Over 3000 local and regional earthquakes were recorded by a compact network of eight ocean bottom seismographs (500 m instrument spacing) from August 1996 to January 1997 in Middle Valley, a sediment-covered rift valley on the northern Juan de Fuca Ridge. Thirteen swarms of small-magnitude microearthquakes (-1.2 < Mw < 0.2) were detected beneath Dead Dog vent field, a major hydrothermal area in Middle Valley with exit fluid temperatures near 270¿C. High precision relative positions for 304 events within swarms were determined using waveform cross-correlation techniques. The events were relocated into small, spatially distinct clusters. The intensity of the swarms is correlated with high heat flow with the largest swarm positioned 1.3 km beneath the Dead Dog vents. Smaller clusters of earthquakes are located up to hundreds of meters outside the vent field. The results suggest that the observed seismicity in the Dead Dog region is triggered by thermal strain (contraction) in the hydrothermal reaction zone as fluids extract heat from hot basement rock. Microearthquake swarms appear to be concentrated in regions where faulting has promoted seawater penetration through the sediment layer, cooling the crust, and yielding larger strain rates than those produced by seafloor spreading.