After a 3-9 April 2001 seismic swarm that was traced to the Jackson Segment of the Gorda Ridge (BGVN 26:03), seismically inferred volcanism remained unconfirmed. The signals detected on 3 April 2001 were located on the S side of the segment, and continued through 9 April. During a six-day period instruments detected over 3,500 earthquakes; 548 epicenters were located. By 11 April seismic activity was at very low levels, possibly below the detection threshold of the T-phase monitoring system.

On 10 April, an NSF- and NOAA-funded response team departed on the ship RV New Horizon to search for mega-plumes from the event, but no plumes were detected. On 26 April the U.S. Coast Guard ship Healy conducted conductivity, temperature, and depth (CTD) probes and took dredge samples on the site. A report made available in late May indicated that investigations from the Healy also failed to find evidence of an eruption at the Jackson Segment and detected no significant thermal anomalies from hydrothermal plumes. Rocks recovered by dredge from the sea floor were clearly old. The entire segment was also resurveyed with multibeam sonar to compare with bathymetry collected before the earthquake swarm. The early April earthquake swarm may have indicated moving magma that never made it up to the sea floor to erupt.

Geological Summary. The Jackson Segment of the Gorda Ridge more than 200 km off the coast of Oregon lies immediately SSW of the North Gorda Ridge, the northermost of five segments forming the Gorda Ridge spreading center. The first recorded activity took place in April 2001, when volcanic seismicity was detected by hydroacoustic monitoring. The seismicity indicated possible dike propagation to the south and was similar to that which was documented at the time of the eruption of a submarine lava flow from the adjacent North Gorda Ridge segment in 1996. The 2001 activity originated from the central axial valley of the Jackson Segment, near the "narrowgate" on the southern part of the segment. Later surveys, however, revealed no evidence for submarine eruptive activity in April 2001.

Information Contacts: Bob Embley (NOAA/PMEL) and Jim Cowen (SOEST, Univ. of Hawaii), NOAA Pacific Marine Environmental Laboratory (PMEL), 2115 SE Osu Drive, Newport, OR 97365 USA (URL: https://www.pmel.noaa.gov/).