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). The earthquake swarm was detected using the U.S. Navy hydrophones in the northeast 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 ~50 km along 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 lateral magma dike injection. The largest three swarm earthquakes occurred within the caldera. Their timing and mechanisms are consistent with adjustment of the caldera floor as magma is removed from beneath the summit. Earthquakes with short T-wave rise times (shallow focal depths) cluster at two spots along the summit and south rift zone, and are considered possible seafloor eruption sites. In situ ground deformation and hydrothermal plume monitors, and later shipboard observations, confirmed the occurrence of a seafloor volcanic eruption at the volcano's summit. ¿ 1999 American Geophysical Union