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Seaquakes on the Nazca-Cocos plate boundary

NAVAL UNDERSEA CENTER, SAN DIEGO, CA 92132

Abstract

Seaquakes were detected by sonobuoys launched from the R/V THOMAS WASHINGTON 20 km south of the Galapagos spreading center at 1°N, 86°W on June 30, 1972. The recordings were analyzed for signal arrival time, sound pressure level and frequency. Three distinct phases were noted for each seaquake. Listed in order of arrival time, they are identified as compressional waves in the ocean having travel paths of : (1) a direct wave (D) between the epicenter and hydrophone, (2) a wave once reflected from the ocean surface and bottom before reaching the hydrophone (R₁), and (3) a wave twice reflected from the ocean surface and bottom before reaching the hydrophone (R₂). Epicentral distances computed from the R₁ minus D arrival times ranged from 6.5 to 14.5 km when a shallow (< 1 km below the ocean floor) focal depth is assumed. Source levels, corrected for spherical spreading to a standard distance of 5 km, ranged from 80 to 100 db re 1 microbar (equivalent to local earthquake magnitudes of –0.25 to +0.75. The time interval between discrete seaquakes varied from 1 to 3 min and the average occurrence rate was 41 per hr. Using these data, a frequency of occurrence versus magnitude curve was drawn, which gives a b value of 2.3. Such a high b value indicates that the seaquakes were associated with seismic activity on a rift zone rather than on a fracture zone.