348 Physics of the Earth and Planetary Interiors, 31(1983) 348—362 Elsevier Science Publishers By., Amsterdam T Printed in The Netherlands Longshot experiments to study velocity anisotropy in the oceanic lithosphere of the northwestern Pacific H. Shimamura ~, T. Asada 2 K. Suyehiro ~, T. Yamada ~and H. Inatani’ / Geophysical Institute, Hokkaido University, Sapporo 060 (Japan) 2 Institute of Research and Development, Tokai University, Hiratsuka 259-12 (Japan) Observation Centre for Earthquake Prediction, Tohoku University, Sendai 980 (Japan) ~ Geophysical Institute, University of Tokyo, Tokyo 113 (Japan) (Received December 11, 1981; revision accepted August 8, 1982) Shimamura, H., Asada, T., Suyehiro, K., Yamada, T. and Inatani, H., 1983. Longshot experiments to study velocity anisotropy in the oceanic lithosphere of the northwestern Pacific. Phys. Earth Planet. Inter., 31: 348—362. Several long-range explosion seismology experiments have been conducted in the northwestern Pacific basin, where one of the oldest oceanic lithospheres is postulated to exist. The experiments were conducted from 1974 to 1980. Highly sensitive ocean-bottom seismographs which had been developed for longshot experiments were used. The lengths of the profiles ranged from 1000 to 1800 km, and the directions were chosen to provide wide azimuthal coverage. One of the aims of this series of experiments was to test the existence of velocity anisotropy on a large, regional scale. The results show that the oceanic lithosphere has anisotropy wherein the velocity changes by 4—7%. The anisotropy extends from a depth of at least 40 to 140 km beneath the sea bottom; however, the magnitude of the anisotropy may vary with depth. The azimuth of the maximum velocity is 150—160° clockwise from north, and coincides with the “fossil” direction of spreading of the Pacific plate, whereas it differs from the present direction of plate motion by — 30°. The azimuth does not seem to depend on depth. In the direction of maximum velocity, the lithosphere is basically two-layered: 8.0—8.2 and 8.6 km s ~. The depth of the interface is 50—60 km beneath the sea floor. 1. Introduction Large-scale velocity anisotropy has also been sug- gested on the basis of natural earthquake data Advances in instrumentation in the field of (Shimamura and Asada, 1983). Probing into the ocean-bottom seismology have led to major find- oceanic lithosphere with seismic body waves has ings regarding the structure of the oceanic litho- thus shown marked complexity in the structure, sphere. The practicality of long-range explosion which inevitably requires wider azimuthal cover- experiments which make use of ocean-bottom age of seismic profiles. seismographs (OBS’s) has been demonstrated Another long-range explosion experiment, (Asada and Shimamura, 1976, 1979; Asada et al., LONGSHOT-6, was carried out in 1980. The ex- 1983). periment was planned to further constrain the Five long-range explosion experiments termed azimuthal variation of seismic wave velocity. The LONGSHOT- 1 through LONGSHOT-5 were car- profile was recorded in the northwestern Pacific ned out in the western Pacific region from 1974 to basin (Fig. 1). 1979. The results revealed regional differences in Obviously, several long-range explosion experi- velocity and/or Q structure between the East ments have been conducted in other regions in the Marianas basin and the northwestern Pacific basin, sea. The first was conducted in the Gulf of Mexico, 003 l-9201/83/$03.00 © 1983 Elsevier Science Publishers B.V.