Yoshibumi Tomoda

Seismographic observation at the bottom of the Central Basin Fault of the Philippine Sea

THE Central Basin Fault of the Philippine Sea is proposed as an extinct mid-oceanic ridge1,2 and seems to be a key to the development of the floor of the Philippine Sea. Though the fault seems to be aseismic on the basis of land networks, it is interesting to know whether microearthquakes occur in the vicinity of the fault. We put a sensitive ocean-bottom seismograph (OBS) in the median valley of the ridge (the width of which is only a few kilometres) and from its recordings have been able to deduce the existence of microearthquakes. It has also been possible to estimate P and S velocities for the top of the mantle and a Q structure for the upper mantle.

Geophysical observations around the northern Yap Trench: seismicity, gravity and heat flow

Abstract Seismicity, gravity and heat flow measurements were taken around the northern Yap Trench, southwest of the Mariana Trench. Subduction of the Pacific Plate at the Yap Trench is thought to have been interrupted by collision of the Caroline Ridge since the Early Miocene. A large number of shallow seismic events were observed below the inner trench wall by four OBSHs deployed for 10 days in 1986. In contrast, only a small number of earthquakes is reported by the ISC (International Seismological Center) for the same area. Free air and Bouguer gravity anomaly profiles across the northern Yap Trench imply convergence or subduction in the area. The heat flow profile shows similar variation as is observed at other trench systems. However, it differs in two respects, namely, the distance between the high heat flow area and the trench axis is small (90 km), and the highest heat flow occurs in the Parece Vela back-arc basin. These findings together with the available petrological data indicate that current plate subduction is probable in spite of the Caroline Ridge collision. However, simple plate subduction can neither explain the distinctive heat flow profile nor the high and shallow seismicity beneath the inner trench wall. With our present understanding, these features can be explained by very slow subduction which accompanies minor thermal activity and shallower slab assimilation than those of normal subduction.

Lithospheric thickness anomaly near the trench and possible driving force of subduction

Abstract An approach to the physical mechanism of initiation of lithospheric subduction is presented. Firstly the subterranean structures of the oceans down to the lithosphere-asthenosphere boundary are estimated, based upon the observed data of seismic refraction and gravity anomalies. Peculiar structures of the lithosphere near the trenches and fracture zones are emphasized. A numerical simulation using the structures thus obtained demonstrates that the lithospheric subduction can be initiated under a certain condition of physical properties in an appropriate time duration, if contrast in lithospheric thickness exists.

Marine geodesy of the northwestern pacific

Abstract Several topics of the marine geodesy of the northwestern Pacific are discussed, based on sea gravimetry and its results: 1) How has gravity measurement in this area been conducted, and how was mapping of the gravity anomaly made? 2) What are the characteristics of the gravity anomaly in the region? 3) What kind of geophysical processes makes such a gravity anomaly distribution? 4) Undulation of the geoid and deflection of the vertical is calculated from the gravity anomaly. What kind of characteristics can be seen? 5) The future aspect of marine geodesy, especially the geodesy of the ocean bottom.

Geophysics of the Pacific Basin

Gravity anomalies show the effects of both the topography of the solid earth and its compensation. Free-air gravity anomalies, therefore, approximately represent isostatic anomalies, and the small range of free-air anomalies of about 800 mgal (1 mgal = 10-5 m sec-2), or 0.08% of the earth’s gravity field, shows that the earth is nearly in isostatic equilibrium (Bowin et al., 1982). Large-amplitude free-air anomalies, or deviations from isostasy, are maintained by tectonic activities (Vening Meinesz, 1932). Isostasy is fairly well achieved in wavelengths longer than about 500 km in the Pacific basin, and the corresponding free-air anomalies are as small as ±20 mgal (McKenzie et al., 1980). The value of 20 mgal is 0.002% of the earth’s gravity field, and the small value indicates that gravity measurements need high precision. It was difficult for the prealtimeter gravity field to discuss such gravity anomalies, because the spatial resolution of the gravity data obtained from satellite orbit perturbations was not sufficient, and because surface observations were sparse and not free from measurement errors of about ±10 mgal.

Seismic velocity structure and gravity anomalies: a comparison

Abstract The question was examined as to whether the same gravity anomaly is produced for a range of crustal velocity structures all of which produce similar seismic record sections. The results indicate that the maximum errors in the computed gravity anomalies for the various velocity structures is around 20–30 mGal for some models and is around 50 mGal for many others. A major source of error is underestimating the effect of a seismic low-velocity zone which is at one end of the correct velocity structure and which was not detected by several of the interpretations presented at a recent workshop. If no low-velocity layer is included, the error in the gravity anomaly is 30 mGal. The choice of a particular velocity-density relationship does not seem to be a serious problem in the analysis, except for the case of the low-velocity zone, as long as we use a continuous velocity-density curve.

Ocean Bottom Gravity Meter

An ocean bottom gravity meter was designed and built to measure gravity at the ocean floor in order to detect up-and-down motions of the sea bottom and density changes in the oceanic crust corresponding to changes preceding a strong earthquake. The gravity sensor is a vibrating-string gravimeter based on the TSSG surface ship gravity meter. The gravity meter was consisted of an underwater part, a land station and an underwater cable. Measured data was transmitted continuously from the sea bottom to the land by means of an optical fiber cable which contains six optical fibers embedded in a grooved aluminum line. The test of the gravity meter was carried out on the bottom in the shallow water at a depth of about 30 meters. The gravity meter had operated in a good condition and measured gravity changes for more than two months.