A swarm of earthquakes on the crest of the Juan de Fuca Ridge was detected in June and July 1993 by a network of hydrophones. The activity migrated 60 km along the crest, suggesting a lateral dike injection and the possibility of a volcanic eruption. Subsequent geologic and oceanographic investigations confirmed that an eruption had taken place. Examination of the individual acoustic arrivals shows changes in the character of the signal that are consistent with an injection of magma. A reduction in the rise time of the wave packet and a proportional increase in high frequency energy was observed and is interpreted to result from a shoaling of the earthquake source region. Second, the source magnitudes were largest at the onset of the swarm and became smaller over time, also consistent with shoaling of the dike. The appearance of the T-wave arrivals changed significantly 5 days after the beginning of the swarm, potentially indicating the onset of a surface eruption. ¿ American Geophysical Union 1995