HYDROTHERMAL MICROSEISMICITY AT THE MEGAPLUME SITE ON THE SOUTHERN JUAN-DE-FUCA RIDGE

Several thousand microearthquakes were recorded by ocean-bottom seismo meters during II. days of deployment on the Cleft segment of the south ern Juan de Fuca Ridge. These microearthquakes primarily occurred in s warms lasting up to I hr, and are characterized by complex time series with typical single-event durations of just over 1 sec. The events we re too small to be detected by more than one instrument at minimum ins trument spacings of about 4 km. Clear P- and S-wave arrivals were not observed for most events, and the majority of the spectra are peaked a t discrete frequencies that are not simply related (i.e., not integer multiples). These peaked spectra are quite similar to those observed f rom geysers and long-period events in volcanoes. Calibration tests per formed during the deployment indicate that the event spectral peaks ar e not explained by instrument resonance or seafloor coupling. Based on these observations, the vigorous hydrothermal activity in the area, a nd thermodynamic considerations, we postulate that the majority of the microseismicity observed is generated within the seafloor hydrotherma l system by pressure transients and hydraulic fracturing.