Seismic activity associated with temperature perturbations at Bio 9 hydrothermal vent on the East Pacific Rise at 9deg50'N

A brief seismic swarm north of high temperature vent Bio 9 on November 19th 2003 appears to have triggered a temperature response on a time scale of ~3 days, similar to previously observed events (Sohn et al., 1998; Wilcock, 2004). Temperatures initially rose by ~5degC over the course of 10 days and then fell by ~14degC over 8 days, before gradually recovering to temperatures slightly higher than before the event over the following 2-3 months. Near-by chimneys showed small or no response to this event suggesting that fluid pathways to individual vents are narrowly defined deep within the crust. The seismic swarm of ~130 relocated events forms a thin near-vertical line approximately 1 km tall with evidence of upward propagation from directly above the axial magma chamber to the top of hydrothermal cracking zone and into the interpreted upflow zone. The earthquakes initiate from an area close to the events associated with a similar temperature perturbation at Bio 9 in 1994 (Sohn et al., 1998), suggesting that fluid pathways are maintained on decadal time scales. However, the character of the temperature perturbation is different from the 1994 event in that after initially rising, temperatures decline to below pre- swarm level before recovering gradually. This may reflect a different phase in the evolution of the hydrothermal cell, since temperatures at Bio 9 in 2003 were higher and more stable than they were in 1994. The interpreted hydrothermal cell beneath Bio 9 shows a decrease in seismic b-value following recovery of the Bio 9 temperatures. A decrease in b-value may reflect an increase in stress, lower pore pressures, or variation in the distribution of fractures within the rock. Analysis of the seismic swarm within the context of the interpreted hydrothermal cell and previous seismic results will be presented, and possible models for triggering the temperature perturbations will be discussed.