A microearthquake survey of the high-temperature vent fields on the volcanically active East Pacific Rise (9 degrees 50'N)

A 3 month deployment of nine ocean bottom seismometers (OBS) on the axis of the East Pacific Rise at 9 degrees 50'N detected 283 local microearthquake s in the spring of 1995. The earthquakes exhibit small, uniform seismic mom ents of 10(14)-10(16) dyne-cm (). Accurate locations were determined for 14 7 of the earthquakes, with hypocenters clustered beneath two high-temperatu re hydrothermal fields (Bio9/P and Tube Worm Pillar/Y) at depths <1.4 km be neath the seafloor. Waveform cross-correlation techniques relocated 76 even ts, 65 of which had lateral standard errors <100 m. Relocated hypocenters l ie along two vertical columns from 0.7 to 1.1 km depth. A coptinuous level of seismic activity was observed beneath the Tube Worm Pillar and Y vent fi elds, 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 degrees C te mperature increase in Bio9 exit fluids. The close correlation between the h ydrothermal systems and the observed seismicity suggests the microearthquak es 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 res tricted to the upper kilometer of crust.