Seismicity at the southern East Pacific Rise from recordings of an ocean bottom seismometer array

Seismic data recorded by ocean bottom seismometers (OBSs) at the southern East Pacific Rise (SEPR) were analyzed to characterize seismicity at fast spreading mid‐ocean ridges. An automated event detection algorithm based on pattern recognition was developed to identify earthquake‐generated T phases. The waveform cross‐correlation and least squares method was found to provide more robust estimates of T phase relative travel times than the method of picking T phase maximum amplitude. The OBSs and subsequent equatorial Pacific hydrophones recorded earthquakes near an earthquake swarm in 1992, which was located ∼300 km west of the ridge axis at about 18°S and 116°W. The process that generated the 1992 swarm either was reactivated 3 years later during the OBS experiment or has been continuously active in the past decade. This long‐term activity suggests renewable stresses, which are consistent with the interpretation that magma injection was the cause of the 1992 swarm. In contrast to the swarm characteristics of earthquakes on the west flank of the ridge axis, earthquakes on the Nazca flank were randomly distributed in time and space. This difference in seismicity may result from the difference in the thermal structure of the Pacific and Nazca plates. The few earthquakes located at or near the ridge axis had very small magnitudes, reflecting the presence of shallow magma chambers and the thin elastic layer of the SEPR. This result suggests that acoustic monitoring of earthquakes associated with dike intrusion is difficult at regional scales at the SEPR. There is no apparent correlation between seismicity and off‐axis seamounts.