are caused by differences in treatment of the distortions of magnetic field and in robustness of the correction equation to a resistivity structure assumed for the topographic correction. The results suggest that the correction equation of Nolasco et al. (1998) should be used because it better accommodates the estimation of unknown resistivity structures. We apply the correction equation of Nolasco et al. (1998) to a response observed in the Mariana area. The one-dimensional resistivity structure estimated from the corrected response explains almost all the components of the observed response.

Magnetic anomaly lineations and fracture zones deduced from vector magnetic anomalies in the West Enderby Basin

Abstract Vector geomagnetic anomaly field data obtained in the West Enderby Basin. Strikes of two-dimensional magnetic structures at the positions of their boundaries were determined using these data. The magnetic strikes were divided into two groups based on structural change and magnetic polarity change. A change in structural trend occurred around 500 km off the Antarctic coast to the east of Gunnerus Ridge. Approximately NNE-SSW structural trends dominate in the northern portion of the study area. In contrast, in the southern part, structural trends range between NW-SE and NNW-SSE directions. The latter trend is dominant just seaward of the continental slope of Antarctica, and is almost perpendicular to the coastline. ENE-WSW and E-W magnetic anomaly lineation trends, possibly belonging to the Mesozoic magnetic anomaly lineation sequence, are also observed in the southern part. Therefore, we surmise that the NW-SE and NNW-SSE-trending structures east of Gunnerus Ridge correspond to fracture zones formed during Mesozoic time and reflect the initial Gondwana break-up trend.

Hydrothermal heat mining in an incoming oceanic plate due to aquifer thickening: Explaining the high heat flow anomaly observed around the Japan Trench

To explain the origin of a high heat flow anomaly observed within 150 km seaward of the Japan Trench, we construct a thermal model for an oceanic plate prior to subduction that includes the effect of hydrothermal circulation within a high-permeability aquifer in its uppermost part. The model includes the effects of aquifer thickening, which is expected to occur near subduction zones where plate bending prior to subduction causes fracturing and faulting within the oceanic plate. Using typical parameter values for the Japan Trench, we find that hydrothermal circulation in the thickening aquifer mines heat from the underlying basement and can account for the observed high heat flow anomaly. The ratio of heat supply below the aquifer as a result of aquifer thickening to the inverse of the thermal resistance of the sediment layer is a control parameter for the system. As long as the aquifer permeability is higher than ∼10−13 m2, a typical value for the uppermost part of the oceanic plate, variations in other details of the hydrothermal circulation such as the exact value of the aquifer permeability and the size of the convection cells do not significantly change model results. Despite its strong influence on seafloor heat flow seaward of the trench, this hydrothermal heat mining does not affect significantly the thermal structure of the subducted oceanic plate. This finding indicates that surface heat flow anomaly around the trench may not correspond to temperature anomaly within the subducted oceanic plate and the megathrust seismogenic zone.

1-D resistivity structures of the oceanic crust around the hydrothermal circulation system in the central Mariana Trough using Magnetometric Resistivity method

We present preliminary results of an electromagnetic survey over a region of hydrothermal circulation in the spreading axis of the central Mariana Trough. The Magnetometric Resistivity (MMR) method is used to determine the electrical resistivity structure of the oceanic crust around the hydrothermal system. 1-D electrical resistivity structure is calculated from the relation between the amplitude of magnetic field and source-receiver separation. The amplitudes of magnetic field variation of ocean bottom magnetometers (OBMs) placed on axis are larger than those of OBMs about 700 m away from the spreading axis, for source-receiver separations larger than 400 m. We estimated two resistivity structure models: one for the axial OBMs, and another for the off-axis OBMs. A region of lower resistivity between depths of 100–300 m is identified in the on-axis model. The hydrothermal source of the Alice Springs Field probably exists beneath the spreading axis, and the size of this source is smaller than 700 m. Lower resistivities at a depth of 50 m are identified in both the on-axis and the off-axis models, suggesting that the porosity in the uppermost oceanic crust is largest at this depth.