A new type of Sea Floor Electro Magnetic Station (SFEMS) has been newly developed by adding a magnetotelluric (MT) variograph to its prototype built previously (Toh and Hamano, 1997). New SFEMS is able to conduct long-term electromagnetic (EM) observations at the seafloor, which is one of the principal goals of the Ocean Hemisphere Project (OHP). Long-term seafloor EM observations enable us to probe into the deep Earth (both the mantle and the core) by improving the spatial coverage of the existing EM observation network. The SFEMS has been tested in three sea experiments to yield 3 components of the geomagnetic field, 2 horizontal components of the geoelectric field and 2 components of tilts in addition to the absolute geomagnetic total force. The SFEMS is designed for measuring these EM signals at the seafloor continuously for as long as 2 yrs. The SFEMS mainly consists of the following three parts: (1) An Overhauser proton precession magnetometer for the absolute measurements of the geomagnetic total force with a possible bias of less than 10 nT. (2) An MT variograph that measures the rest of the EM components and tilt. (3) An Acoustic Telemetry Modem (ATM) that allows us to control/monitor the seafloor instrument as well as data transmission at the maximum rate of 1200 baud. Construction of seafloor EM observatories in regions where significant EM data have never been collected is now quite feasible by development of the SFEMS.

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We have developed systems for measuring differential displacements across a fault zone, and examined their resolutions through seafloor experiments at relatively short baselines. A system for a seafloor extensometer makes use of precise acoustic ranging with a linear pulse compression technique. The system has a resolution better than 1 cm in acoustic ranging over a baseline of at least 1 km. The most critical problem is correction for temperature variations, and we estimate that the effect can be corrected with cm-order accuracy in the case of a deep-sea experiment. We have also examined a leveling system on the seafloor using an array of ocean bottom pressure gauges and an ocean bottom gravimeter to detect differential vertical motion. The system is estimated to have a resolution of several centimeters in vertical displacement. These system will be useful for triangulation and leveling on the seafloor, but we need further studies over a longer baseline and to achieve better long-term stability.

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In order to better understand earthquake generation, tectonics at plate boundaries, and better image the Earth's deep structures, real-time geophysical measurements in the ocean are required. We therefore attempted to use decommissioned submarine cables, TPC-1 and TPC-2. An OBS was successfully linked to the TPC-1 on the landward slope of the Izu-Bonin Trench in 1997. The OBS detected co-seismic and gradual changes during a Mw 6.1 earthquake just below the station at 80 km depth on November 11, 1997. A pressure sensor co-registered a change equivalent to 50 cm sea-level change. This suggests a high possibility detecting silent earthquakes or earthquake precursors if they exist. A multi-disciplinary geophysical station has been developed for deep-sea floor using TPC-2 since 1995. The station comprises eight instrument sets: broadband seismometers, geodetic measurements, hydrophone array, deepsea digital camera, CTD, etc. These activities are examples that decommissioned submarine cables can be great global resources for real-time cost-effective geophysical measurements on a deep-sea floor.

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We describe the scientific purposes and experimental set-up of an international deployment of a 3 component broadband seismometer package on the ocean floor in Monterey Bay which took place during the summer of 1997. Highlights of this experiment were the installation, performed using a remotely operated vehicle (ROV), the underwater connection of the different components of the package, and the successful retrieval of 3 months of broadband seismic and auxiliary data. Examples of recordings of teleseisms and regional earthquakes are presented and the background noise characteristics are discussed, in comparison with those of near-by broadband land sites, current-meter data from the vicinity of the ocean bottom package, as well as pressure data from deeper ocean sites.

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The three-dimensional shear wave velocity structure in the upper mantle beneath the Philippine Sea was investigated with Rayleigh wave phase velocities in the periods 30-100 s. More than 900 Rayleigh wave phase velocity curves were obtained for this region with good path coverage. The phase velocity data were inverted for the phase velocity distribution maps in the Philippine Sea with 2-D tomographic technique without any a priori regionalization. The resolutions of the tomographic analysis were quite good in almost of the target region. The phase velocity maps were inverted for the 3-D shear wave velocity structure in the upper mantle down to 220 km. In the shallow depths lateral heterogeneities with short wavelength were seen in the shear wave velocity maps. This might be related with complicated surface structures. In the middle depths the shear wave velocity was well correlated to the main tectonic features seen at the surface and well explained by the evolution history of the Philippine Sea. The older western Philippine Sea had higher shear wave velocities than the younger eastern Philippine Sea. In the western Philippine Sea the central basin ridge, which is the youngest in this area, showed the low velocity anomaly. This is supported by the fact that the West Philippine Basin was formed in this area. In the depths 150-200 km the low velocity anomaly was dominant inside the Philippine Sea, which might suggest the existence of the mantle return flows. The thickness of the lithosphere in the south of the West Philippine Basin reached about 100 km, which is much thicker than the results of previous studies for this region.

