Several thousand microearthquakes were recorded by ocean-bottom seismometers during 11 days of deployment on the Cleft segment of the southern Juan de Fuca Ridge. These microearthquakes primarily occurred in swarms lasting up to 1 hr, and are characterized by complex time series with typical single-event durations of just over 1 sec. The events were too small to be detected by more than one instrument at minimum instrument 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 at discrete frequencies that are not simply related (i.e., not integer multiples). These peaked spectra are quite similar to those observed from geysers and long-period events in volcanoes. Calibration tests performed during the deployment indicate that the event spectral peaks are not explained by instrument resonance or seafloor coupling. Based on these observations, the vigorous hydrothermal activity in the area, and thermodynamic considerations, we postulate that the majority of the microseismicity observed is generated within the seafloor hydrothermal system by pressure transients and hydraulic fracturing.

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