Since May 1996, an array of autonomous hydrophone moorings has been continuously deployed in the eastern equatorial Pacific to provide long‐term monitoring of seismic activity, including low‐level volcanic signals, along the East Pacific Rise between 20°N and 20°S and the Galapagos Ridge. The instruments and moorings were designed to continuously record low‐frequency acoustic energy in the SOFAR channel for extended periods and produce results comparable to those previously derived by using the U.S. Navy Sound Surveillance System (SOSUS) in the northeast Pacific. The technology and methodology developed for this experiment, including instrument design, mooring configuration, analysis software, location algorithms (with an analysis of errors), and a predicted error field, are described in detail. Volcanic activity is observed throughout the Pacific, along with seismicity along transform faults, subduction zones, and intraplate regions. Comparison data sets indicate detection thresholds and accuracy better than the land networks for open ocean areas and results comparable to, or better than, SOSUS. Volcanic seismicity along the fast spreading East Pacific Rise appears similar to documented examples in the northeast Pacific but with much shorter durations. One example from the intermediate spreading Galapagos Ridge is comparable to northeast Pacific examples, and several episodes of activity were observed in the Wilkes Transform Fault Zone. A site of continuing off‐axis seismicity is located near 18°S and 116°W. Isolated intraplate earthquakes are observed throughout the study area. Earthquake information from this experiment and future observations will be provided through the World Wide Web and earthquake data centers.

Until recently, no practical method has been available to continuously monitor seismicity of seafloor spreading centers. The availability of the U.S. Navy's SOund SUrveillance System (SOSUS) for environmental research has allowed the continuous monitoring of low‐level seismicity of the Juan de Fuca Ridge in the northeast Pacific. On June 22, 1993, NOAA installed a prototype system at U.S. Naval Facility Whidbey Island to allow real‐time acoustic monitoring of the Juan de Fuca Ridge using SOSUS. On June 26, 2145 GMT, a burst of low‐level seismic activity, with accompanying harmonic tremor, was observed and subsequently located near 46°15′N, 129°53′W, on the spreading axis of the Juan de Fuca Ridge. Over the following 2 days, the activity migrated to the NNE along the spreading axis with the final locus of activity near 46°31.5′N, 129°35′W. The nature of the seismicity was interpreted to represent a lateral dike injection with the possibility of eruption on the seafloor. Based on this interpretation, a response effort was initiated by U.S. and Canadian research vessels, and both warm water plumes and fresh lavas were subsequently identified at the site.

[1] Extraordinary video and hydrophone observations of a submarine explosive eruption were made with a remotely operated vehicle in April 2006 at a depth of 550-560 m on NW Rota-1 volcano in the Mariana arc. The observed eruption evolved from effusive to explosive, while the eruption rate increased from near zero to 10-100 m 3 /h. During the peak in activity, cyclic explosive bursts 2-6 min long were separated by shorter non-eruptive pauses lasting 10-100 s. The size of the ejecta increased with the vigor of the explosions. A portable hydrophone deployed near the vent recorded sounds correlated with the explosive bursts; the highest amplitudes were $50 dB higher than ambient noise at frequencies between 10 and 50 Hz. The acoustic data allow us to quantify the durations, amplitudes, and evolution of the eruptive events over time. The low eruption rate, high gas/lava ratio, and rhythmic eruptive behavior at NW Rota-1 are most consistent with a Strombolian eruptive style. We interpret that the eruption was primarily driven by the venting of magmatic gases, which was also the primary source of the sound recorded during the explosive bursts. The rhythmic nature of the bursts can be explained by partial gas segregation in the conduit and upward migration in a transitional regime between bubbly flow and fully developed slug flow. The strongest explosive bursts were accompanied by flashes of red glow and oscillating eruption plumes in the vent, apparently caused by magma-seawater interaction and rapid steam formation and condensation. This is the first time submarine explosive eruptions have been witnessed with simultaneous near-field acoustic recordings.

Volcanoes at spreading centres on land often exhibit seismicity and ground inflation months to years before an eruption, caused by a gradual influx of magma to the source reservoir [1][2][3][4] . Deflation and seismicity can occur on time scales of hours to days, and result from the injection of magma into adjacent rift zones [5][6][7][8] . Volcanoes at submarine rift zones, such as Axial Seamount in the northeast Pacific Ocean, have exhibited similar behaviour [9][10][11][12] , but a direct link between seismicity, seafloor deformation and magma intrusion has never been demonstrated. Here we present recordings from ocean-bottom hydrophones and an established array of bottom-pressure recorders that reveal patterns of both microearthquakes and seafloor deformation at Axial Seamount on the Juan de Fuca Ridge, before it erupted in April 2011. Our observations show that the rate of seismicity increased steadily during a period of several years, leading up to an intrusion and eruption of magma that began on 6 April 2011. We also detected a sudden increase in seismo-acoustic energy about 2.6 h before the eruption began. Our data indicate that access to real-time seismic data, projected to be available in the near future, might facilitate short-term forecasting and provide sufficient leadtime to prepare in situ instrumentation before future intrusion and eruption events.