S U M M A R YA detailed study of the physics of a 1-D sedimentary compaction of a viscous medium was carried out both numerically and analytically for columnar and self-gravitating spherical cases, in view of applying it to the inner-core growth process of the Earth. The effects of sedimentation rate and surface porosity upon the porosity profile were investigated. It was found that the porosity profile differs depending on whether or not the sedimentation rate is larger than the Darcy velocity (velocity of the solid matrix when the fluid flows by buoyancy alone). When the sedimentation rate is larger than the Darcy velocity, a thick, constant-porosity layer develops at the surface, and below it, the porosity decreases gradually towards the bottom. When the sedimentation rate is smaller than the Darcy velocity, the porosity profile is characterized by a mushy layer at the top, where the fluid is expelled by the deformation of the solid, underlain by a thick layer of constant porosity, termed the residual porosity. Such a porosity profile can be understood as the propagation of a half-sided solitary wave. The study was extended further for the self-gravitating spherical case. Formation of an unstable porosity structure and the appearance of solitary waves were discovered for the case of monotonically decreasing sedimentation rate. Given the size of the sphere formed by sedimentary compaction, according to the magnitude of the ratio of sedimentation rate to Darcy velocity, three types of porosity structure, which differ in force balance and the typical length scale required for porosity decrease, were discovered. One such structure is where a low-porosity layer forms at the top, accompanied by solitary waves beneath it, indicating that a crust-like region can develop at the surface of the inner core.

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A seafloor geomagnetic observatory in the northwest Pacific detected clear electromagnetic (EM) variations associated with tsunami passage from two earthquakes that occurred along the Kuril Trench. Previous seismological analyses indicated that the M8.3 earthquake on 15 November 2006 was an underthrust type on the landward slope of the trench, while the M8.1 earthquake on 13 January 2007 was a normal fault type on the seaward side. The EM measurements enabled precise monitoring of the tsunami propagation direction as well as particle motion of the seawater. The estimated horizontal water velocity differs significantly for the 2006 and 2007 tsunamis, in terms of initial motion and dispersive characters, being consistent with the hydrodynamic simulation results of the tsunamis. Namely, the tsunami‐induced horizontal geomagnetic components showed opposite signs for the rise and retreat waves as expected from the “electric current wall hypothesis.” The dispersion effect is more remarkable in the 2007 event with a smaller source region of its tsunamigenic earthquake. The 2007 tsunamis, therefore, tend to violate the long‐wave approximation. The Boussinesq approximation was required to reproduce the dispersive character of the 2007 event in our numerical simulation. In terms of tsunami forecast, an important advantage of EM sensors over conventional tsunami sensors, such as seafloor pressure gauges, is their capability of vector measurements: in addition to their ability to monitor particle motions, the first peak of the downward magnetic component always precedes the tsunami peak, suggesting a significant improvement in global tsunami warning systems if vector EM sensors are integrated into the existing systems.

We performed laboratory experiments of Rayleigh-Bénard convection with liquid gallium under various intensities of a uniform imposed horizontal magnetic field. An ultrasonic velocity profiling method was used to visualize the spatiotemporal structure of the flows with simultaneous monitoring of the temperature fluctuations in the liquid gallium layer. The explored Rayleigh numbers Ra range from the critical value for onset of convection to 10 5 ; the Chandrasekhar number Q covers values up to 1100. A regime diagram of the convection patterns was established in relation to the Ra and Q values for a square vessel with aspect ratio 5. We identified five flow regimes: (I) a fluctuating large-scale pattern without rolls, (II) weakly constrained rolls with fluctuations, (III) a continuous oscillation of rolls, (IV) repeated roll number transitions with random reversals of the flow direction, and (V) steady two-dimensional (2D) rolls. These flow regimes are classified by the Ra/Q values, the ratio of the buoyancy to the Lorentz force. Power spectra from the temperature time series indicate that regimes I and II have the features of developed turbulence, while the other regimes do not. The region of steady 2D rolls (Busse balloon) extends to high Ra values in the present setting by a horizontal magnetic field and regime V is located inside the Busse balloon. Concerning the instabilities of the steady 2D rolls, regime III is the traveling wave convection developed from the oscillatory instability. Regime IV can be regarded as a state of phase turbulence, which is induced by intermittent occurrences of the skewed-varicose instability.

Locations of the geomagnetic pole over the past 10,000 years have been calculated by averaging the VGP positions obtained from paleomagnetic data. The distribution of the geomagnetic pole was elongated to the direction parallel to the meridian of 45° and 225° longitude, and westward movement of the pole was predominant throughout this period. The time sequence of the polar motion can be divided into three intervals, the intervals between ca. 10,000 B.P. and ca. 7000 B.P., between ca. 7000 B.P. and ca. 3700 B.P., and between ca. 3700 B.P. and the present. During the period between ca. 7000 and ca. 3700 B.P., the range of the movement of the geomagnetic pole was limited within 5 degrees around the geographical pole. Before and after this period, the movement was very active, fluctuating over 10 degrees. The results of the last 2000 years show good consistency with the geomagnetic pole calculated from archaeomagnetic data by Merrill and McElhinny [1983].

