Sonobuoy measurements of seaquakes on the East Pacific rise near San Benedicto island

Sonobuoy modified to receive low (< 100 Hz) frequency sounds in the deep ocean were deployed from the R.V. Yaquina on the Rivera fracture zone and 4·3 kmoff San Benedicto Island. An array of at least two operating sonobuoys was maintained for 24 h at each station. Calculated earthquake magnitudes were 2·2 to 3·7 on the Richter scale. Most of the acoustic energy in the signals was below 40 Hz and recorded well on the modified sonobuoys but poorly on simultaneously monitored unmodified buoys. A swarm of microearthquakes following a magnitude 4·3 earthquake was recorded on the Rivera fracture zone site while seismic activity at the San Benedicto Island was more sporadic. Epicentral distance from the sonobuoys, calculated from the T-P interval (when present), ranged from 10 to 120 km, most less than 20 km. The earthquake magnitude versus occurrence rate curve for the Rivera fracture zone events has b value of 0·6, indicating that the earthquakes were associated with the fracture zone rather than the median rift of the East Pacific Rise.