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March 2005
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Endeavour Rapid Response Cruise
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March
9: Five earthquakes occurred overnight on the ridge
between the southern west valley and the northern Endeavour
segment at ~48-18'N, 129-02W. Overall, however, earthquake
remain at background levels.
March
8: Weather forecast looks good for us to do a camtow
today. Winds are still around 30 but are supposed to start
coming down soon; they never got above 30 last night. We're
planning to do a camtow after the 3rd CTD tow-yo comes up
around mid-day today. The camera tow is planned for on-axis
(on Endeavour), south of where the 1st CTD ended, and over
the northern most AMC reflectors seen by the MCS survey (~10
km north of the Endeavour vent fields).
The
3rd CTD is still underway. it started at 48 12.5' and is N->S
between the other 2 tows, then it dog-legs to the SSW to paralled
the Endeavour high, and offset ~2 miles to the west (looking
for plumes downcurrent from the axis). It will go to about
47 56' or so. Right now we're at 48 02' and they haven't seen
much so far. (courtesy, B. Chadwick).
March
7: The first tow of the CTD package from the R/V
Thompson was completed last night. The tow survey was conducted
on a line from ~48-12'N to 48-05'N on the axis of northern
Endeavour segment. A few small signals were detected on the
nephelometer, and these will have to be investigated further
in the water chemistry. These signals could have been resuspension,
or perhaps the edge of some bigger source, but no major signals
yet. Conductivity sensor did not work on the first tow, and
therefore temperature anomaly was not available. EM300 bathymetric
data was collected during the tow and more survey lines will
be added between tows (~1.5-2 hrs in duration). Next up is
another North-South CTD tow, this time further west along
the 48-13'N to 48-05'N line but sampling the southern end
of West Valley segment. The winds were 20-30 knots early in
the tow, but are now under 20 knts and are supposed to stay
that way until tomorrow afternoon (courtesy, B. Chadwick).
Three
magnitude 5.2, 5.1 and 4.3 earthquakes occurred on the northern
Gorda Ridge segment over a 7.5 hr period beginning at
02:34Z on March 7. The SOSUS locations of these earthquakes
is 42-32.1'N 126-55.2'W which is along a section of the Gorda
Ridge that experienced a dike injection and seafloor eruption
in 1996. As of 1400Z, SOSUS has detected 228 earthquakes from
this sequence with event counts in the range of 13-27 per
hour. This is significantly smaller than the Endeavour swarm
during its peak in activity, and the extent of the magmatic
component to this earthquake sequence (if any) has yet to
be determined.
March
6: Earthquake activity at the Endeavour segment remains
at background levels and did not increase overnight. The R/V
Thompson is scheduled to be onsite at noon today. The initial
plan is to perform water-column surveys along the northern
end of Endeavour (on-axis) from 48 12' to 48 05'. If no temperature
anomaly is detected, the ship will move west over to the southern
end of West Valley and perform another North-South survey
there.
March
5:The R/V Thompson departed Seattle at 09:00 PST
today and should arrive at the earthquke swarm location on
the Endeavour on Sunday morning March 6. Earthquake activity
on the Endeavour segment has decreased to background levels,
with events occurring at a rate of 0-5 earthquakes per hour.
We will continue, however, to closely monitor all seismicity
on the Endeavour while the research ship is in the area.
March
4 Update: A research cruise on board the R/V Thompson
will leave from Seattle Saturday morning to investigate the
seafloor and water-column above the earthquake swarm.
Earthquake
activity has decreased significantly in the last 12 hours
to less than 10 earthquakes per hour. The earthquakes over
the last day have been locating along both the southwestern
part of the basin (west of the Main Endeavour Vent Field)
and in the saddle region of the Heck Seamount intersection.It
appears now that during the first 3-4 days, the earthquake
swarm was focused within the basin north and west of the MEF.
Today's earthquakes along the south part of the basin and
in the saddle region may be a response to the stress changes
induced by the initial intrusion event.
Review
of the Multichannel Seismic data from the MEF (courtesy R.
