Microearthquakes on the East Pacific Rise at 21°N and the Rivera Fracture Zone

Abstract

During March and April of 1974, three ocean bottom seismometer (OBS) capsules were deployed in arrays on the East Pacific Rise (EPR) at 21°N for 8 days, on the Rivera Fraction Zone (RFZ) for 9 days, and at the EPR/RFZ intersection for 12 days. Seismicity on the EPR is characterized by extremely small events, none large enough to trigger all of the instruments simultaneously. A weak 24‐hour periodicity in OBS event trigger rate is observed. This could be related to earth strain tides or a modulation in the trigger sensitivity due to a varying noise level. On the Rivera Fracture Zone, an en echelon pattern of earthquakes with a maximum depth of 10 km was observed. Seismic activity at the junction of the EPR and RFZ becomes more diffuse and terminates abruptly at the EPR. The junction array was slightly offset from the active zone of seismicity, and the depth of only one event is reliably determined at 5.4±2 km. The S wave spectra of microearthquakes at the junction and on the RFZ are computed, and seismic moments and corner frequencies determined. Stress drops are between 0.1 and 10 bars and show no systematic difference at the two deployment sites. Larger earthquakes observed at teleseismic distances do show markedly lower stress drops when they originate near to spreading centers, but these differences are not observed at the microearthquake level in this study. It is suggested that this is due to a natural tendency for small earthquakes to have small stress drops and a lack of large earthquakes during the RFZ deployment.