A microearthquake survey of the high‐temperature vent fields on the volcanically active East Pacific Rise (9°50′N)

A 3 month deployment of nine ocean bottom seismometers (OBS) on the axis of the East Pacific Rise at 9°50′N detected 283 local microearthquakes in the spring of 1995. The earthquakes exhibit small, uniform seismic moments of 10 14 –10 16 dyne‐cm (). Accurate locations were determined for 147 of the earthquakes, with hypocenters clustered beneath two high‐temperature hydrothermal fields (Bio9/P and Tube Worm Pillar/Y) at depths <1.4 km beneath the seafloor. Waveform cross‐correlation techniques relocated 76 events, 65 of which had lateral standard errors <100 m. Relocated hypocenters lie along two vertical columns from 0.7 to 1.1 km depth. A continuous level of seismic activity was observed beneath the Tube Worm Pillar and Y vent fields, while activity beneath the Bio9 and P vent fields was dominated by a vigorous swarm of 162 events during 3 hours that triggered a 7°C temperature increase in Bio9 exit fluids. The close correlation between the hydrothermal systems and the observed seismicity suggests the microearthquakes were generated by thermal strain associated with cooling of the shallow crust and that the water‐rock reaction zone is also a seismogenic zone. The earthquake depths suggest that brittle deformation on the rise axis is restricted to the upper kilometer of crust.