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Recent P wave travel-time tomographic studies using data from the International Seismological Centre (ISC) catalog determine a large-scale subhorizontal high velocity anomaly in the northwestern Pacific subduction zones and it has been interpreted as imaging stagnant slab in the upper mantle transition zone (-400 to 700 km). The limited resolution of the travel time tomographic studies in this depth range, however, makes it difficult to evaluate accurately the vertical and lateral extent of a stagnant slab. A broadband waveform modeling of triplicated regional seismic waves which are very sensitive to the transition zone structure is useful to evaluate the velocity structure along the propagation paths and therefore to constrain the spatial distribution of anomalies. This study thus compares tomographic images from the model of Obayashi et al. (1997) with results of the regional waveform modeling by Tajima and Grand (1998). The ISC tomographic model shows the largest lateral extent of high velocity anomaly in the layer of 478 to 551 km depths although part of this spread is likely due to the deteriorated resolution in that depth range. The waveform modeling suggests that the strong high velocity anomaly associated with a stagnant slab exists below 525 km with its maximum intensity in the top 50 km and decreases with increasing depth to vanish at 660 km. These results along with a recent global SH velocity model SAW12D of Li and Romanowicz (1996) which has the strongest high velocity anomaly in a depth range 500-550 km may be integrated into an image of a stagnant slab. The anomalous velocity structure associated with a stagnant slab has its maximum intensity not immediately above the 660 km discontinuity but in a depth range -100 km above it. This feature appears to be consistent with a thermochemical model of down-going slab in which a larger velocity contrast with the surrounding mantle is expected at a shallower depth of the transition zone. The ISC tomographic model and waveform modeling consistently show that the deflected slabs are not laterally continuous but are separated into a few subregions. Beneath the northeastern China where the resolution is good, the slab related anomaly above the 660 km discontinuity is accompanied by its downward extension into the lower mantle.

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Broadband seismic waveform data are stacked to investigate the mantle discontinuities beneath a station. A polarized filter is devised to remove pseudo-signals in the stacked traces, which might be otherwise misinterpreted as a discontinuity. The depth of a mantle discontinuity determined in previous studies depends on the reference model. We suggest the use of data sets which have a range of epicentral distances to the investigated station. The observed travel time of the P-to-S converted phases as a function of epicentral distance can be used to constrain the proper reference model. When the technique is applied to real data, we can determine the absolute depth of a discontinuity with an accuracy of approximately ±10 km. The method is applied to the broadband data of the CDSN stations. There is no significant depression observed for any of the stations except BJI, implying that the lateral scale of the trough in the ‘660-km’ discontinuity under northeast China is smaller than suggested in previous SS precursors studies. Beneath station BJI, the ‘410-km’ and ‘660-km’ discontinuities are elevated 10 km and depressed 30 km, respectively, resulting in an extremely thick transition zone. This may be attributed to the cold pacific plate that exists in the transition zone of the same region. Meanwhile, at station MDJ, where the subducted pacific plate is also found in the mantle transition zone, a multiple-discontinuity structure is observed rather than a depressed ‘660-km’ discontinuity. At station SSE, there is no depression of the ‘660-km’ discontinuity, suggesting that there is no significant difference of temperature at depths around 660 km between SSE and the average mantle.

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Along the Kamchatka-Kuril-Japan-Izu-Bonin-Mariana subduction zones, the old age of the subducting Pacific Plate and the rapid subduction rate together suggest that earthquakes should occur to the bottom of the transition zone. However, the seismicity cutoff varies in depth between 350 km and 650 km. Along these subduction zones, the largest deep-focus earthquakes invariably occur near the depth of the local seismicity cutoff regardless of its depth. The events near the seismicity cutoffs also have systematically different focal mechanisms than shallower events. Furthermore, data from S₆₆₀P arrivals, residual sphere analysis, and tomographic studies all show that the slab dip consistently steepens to a near-vertical orientation at the seismicity cutoff. This change in slab dip indicates a strength loss in the slab. We hypothesize the following causal connection among all these observations: The cold temperatures in the slab kinetically hinder the olivine to spinel phase change and allow the olivine to persist metastably to depths well below its equilibrium pressure. When the phase transition occurs, it nucleates very finegrained spinel which acts as a lubricant, allowing the initiation of earthquake faulting at high confining pressures which further nucleates additional fine-grained spinel. The cold anomaly of the slab severely inhibits the growth of the nucleated spinel crystals. The presence of the fine-grained spinel crystals reduces the strength of the coldest part of the slab by several orders of magnitude, allowing high slab deformation rates. Additionally, the phase change, by increasing the density, provides a negative buoyancy force. Combined, these processes reduce the slab membrane strength and allow the slab to descend at a steeper dip.