Detrick, E. Griffen, WHOI, S. Carbotte, LDEO) indicates that
although there was not an Axial Magma Chamber (AMC) as far
north as where the recent earthquake swarm is located, there
is possibly a weak AMC reflection about 10 km north of the
"Sasquatch" vent at the MEF, which is ~5-10 km east
of the earthquake swarm. In addition, seismic data has been
made available from a real-time buoy located on the Nootka
Fault (J. Collins, WHOI). This data will be incorporated with
SOSUS to help refine the locations of the swarm earthquakes.
March
3 Update: As of 1200Z, earthquake activity continues
at a moderate pace, with between 4-45 events per hour occurring
in the last 24 hrs. Earthquakes are still concentrated within
the basin north of the main Endeavour Vent field, with new
events occurring in the saddle region where the Heck Seamounts
intersect the north Endeavour segment.
March 2 Update: The earthquakes from the
Endeavour swarm continue at a somewhat reduced, but high level,
still reaching peaks of between 10-30 events per hour. The
earthquake locations are continuing to form an ~18-20 km long
cluster of seismicity with a north-east trend. The earthquake
cluster is centered along the west side of a deep basin to
the north and west of the main Endeavour vent field. Models
of the event decay rate (courtesy of D. Bohnenstiehl; LDEO)
indicate this earthquake swarm cannot be described by an Omori
Law decay rate of 1/t^p, suggesting the earthquake swarm's
behavior is inconsistent mainshock-aftershock (tectonic) activity
and may have a magmatic component. In addition, the large
M~5 earthquakes have unusually high numbers of aftershocks
(which is unexpected from tectonic events), and the periods
of near-linear event rate are similar to the diking event
observed at Endeavour in 1999. Recent MCS surveys of the region
(R. Detrick, WHOI and S.Carbotte, LDEO) show the presence
of an Axial Magma Chamber beneath the Main Endeavour Vent
Field. The southern end of the earthquake swarm, however,
is located 5-10 km north-northwest of the main Endeavour AMC.
The movement of the earthquake locations from the north to
southwest side of the basin seems inconsistent with this AMC
as a possible dike source. Also the strike-slip mechanisms
of the early large earthquakes support the possibility a deep
source for the earthquake swarm.
March
1 Update: As of 1200Z, earthquake activity recorded
on SOSUS from the central Endeavour segment continues at a
high level with rates of 20-50 events per hour. Earthquakes
are currently locating near the southern part of the basin
(~48-05'N; 129-07'W), concentrating south-southwest of the
initial earthquake locations on 27 February. In addition,
two more large (mb=4.4, 4.8) events occurred on March 1 and
were located on the northeast side of the basin. While the
earthquake swarm has a high event count consistent with volcanic
activity, the extent of the magmatic component to the swarm
remains unclear.
Feb. 28 Update: Since Sunday morning 27 February
at 0031Z, there has been an ongoing, intense earthquake swarm
on the Endeavour segment of the northern Juan de Fuca Ridge.
SOSUS has detected 1,498 earthquakes in the first 36 hrs of
the swarm, with event counts as high as 50-70 per hour which
is very similar in scale to past sea floor spreading events
at Middle Valley in 2001 and Endeavour in 1999. We are currently
working through the data, but have a preliminary location
of 48-14.5'N 128-57.6'W which is ~36 km north-northeast of
the Main Endeavour vent field and a few kms east of the intersection
of the Heck Seamounts with the JdF Ridge axis. We have not
located enough events yet to evaluate whether or not the earthquakes
have migrated, or are currently migrating along the ridge
axis. The sequence has also produced three large earthquakes
(mb=4.5, 4.8, 4.9) which have been detected by NEIC, UW, PNSN.
NEIC
lists the location of the largest event a little east of the
SOSUS location at 48-18.6'N 128-49.2'W. The Canadian Geological
Survey/Pacific Geoscience Center has computed focal
mechanisms for the largest earthquakes.
Back
to main event page: Endeavour,
March 2005
Note
there was another swarm of earthquakes in this same area in
October 2004.
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