Shallow sub-bottom structures of Guyots on and around Ogasawara Rise are determined from the Bouguer anomalies over the top plane of the Guyots. It was found from the result that the Guyot east of Ogasawara Rise is covered with a relatively thin sedimentary layer, while the seamount on the Rise has a thick sedimentary layer of about 1km in thickness. Moreover, it was concluded from the inclination of the Rise and of the seamount on it that the whole rise is inclined from the horizontal plane by an angle of about 1 degree although nothing is known about the cause of such an inclination.

View full abstract

Maps of free air and Bouguer gravity anomalies in the western Pacific are compiled on the basis of the sea gravity data obtained during a period 1963-1980 by use of the Tokyo Surface Ship Gravity Meter.
Outer gravity high seaward of trench and negative gravity moat around seamounts can be interpreted as caused by anomalous thickness of the lithosphere.
Mutual interference between trench and seamount or rise is discussed from the view point of thickness of the lithosphere. Seamounts or rises can be classified into three types;
1) seamount which can easily subduct as represented by Kashima No. 1 Seamount,
2) sizable seamounts which take a long time to subduct,
3) rises which will never subduct.
It is suggested that a new subduction zone seaward of the old trench is formed if the rises of the type 3 encounter a trench. In such a case a relic of trench and a fore-arc ridge may be left in topography. After seamount of type 2 eventually subduct, they give rise to an acute bending of trench accompanied by a zone of negative gravity anomaly in the landward side of the trench.

View full abstract

Ocean-bottom pressure at a depth of 2, 200m has been observed on real-time at an observation point 40km northwest of the Nankai trough since August 1978. The pressure data show a linear drifting trend of nearly +6 cmH₂O/year, which is considered to be an instrumental drift. Some fluctuations having a period longer than a week with amplitudes less than 10 cmH₂O are also seen on the data recorded. These must be caused by the oceanographic variations associated with the meandering of the Kuroshio. A large crustal level change just prior to (several days to several hours) a great earthquake, if any, could be detected on real-time by use of the tidal residuals. The threshold of ±10 cmH₂O is proposed here on the basis of a simulation for a year. The threshold can further be improved by possible suppression of thermally induced instrumental noise, more precise tidal analysis and correction of the oceanographic variations, although it is no easy matter to achieve these points. This threshold is comparable to that for the coastal differential tide observation between Tago and Omaezaki. It is expected that the bottom pressure observation will serve as one of the monitoring tools for an earthquake occurrence in the sea area.

View full abstract

In this work the author studies scattering of Rayleigh wave at a vertical interface, which is the boundary between two quarter spaces with different densities and elastic constants welded together to form a half-space. The incident wave is a two-dimensional periodic Rayleigh wave. The method of approach is such that integral equations are derived by use of the Fourier transform technique and that they are solved by deforming their integration path along which the integrals vary smoothly in magnitude. Energy fluxes of the transmitted and reflected Rayleigh waves, the scattered body waves and Stoneley wave (if exists) are numerically computed. The accuracy of computed results is sufficiently high for the purpose.
The models treated are then sorted out into four cases, i.e., cases of the discontinuity of density (i), P velocity (ii), and S velocity (iii)-in these three cases, no Stoneley waves exist-, and further a case for which Stoneley wave is present (iv). Through all the above four cases, the following common features are found: (a) the energy flux of the transmitted Rayleigh waves is the most remarkable among all the partitioned waves, (b) the forward and backward transmissions of Rayleigh waves for a model are almost the same in energy and (c) the scattered P waves are propagated along the boundaries as if they are trapped there. Meanwhile, the effect of the existence of Stoneley wave upon partitioned energies are discussed by comparing them with those in the case without Stoneley wave. In order to explain the features thus brought out, the author directed his attention to the difference in the energy profiles between the Rayleigh waves in the respective two media; the difference is believed to reflect the converging and diverging processes of waves transmitted through the vertical interface.
For the case without Stoneley wave, the author introduced an approximate expression which enables us to evaluate the energy flux of the transmitted Rayleigh waves with an error of 10% or so.

View full abstract