The January 1998 earthquake swarm at axial volcano, Juan de Fuca Ridge: Hydroacoustic evidence of seafloor volcanic activity.

On January 25, 1998 at 11:33Z, an intense swarm of earthquakes began within the summit caldera of Axial Volcano, central Juan de Fuca Ridge (JdFR). Th e earthquake swarm was detected using the U.S. Navy hydrophones in the nort heast Pacific Ocean. The earthquake swarm lasted 11 days and produced 8247 detected (1037 located) earthquakes. During the first two days of the swarm , earthquake activity migrated from the summit caldera similar to 50 km alo ng the south rift zone of Axial Volcano at rates of 0.92 +/- 0.13 and 0.23 +/- 0.09 m s(-1). Earthquake epicenter migration is characteristic of a lat eral magma dike injection. The largest three swarm earthquakes occurred wit hin the caldera. Their timing and mechanisms are consistent with adjustment of the caldera floor las magma is removed from beneath the summit. Earthqu akes with short T-wave rise times (shallow focal depths) cluster at two spo ts along the summit and south rift zone, and are considered possible seaflo or eruption sites. In situ ground deformation and hydrothermal plume monito rs, and later shipboard observations, confirmed the occurrence of a seafloo r volcanic eruption at the volcano's summit.