Epicenter maps of the Middle Valley event. Left image has epicenters through 9/12/01 and right has epicenters through 9/20/01, indicating a southern migration of events. Click for full-size images.

At about 0330Z on JD 250 (Thursday, September 6, 2030PDT), a large seismic swarm was detected by the PMEL T-phase Monitoring System, which accesses the U.S. Navy's SOund SUrveillance System (SOSUS) to monitor ocean sound. The activity is located on the northern terminus of the Juan de Fuca Ridge. The general location (48.78N, 128.64W or 48 46.7'N; 128 38.6'W) is on the eastern edge of a large sedimented feature called Middle Valley.

This diagram shows earthquakes that occurred from Julian Day 250-262 (with position errors of < 5 minutes). The earthquakes have continued to propagate southward over time, and are now locating within a few kms of the hydrothermal vents and drill sites. More...

Event Description:
The land seismic network operated by the Pacific Geoscience Center of the Geological Survey of Canada has recorded and produced moment tensor solutions for several of the larger events, indicating a mixture of normal and strike-slip motion. Strike-slip events are oriented parallel to Juan de Fuca orientation (15 deg) and are likely associated with the ridge system. The complex interplay between the Nootka fault and the JdF volcanic system results in a diffuse triple junction with correspondingly complex stress fields. More...

UPDATE (9/27/01; 1330 PDT): Activity has slowed to near background levels. Nearly 14,000 earthquakes have been detected by NOAA/PMEL in the 21 days of activity. The Canadian R/V Tully will be at the site October 4-7 for water column sampling.

(10/3/01; 1000 PDT): R/V Tully sailed today with Chief Scientist Marv Lilley of the University of Washington.

Event Contacts/Inquiries

Event Detection:
Robert Dziak - Seismologist, Vents Acoustics Program robert.p.dziak@noaa.gov

Response Planning:
Jim Cowen - Chief Scientist, University of Hawaii jcowen@soest.hawaii.edu
Ed Baker - Chief Scientist, NOAA Vents Program edward.baker@noaa.gov