Additional Report for Jackson Segment

Jackson Segment

Gorda Ridge, NE Pacific Ocean

42.15°N, 127.05°W; summit elev. -3,000 m (submarine)

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 eruptive activity from the Jackson Segment 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.


Index of Monthly Reports

Reports are organized chronologically and indexed below by Month/Year (Publication Volume:Number), and include a one-line summary. Click on the index link or scroll down to read the reports.

03/2001 (BGVN 26:03) Volcanic seismicity along the Jackson segment, early April 2001

08/2001 (BGVN 26:08) Additional investigations show no evidence of April eruption


Contents of Monthly Reports

All information contained in these reports is preliminary and subject to change.

All times are local (= UTC + 8 hours)

03/2001 (BGVN 26:03) Volcanic seismicity along the Jackson segment, early April 2001

Volcanic seismicity along the Jackson Segment of the Gorda Ridge (figure 1) was detected by the NOAA/PMEL T-Phase Monitoring System beginning at ~1750 on 3 April 2001. Data were recorded using the U.S. Navy's Sound Surveillance System (SOSUS) in the NE Pacific, and were similar to the seismicity produced by the February 1996 eruption along the North Gorda Ridge segment of the ridge (BGVN 21:02).

Figure 1. Map of Gorda Ridge segments (index map in upper left corner), with arrows locating the 2001 eruption along the Jackson segment; this segment lies ~60 km SW of the site where the February 1996 eruption initiated, along the North Gorda segment. Courtesy of NOAA/PMEL.

For the current episode, epicenters were inferred from hydroacoustic analysis (figure 2). Multibeam bathymetry from the area indicated that the signal source came from within the central axial valley. The activity was located near the summit of the "narrowgate" on the S side of the ridge, and indicated possible dike propagation.

Figure 2. Earthquake locations for the Gorda Ridge derived from hydroacoustic monitoring. The time interval for these events is undisclosed. Courtesy of NOAA/PMEL.

The source of the recorded signals was analogous to the 1996 event's relative location along the ridge. The nature of the seismic character was consistent with other examples of volcanic seismicity that were detected hydroacoustically. These events were characterized by low-magnitude earthquakes emergent from background ambient ocean noise without a large "mainshock" to initiate the sequence. These were then followed by over a week of elevated seismicity, with rapidly repeating (up to 100 events/hour) small earthquakes and nearly continuous volcanic tremor in the range of 2-30 Hz (figure 3). The events were heard on multiple SOSUS arrays without the benefit of beamforming, indicating that they were relatively loud. Recorded epicenter locations as of 4 April indicated lateral migration of events to the S, typical of dike injections. As of 14 April, no hydrothermal plumes had been successfully detected.

Figure 3. Histogram showing seismic events per hour along the Gorda Ridge during 3-12 April 2001. After a figure of NOAA/PMEL.

Background. The Jackson Segment lies immediately SSW of the North Gorda Ridge, the northernmost of five segments forming the Gorda Ridge. The April 2001 activity reported here was the first documented at the Jackson Segment. A submarine lava flow was erupted in late February and early March 1996, near the center of the North Gorda Ridge segment. The eruption was initially detected through acoustic T-waves from a seismic swarm and the emission of large thermal plumes. Camera tows in April 1996 revealed new lava flows about 100-200 m wide along a fissure that was at least 3.5 km long. A seismic swarm of uncertain origin also occurred at this location in January 1998.

Information Contacts: Chris Fox, Bob Dziak, and Paul Will, NOAA Pacific Marine Environmental Laboratory (PMEL), 2115 SE OSU Drive, Newport, OR 97365 USA (Email: fox@pmel. noaa.gov, dziak@pmel.noaa.gov, will@pmel.noaa.gov, URL: http://www.pmel.noaa.gov/).

08/2001 (BGVN 26:08) Additional investigations show no evidence of April eruption

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.

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 (Email: embley@pmel.noaa.gov, jcowen@soest.hawaii.edu, URL: http://www.pmel.noaa.gov/vents/acoustics/seismicity/ seismicity.html).