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Surface-Wave Studies beneath the Pacific Ocean

¹ Geodynamic Research Center
Ehime University
Matsuyama 790-8577, Japan
 (O.P.M., D.Z.)
² National Geophysical Research Institute
Hyderabad 500007, India
 (D.D.S)

The fundamental mode Rayleigh, R2, and R3 waves were used to study the phase velocity tomography and surface-wave attenuation variation beneath the Pacific Ocean. The Rayleigh-wave phase velocity estimation for about 4000 propagation paths across the Pacific Ocean were made at time periods of 70, 100, 150, and 230 sec. More than 100 earthquakes, which occurred in the Pacific Ocean and were recorded at about 60 broadband digital seismic networks ascribed to IRIS, OHP, TERRAscope, Pacific 21 (formerly POSEIDON), and the Pacific Northwest Region (UW), are used in this study. The resulting phase velocity maps at varying periods showed a good correlation with known geological and tectonic features of the region. A fast-velocity anomaly is surrounded by a slow-velocity anomaly over the eastern and western parts of the Pacific Ocean, while the central part is associated with a fast-velocity anomaly. Subduction zones and spreading regions are demarcated by slow-velocity anomalies. A slow-velocity anomaly is also clearly visible over the Hawaiian hot spots and the French Polynesia, Micronesia, Japan, and Java–Sumatra trench axis zones.

A high-attenuation zone (low Q-value) is visible over the East Pacific rise and Nazca plate and Cocos plate regions of younger lithospheric age, and a low-attenuation anomaly (high Q-value) marks the zones of older lithosphere in the western part of the Pacific Ocean. High-attenuation regions are surrounded by low-attenuation zones in the eastern and western Pacific Ocean. Subduction zones with lower attenuation are flanked by high-attenuation anomalies on both sides, reflecting the plate interaction. The hot spots are associated with a high-attenuation zone of the Pacific Ocean at a depth of 100–300 km.