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 western Blanco Transform Fault Zone (BTFZ), was the site of an intense earthquake T‐wave swarm that began at 1317Z on January 9, 1994. Although technically 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 eruption near Socorro Island, a seamount several hundred km west of Baja, California. The ∼69 earthquakes and ∼400 tremor‐like events at the EBD occurred sporadically, with two periods of peak activity occurring between January 5–16 and 27–31. The swarm‐like character of the earthquakes and the similarity of the tremor activity to the Socorro eruption indicated that the EBD was undergoing an intrusion or eruption episode. On January 27, six CTD/rosette casts were conducted at the site. Water samples from two of the stations yielded anomalous ³He concentrations, with maxima at ∼2800 m depth over the main basin. In June 1994 two camera tows within the basin yielded evidence of pillow‐lava volcanism and hydrothermal deposits, but no conclusive evidence of a recent seafloor eruption. In September 1994, deployments of the U.S. Navy's Advanced Tethered Vehicle resulted in the discovery of an active hydrothermal mound on the flanks of a pillow‐lava volcano. The hydrothermal mound consists of Fe‐rich hydrothermal precipitate and bacterial mats. Temperatures to 60°C were measured 30 cm below the surface. This is the first discovery of active hydrothermal vents along an oceanic fracture zone. Although no conclusive evidence of volcanic activity associated with the T‐wave event swarm was found during these response efforts, the EBD has been the site of recent seafloor eruptions.