FY 1988

A microseismicity survey of Axial Seamount, Juan de Fuca Ridge

Jacobson, R.S., L.D. Bibee, R.W. Embley, and S.R. Hammond

Bull. Seismol. Soc. Am., 77(1), 160–172 (1987)

An array of ocean bottom seismometers and hydrophones were deployed within the caldera of Axial Seamount, located at the intersection of the Cobb-Eickelberg Seamount Chain and the Juan de Fuca Ridge. Recent manned submersible dives have discovered the presence of two distinct hydrothermal vent fields. The objective of the microseismicity survey was to determine baseline information necessary for designing an optimal experiment to monitor seismicity associated with submarine volcanic and hydrothermal systems. With an instrument deployment period of eight days, over 1500 events were recorded and examined. Most events occurred within three periods of swarm activity. The location of the swarm events is believed to be within the low temperature vent field in the northwest portion of Axial's caldera. These impulsive, high-frequency events are interpreted to be tectonic in origin. No harmonic tremor-like events were observed on any of the instruments, indicating a lack of magmatic movement. Only nine local events were recorded on more than one instrument, and none of these could be located unambiguously. Monochromatic 20-Hz signals were also recorded and are interpreted to originate from whales observed in the immediate vicinity.