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A technique for detecting intermediate-period (6-12 s) Sd P phases converted from S to P at a depth d in the source region is described. Previously, these phases were detected in short-period array recordings of deep events. The main idea of our technique is to deconvolve the vertical component of a single record by the S waveform, and to stack the deconvolved components of a number of records, with appropriate time-shift corrections accounting for the difference of epicentral distance. Using this technique, the phases converted from discontinuities at around 660 km, 860 km, 1070 km, and 1170 km depths beneath Sunda arc are detected at seismograph stations in central and eastern Asia. Our data on ‘1070 km’ discontinuity are very consistent with those inferred from short-period recordings of the same events at the J-array in Japan (Niu and Kawakatsu, 1997), but favour a few different discontinuities in the midmantle, rather than one with a strongly variable depth. When compared with a tomographic model of the mantle for the same region, our data suggest that ‘1070 km’ discontinuity may act as a barrier for the downgoing lithospheric slabs.

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Seismic tomography using S wave travel times faces the difficulty imposed by the interference between S and SKS phases near 83° epicentral distance, as the SKS phase overtakes the S waves in the mantle. If the cross-over is avoided completely by excluding S data beyond 82° then no resolution is available below 2200 km in the lower mantle. A partial solution is to try to pick up the S phase beyond the cross-over which improves coverage and resolution in depth. However, a much larger improvement can be made by following the first arrival with S character and including SKS information with S. Arrival times for both S and SKS phases and the event hypocentres have been taken from the reprocessing of data reported to international agencies. Each event has been relocated, including depth phase information, and later phases re-associated using the improved locations to provide a set of travel times whose variance is significantly reduced compared with the original data catalogues. S travel-time tomography including SKS information out to 105., provides tomographic images with improved rendition of heterogeneity in the lower mantle. The three-dimensional models of SV wavespeed relative to the ak135 reference velocity model show a significant increase in heterogeneity at the base of the mantle which matches the behaviour seen in results derived from waveform inversion. For most of the mantle there is a considerable similarity between the patterns of heterogeneity in the S wave images and recent P wave tomographic results, but greater differences develop in the lowermost mantle. In the D″ region the SV wavespeed patterns also show some differences from recent SH wavespeed results which mostly correlate with regions of recognised structural complexity.

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Proposals that the seismic structure of the Earth's inner core is time-dependent are investigated using the splitting functions of free oscillations, determined from seismic data recorded over some 20 years. It is shown that the data support some time-dependence, although the paucity of the data for the 1970's and 1980's makes it difficult to be certain of the precise form of such time dependence. The data do not support a positive (i.e. West-to-East) rotation; rather, the inferred rotational component of the time-dependent inner core structure is negative (i.e. East-to-West), and varies according to the mode under consideration from less than 1° per year to more than 2° per year. The analysis suggests that inner core structure is time-dependent but that the dependence is not well explained by differential rotation of the inner core. This kind of analysis will become much more powerful with increasing quantities of data over a longer time span.

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Physical mechanisms of seismic anisotropy in the D″ layer are examined based on seismological and mineral physics observations. The results of body-wave seismology on the fine structure of the D″ layer and of mineral physics studies on the elastic constants and the lattice preferred orientation in lower mantle minerals as well as the shape preferred orientation of melt pockets are taken into account. Evidence of large but depth (pressure)-dependent elastic anisotropy of lower mantle minerals, particularly (Mg, Fe)O, and of tilted shape preferred orientation of sheared partial melts is summarized. It is shown that both shape preferred orientation of partial melts (or iron-rich secondary phases) and lattice preferred orientation of minerals with well-documented slip systems are difficult to reconcile with seismological observations. However, lattice preferred orientation of highly anisotropic mineral, (Mg, Fe)O, is consistent with most of the seismic observations if the dominant glide plane under the D″ layer conditions is {100} rather than {110} as observed at lower pressures. Such a change in glide plane in MgO (or (Mg, Fe)O) is likely to occur as a result of pressure-induced change in elastic anisotropy and/or in the nature of chemical bonding (and possibly due to high temperatures). Both solid-state and partial melt mechanisms of anisotropy imply that the V_SH > V_SV (V_SV > V_SH) polarization anisotropy means horizontal (vertical) flow. In the solid-state mechanism, significant V_SH > V_SV in the D″ layer beneath the circum-Pacific (Alaska and the Caribbean) implies horizontal shear at high stress caused presumably by the collision of subducting materials with the core-mantle boundary. Highly variable anisotropy beneath the central-Pacific can be attributed to solid-state fabrics caused by a complicated threedimensional flowpresumably related to the upwelling of plumes, but anisotropy in this region could also be attributed to the shape preferred orientation of melt pockets the presence of which is suggested by very low average velocities.

