ACOUSTIC DETECTION OF A SEA-FLOOR SPREADING EPISODE ON THE JUAN-DE-FUCA RIDGE USING MILITARY HYDROPHONE ARRAYS

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 r esearch 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 Islan d to allow real-time acoustic monitoring of the Juan de Fuca Ridge usi ng SOSUS. On June 26, 2145 GMT, a burst of low-level seismic activity, with accompanying harmonic tremor, was observed and subsequently loca ted near 46 degrees 15'N, 129 degrees 53'W, on the spreading axis of t he Juan de Fuca Ridge. Over the following 2 days, the activity migrate d to the NNE along the spreading axis with the final locus of activity near 46 degrees 31.5'N, 129 degrees 35'W. The nature of the seismicit y was interpreted to represent a lateral dike injection with the possi bility of eruption on the seafloor. Based on this interpretation, a re sponse effort was initiated by U.S. and Canadian research vessels, and both warm water plumes and fresh lavas were subsequently identified a t the site.