Toshi Asada

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.

Apparent velocity measurements on an oceanic lithosphere

Abstract A direct measurement of apparent velocities for oceanic paths was made with an array of sensitive ocean bottom seismographs. The measurement was performed by recording waves from shallow earthquakes which occurred in the area close to trench axes and which were accurately located by the land seismological network in Japan. The range of epicentral distances is from 500 to 1,800 km. The observed P travel times are less than those in the Jeffreys-Bullen tables by 6–10 s for the range of distances. Since the dimension of the OBS array is about 400 km, the apparent velocities are determined quite precisely and show little dependence on the epicentral distances. The average value of the apparent velocities for the range 500–1,700 km is 8.64 ± 0.13 km/s. An offset of travel times, which is thought to be associated with a low-velocity layer underneath the oceanic lithosphere, has been observed. These results indicate that a high-velocity layer with a velocity of 8.6 km/s exists in the lower part of the oceanic lithosphere. Beneath the 8.6-km/s layer there is a thin low-velocity layer under which the velocity of the P wave is again 8.6 km/s.

Long-range refraction experiments in deep ocean

Abstract Long-range refraction experiments as a potentially powerful tool to elucidate the structure of the oceanic lithosphere are reviewed. Although the necessity for long-range experiments has been well appreciated for some time, difficulties of undertaking such experiments, especially the lack of access to reliable and low-noise OBS (ocean-bottom seismograph), have prevented full implementation of these experiments. However, several long-range experiments have been performed by utilizing land seismographs, which led to various interesting findings: the existence of a high-velocity layer (Vp of 8.4–8.7 km/s) beneath the Moho discontinuity, and offsets of travel times that suggest the existence of a low-velocity layer beneath the lithosphere. Nevertheless, the use of land stations inherently limits the location of the experiments, as well as the resolution of their analyses, since the profile inevitably includes an ocean—continent (or island) transition structure that is probably complex. After examining ambient noise spectra at ocean bottoms and a reconnaissance survey by a low-noise OBS system, four long-range experiments using OBSs have been performed to date by Japanese scientists. Although not all were successful, some interesting results were obtained. The profiles lay over the western Pacific basin where the oldest oceanic lithosphere is postulated. The existence of an 8.6-km/s layer in the lower part of the lithosphere, and a rather thin low-velocity layer that is bottomed by a 8.6-km/s layer, have been found from a N—S profile east of the Japan trench (1974 profile). However, a profile which is rougly perpendicular to the 1974 profile suggests considerably different velocities; the difference probably comes from an anisotropy. From a study of the profile east of the Mariana trench, which is about 2000 km from the 1974 profile, a variability in lithospheric structures is indicated. Groups of scientists in Europe and the U.S.A. have initiated long-range programs which also make use of their own OBSs; they have been making good progress in elucidating the fine structure of the oceanic upper mantle. Some problems which current long-range experiments are encountering are also reviewed.

T waves from deep earthquakes generated exactly at the bottom of deep-sea trenches

Abstract T waves (seismic water waves), which were generated by deep-focused earthquakes, have been found by an array of sensitive ocean-bottom seismographic observations depolyed on the western Pacific basin. The points of generation of T waves have been exactly located by use of the accurate velocity of water waves which were known from explosions. The positions obtained are at the bottom of deep-sea trenches; however, the positions are slightly (10–35 km) ocean-side of the trench. T waves have been known to be generated by seismic waves which were transmitted from the focus to the trench bottom along the descending lithosphere. The intensity of the observed T waves implies that the Q value along the descending lithosphere is more than 4000. The positions of T-wave generation are consistent with the 8.2- to 8.6-km/s stratified structure of the oceanic lithosphere. T waves from shallow earthquakes beneath the lower continental slope are also clearly observed by bottom seismography.