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Red
star shows approximate location of seismic event. See epicenter
map for detailed location maps.
Histogram
of seismic events, click
for full-size image.
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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.
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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...
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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.
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