Scientists aboard R/V Wecoma returning to Newport, Oregon. (full-size)
The North Gorda swarm essentially stopped as of last week, but interestingly there have been two large events (magnitude 4.6 and 4.5) in the north segment valley (42-52'N, 126-42'W) in the last two days. These large earthquakes were fairly isolated events, probably representing continued crustal deformation in response to the swarm of 22-25 April.
Synopsis as of 2 May 2008-
The earthquake swarms north of the Blanco Transform and
on the northern Gorda Ridge segment have subsided and seismicity is at background
levels. Overall, it was a very interesting progression of seismicity, beginning
with an intense, 10-day intraplate swarm north of the central Blanco Transform.
This was followed over the next week by two magnitude 5+ earthquakes
(with aftershocks) at the Cascadia basin (central BTF) and along the eastern
Blanco Transform. Then surprisingly during the response cruise, another
intense earthquake swarm began on the northern Gorda Ridge segment adjacent
to the Blanco Transform. The Gorda swarm produced >1000
detected earthquakes over the 5 day duration of the event...full
Wecoma event response was led by Ron Greene,
Matt Fowler, and Susan Merle of the NOAA Vents Program. Wecoma response
cruise operations log now available (pdf).
Complete listing of scientists aboard the expedition available here.
R/V Wecoma returns
to Newport. 11 CTD casts are being analyzed for final results in
NOAA Vents laboratories. Hydrophone remains on seafloor to continue monitoring
seismic activity. Figure
(below) updated to show final cruise track and seismicity at both sites.
Northern Gorda seismicity
histogram chart and time
vs. latitude plot now available.
Researchers are investigating
another earthquake swarm located on the northern Gorda Ridge which
began early Tuesday morning. Wecoma researchers conducting CTDs at
new swarm area. (see map at
CTD casts conducted at East Blanco Depression,
Cascadia Depression (CD)and half-way between swarm center and CD.
Although the realtime sensors on the CTD do not show any evidence
of hydrothermal emissions from the seafloor, only shore-based measurements
will provide definitive analyses of the water samples. Ship ahead
of schedule allowing further CTD operations. Hydrophone will not
be recovered on this expedition.
Hydrophone deployed. At-sea analysis
of CTD samples from the swarm site show do not show evidence of
a hydrothermal plume. Conducting further CTDs at other nearby quake
logbook from Bill Hanshumaker, educator
Animation of earthquakes through
Figure 1: Map
showing the earthquakes, cruise
track line and CTD locations. (click
Courtesy of Del Bohnenstiehl,
North Carolina State University.
cruise aboard the R/V Wecoma is scheduled to depart Newport, Oregon
on Sunday, April 20th. The focus of the expedition is
to search for water-column plumes that might indicate expulsion
of hydrothermal or crustal fluids and/or seafloor volcanic activity.
NOAA Vents research scientists will be conducting CTD operations
at several sites where there has been recent earthquake activity,
both on the Juan de Fuca plate and within the Blanco Fracture Zone.
Sunday March 30, 2008, a large earthquake
swarm began within the central Juan de Fuca plate, located ~150 nautical
miles west of the Oregon coast and ~
70 km north of the Blanco Transform Fault (Figure
1). This earthquake
swarm (with red, yellow, brown, purple dots representing different
days) is located on a basin between two faulted basement
highs that rise above the surrounding, deep abyssal sediments.
The swarm, located using the SOSUS hydrophone arrays, has produced
more than 600 earthquakes in the past 10 days. This earthquake
swarm is unique since it is the first time in 17 years of SOSUS
recordings that a set of earthquakes this focused and intense
has occurred within the middle of the Juan de Fuca plate away from
the major, regional tectonic boundaries. The abyssal hill
nearest the swarm location appears to be on a curved structure
that trends northwestward from the Cascadia Depression in the
BTFZ toward the Juan de Fuca Ridge. However, seismicity decay
rates indicate this is not a standard mainshock-aftershock sequence
and some process is sustaining a high stress rate in the crust (Figure
the midplate location of the swarm, a volcanic event response effort
does not seem warranted.* However, this situation represents a rare
opportunity to learn more about this very unusual event. Accordingly,
we are exploring options for ships-of-opportunity that might be able
to take some key samples and measurements with a CTD over the earthquake
swarm site with a minimum of time and effort. (*See
above update, response cruise is departing April 18 from Newport).
Dziak - Seismologist - Email
Embley - Geologist - Email
William Chadwick -
Geologist - Email
Analyst - Email
John Lupton -
Studies - Email