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Latent heat release by equilibrium mineralogical transformations in an adiabatically subducting slab reversibly perturbs temperatures and pressures so as to conserve entropy. However, latent heats of metastable transformations in such a slab yield irreversible isobaric temperature changes which increase entropy despite adiabatic constraints. As a result, latent heat release by metastable exothermic transformations can yield local superheating above the background adiabat, with the degree of potential superheating increasing with extent of metastable overstep. In real slabs, however, regions of metastably persisting low-pressure phases should undergo conductive warming from surrounding transformed material. Such warming should proceed more rapidly than warming of the bulk slab from the surrounding mantle, and the resulting decrease in metastable transition pressures will slightly decrease the degree of local superheating. Nonetheless, such local temperature increases may trigger seismic release of accumulated strain energy via a number of proposed mechanisms of shear instability. Adiabatic instability, in the form of shear localization in material with temperature-dependent rheology, is one mechanism which may be triggered by such latent heat release in metastable regions yet produce rupture that extends beyond the boundaries of such regions.

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Recent geophysical evidence from seismology, mineral physics, viscosity inversion shows that the mantle between 400 and 1000 km is extremely complicated, with intermediate scale structures present regionally as seismic reflectors under the 660 km discontinuity and bent plume-like structures under the transition zone. We have studied the dynamics of the transition zone with two models, an axisymmetric spherical-shell (2-D) model with a horizontally averaged temperature- and pressure-dependent viscosity and a 3-D Cartesian model with a depth-dependent viscosity. Two mantle phase transitions have been employed in both models. Results of the 2-D axisymmetric model show that the interaction of the lower mantle plumes with the transition zone can result in a horizontal channel flow right underneath the 660 km and in the birth of secondary plume some distance away from the lower mantle plume. The strength of the secondary plume increases in strength with larger viscosity contrast across the 660 km discontinuity. In the 3-D model we have found that with the presence of a second low viscosity zone somewhere between 660 and 1000 km, many secondary instabilities are developed in the second asthenosphere and the mesoscale thermal structure developed can become quite complex. Many small-scale plumes can emanate from the transition zone. Occasionally a very large plume burst, with a near-surface radius exceeding 1000 km, can develop from the hot lower-mantle material trapped in the second asthenosphere. Both the viscosity and the phase transition structure between 660 km and 1000 km can exert a significant influence on the plume distribution and cause singular plume eruption events in the upper mantle. Plume instabilities originating below the 660 km discontinuity in the western Pacific might have launched a large hot upwelling into the upper mantle, thus precipitating the massive flood basalt volcanism in the Ontong-Java region.

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Recent seismological evidences imply that the boundary between the lithosphere and asthenosphere is a compositional boundary in the oceanic upper mantle, and a rapid increase of viscosity at this boundary is suggested. We modeled a thermal convection in the oceanic mantle numerically using the finite element method, and investigated geodynamical consequences of such a rheological layering. Early results from both quasi-steady state flows and time-dependent flows are presented in this report. We assumed a temperature- and depth-dependent viscosity law so that both the thermal effects and those of layering are taken into account. The effect of a high-viscosity layer (HVL) is small on the flow and the temperature field. Velocity gradients in the HVL are small in both directions, and the velocity field is well approximated by a one-dimensional channel flow. The HVL acts as a low-pass filter of the dynamic topography.

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We revisited the resolving power of viscosity inversion in terms of geoid misfit in a 2-D Cartesian geometry under the assumption that the mantle viscosity is laterally stratified. Firstly, we considered a simple case of two viscosity layers only, which is described by two parameters of the amount and the depth of the viscosity jump. The uniqueness of the inversion was examined by evaluating misfits between the reference geoid for a reference viscosity and that for a viscosity described by the changing two parameters. The misfits are mapped into 2-D model space as a function of the two parameters. Three types of density distribution are tested; they are vertically constant (1), taken from a tomographic model (2), and the same but includes artificial noise (3). We found that, at least for this simple case, the viscosity solution keeps unique in the entire 2-D model space using whole degree band (1-8) of geoid. This holds even if the artificial noise is rather large (70%), though the solution is slightly different from the reference viscosity. However, we also observed non-uniqueness, such as trade-off between the two parameters, when individual degree components of geoid are concerned. In the next, we employed a more realistic viscosity structure, having seven iso-viscous layers. It is no longer possible to describe 6-D model space easily. Therefore we tried to reconstruct a reference viscosity from the reference geoid using genetic algorithm search. According to this analysis, nearly the same solution with the reference viscosity can be reconstructed, while solutions apart from the reference viscosity with increase of noise in the density distribution.

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