SEISMICITY AND TREMOR IN A SUBMARINE HYDROTHERMAL FIELD - THE NORTHERN JUAN-DE-FUCA RIDGE

We have made the first multi-year deployments of a small number of oce an bottom seismometers on a mid-ocean ridge. We located our instrument s in and around a vigorous hydrothermal vent field on the Endeavour Se gment of the Juan de Fuca Ridge. The deployments have revealed a consi stent pattern of activity comprising several types of events. Most abu ndant are swarms of extremely small microearthquakes that we suggest a re associated with cracking of the oceanic crust. We also observe loca l tectonic earthquakes that occur about four times per day. Preliminar y hypocentral locations place these tectonic events away from the ridg e axis and outside the hydrothermal field. The seismicity on the Endea vour Segment is remarkably different from the southern Juan de Fuca Ri dge where little, if any, tectonic activity is occurring. We speculate that the Endeavour and southern Juan de Fuca Ridge are in two differe nt phases of their long-term evolution. The latter has only rare activ ity that is only beginning to focus hydrothermal circulation along fau lts. In contrast, the Endeavour Segment is in an active tectonic phase ; and the faults have had time to develop robust hydrothermal circulat ion, resulting in large hydrothermal vent deposits.