Update: Multibeam survey and analysis -
A second expedition in September 2008 aboard the R/V Melville (S. Merle, Chief Sci) collected EM120 bathymetry and backscatter data over the swarm site and down the Gorda Ridge (white outline). The EM300 backscatter and 3.5 kHz sub-bottom profile data indicate there are recently (Holocene) active faults within the Juan de Fuca intraplate, which is consistent with seismic refraction data from the area. The EM120 data provides a new, high resolution image of the seafloor that had not been previously available from the existing low resolution bathymetry. These intraplate faults are probably relict normal faults from the ridge re-activated by the current stress regime. The March intraplate earthquake swarm could have resulted from slip on these faults. It is important to note that the amount of offset observed on these faults did not occur only during the March 2008 earthquake swarm, but represents long-term slip along these faults. The March 2008 swarm of earthquakes were all relatively low magnitude earthquakes and were not felt on land.
An additional high resolution seafloor survey of the eastern Blanco Transform fault was obtained during the summer 2009 by the NOAA R/V Okeanos Explorer.
2009 Media Links:
Oregon Public Broadcasting:
Temporal history of earthquake activity in the vicinity of the Blanco Transform, 30 March - 6 April 2008. Black line shows the cumulative number of earthquakes located by SOSUS, with an average rate of ~120 events/day observed during the initial four days of the swarm. Stem plot indicates the acoustic source level (dB re 1μPa @ 1m) of each event. Data shown related to activity associated with the intraplate swarm Earthquakes having magnitudes Mw ≥ 5 are identified. (click for larger size)
News Guard (4/30/08): Four days aboard the Wecoma
National Public Radio (4/16/08): Earthquake Swarm Worries Oregon
National Geographic (4/16/08): Mysterious "Swarm" of Quakes Strikes Oregon Waters
Science Daily (4/14/08) : Unusual Earthquake Swarm Off Oregon Coast Puzzles Scientists
MSNBC (4/13/08):Undersea quake swarm puzzles the experts
Seattle Times (4/13/08): Scientists investigate rash of earthquakes off Oregon's coast
Oregonian (4/12/08): Quakes off Oregon coast shake up a mystery
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 synopsis here.
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 1 (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 left)
|Map showing the earthquakes of the North Gorda swarm from 23-24 April 2008. (click for full size map)|
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 sites. View logbook from Bill Hanshumaker, educator at sea.
Figure 1: >Map showing the earthquakes, cruise track line and CTD locations. (click for full-size)
A response 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.
On 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 2). Given 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).