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[1] We studied undeformed sediment and accreted strata recently recovered by Ocean Drilling Program/ Integrated Ocean Drilling Program (ODP/IODP) drilling in Nankai Trough convergent margin to unravel the changes in physical properties from initial deposition to incipient deformation. We have derived acoustic (V p ) and mechanical (uniaxial poroelastic compliance, compaction amplitude) properties of samples from various drill sites along the Muroto (ODP 1173) and Kii transects (IODP C0001, C0002, C0006, and C0007) from isotropic loading tests where confining and pore pressure were independently applied. We quantified the dependence of V p on both effective (P eff ) and confining (P c ) pressure, which can be used to correct atmospheric pressure measurements of V p . Experimental V p obtained on core samples extrapolated to in situ conditions are slightly higher than logging-derived velocities, which can be attributed either to velocity dispersion or to the effect of large-scale faults and weak zones on waves with longer wavelength. In the high-porosity (30%-60%) tested sediments, velocities are controlled at first order by porosity and not by lithology, which is in agreement with our static measurements of drained framework incompressibility, much smaller than fluid incompressibility. Rather than framework incompressibility, shear modulus is probably the second-order control on V p , accounting for most of the difference between actual V p and the prediction by Wood's (1941) suspension model. We also quantified the mechanical state of Nankai samples in terms of anisotropy, diagenesis, and consolidation. Both acoustic and mechanical parameters reveal similar values in vertical and horizontal directions, attesting to the very low anisotropy of the tested material. When considering the porous samples of the Upper Shikoku Basin sediments (Site 1173) as examples of diagenetically cemented material, several mechanical and acoustic attributes appeared as reliable experimental indicators of the presence of intergrain cementation. We also detected incipient cementation in samples from IODP Site C0001 (accretionary prism unit). In terms of consolidation, we distinguished two classes of material response (shallow, deformable samples and deep, hardly deformable ones) based on the amount of compaction upon application of a P eff large with respect to the inferred in situ value, with a transition that might be related to a critical porosity.Components: 15,300 words, 15 figures, 1 table.

Attslract. We have made a paleomagnetic study of Tertiary rocks from the Oga Peninsula, northers Honshu Island. Vokanic and sedimentary rocks were sampled from 26 sites in the peninsula. The ages of the rocks range from 62 Ma to the present. The resuRs indicate a couaterclockwise rotation of Northeast Japan with respect to eastern Asia around a pole at 58øN, 149øE, between about 22 Ma and 15 Ms. The amount of the rotation is about 20 ø. Before the rotation, Northeast Japan was situated along the east coast of the Asian continent. INTRODUCTION More than two decades ago, Kawai et aJ. [1961] demonstrated the bending of Honshu Island on the basis of the comparison of the paleomagnetic field directions from Northeast and Southwest Japan. Recent paleomagnetic studies in Japan have aimed at determining the mode and the timing of the bending and at relating the bending to the opening of the Sea of Japan. For Southwest Japan, Otofuji and Matsuds [1983] performed paleomagnetic investigations on the acidic rocks exposed on the Sea of Japan side of Southwest Japan and concluded that the clockwise rotation of that region took place from 28 to 12 Ms. The timing of the rotation was narrowed to around 15 Ma by Otofaji and Matsuds [1984]. For Northeast Japan, Tosha [1983] and Otofuji et aL [1985] recently made paleomagnetic studies. Otofuji et aL [1985] collected samples from a wide area of Northeast Japan, and their results indicate that the region north of the Tanakura Tectonic Line (see Figure la) behaved as one block, at least during the Tertiary period. However, the timing of the rotation of Northeast Japan is not clear from their data, since the relative ages of the rocks are usknown, partly because of the wide spread of the sampling sites. On the other hand, Tosha [1983] collected samples from the Oga Peninsula, where the sequence of Tertiary rocks is well preserved. Therefore his data are suitable for determining the chronologic progression of the movement of Northeast Japan. Recently, Celaya and McCabe [1987] and Morean et aL [1987] used Tosha's [1983] data and interpreted the tectonic evolution of Japan. These workers discussed the opening of the Sea of Japan by using the temporal declination changes in Japan, and later workers argued the remagnetization of pre-Tertiary rocks. Since the thesis by Tosha [1983] is not widely known and the above authors' iaterpretation of his data is quite different from his own interpretation, we here present the original data and results of recent studies, and discuss the tectonic implications. Some geological and geophysical studies suggest the tectonic continuity between the northeastern part of Honshu Island and the western part of Hokkaido Island in Cretaceous time [Takahashi, 1983; Takigaml; 1983; Segawa and Furuta, 19T8]. In the present study we assume the area north of the Tanakura Tectonic Line and the western part of Hokkaido to be one tectonic block since the Early Cretaceous, and we call the block Northeast Japan. GEOLOGICAL SETTING, AND SAMPLINGIn Northeast Japan, Tertiary an...