DETECTION OF AND RESPONSE TO A PROBABLE VOLCANOGENIC T-WAVE EVENT SWARM ON THE WESTERN BLANCO TRANSFORM-FAULT ZONE

The East Blanco Depression (EBD), a pull-apart basin within the wester n Blanco Transform Fault Zone (BTFZ), was the site of an intense earth quake T-wave swarm that began at 1317Z on January 9, 1994. Although te ctonically generated earthquakes occur frequently along the BTFZ, this swarm was unusual in that it was preceded and accompanied by periodic , low-frequency, long-duration acoustic signals, that originated from near the swarm epicenters. These tremor-like signals were very similar in character to acoustic energy produced by a shallow-submarine erupt ion near Socorro Island, a seamount several hundred km west of Baja, C alifornia. The similar to 69 earthquakes and similar to 400 tremor-lik e events at the EBD occurred sporadically, with two periods of peak ac tivity occurring between January 5-16 and 27-31. The swarm-like charac ter of the earthquakes and the similarity of the tremor activity to th e Socorro eruption indicated that the EBD was undergoing an intrusion or eruption episode. On January 27, six CTD/rosette casts were conduct ed at the site. Water samples from two of the stations yielded anomalo us He-3 concentrations, with maxima at similar to 2800 m depth over th e main basin. In June 1994 two camera tows within the basin yielded ev idence of pillow-lava volcanism and hydrothermal deposits, but no conc lusive evidence of a recent seafloor eruption. In September 1994, depl oyments of the U.S. Navy's Advanced Tethered Vehicle resulted in the d iscovery of an active hydrothermal mound on the flanks of a pillow-lav a volcano. The hydrothermal mound consists of Fe-rich hydrothermal pre cipitate and bacterial mats. Temperatures to 60 degrees C were measure d 30 cm below the surface. This is the first discovery of active hydro thermal vents along an oceanic fracture zone. Although no conclusive e vidence of volcanic activity associated with the T-wave event swarm wa s found during these response efforts, the EBD has been the site of re cent seafloor eruptions.