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wilcock

HR: 16:15h
AN: V54B-02
TI: An Automatically Generated Earthquake Catalog for the Endeavour Segment of the Juan de Fuca Ridge: Linkages Between Segment and Vent-Field Scale Seismicity
AU: * Weekly, R T
EM: rtweekly@u.washington.edu
AF: University of Washington, Department of Earth and Space Sciences Box 351310, Seattle, WA 98115, United States
AU: Wilcock, W S
EM: wilcock@u.washington.edu
AF: University of Washington, School of Oceanography Box 357940, Seattle, WA 98115, United States
AU: Toomey, D R
EM: drt@uoregon.edu
AF: University of Oregon, Department of Geological Sciences, Eugene, OR 97403, United States
AU: Hooft, E E
EM: emilie@uoregon.edu
AF: University of Oregon, Department of Geological Sciences, Eugene, OR 97403, United States
AU: McGill, P R
EM: mcgill@mbari.org
AF: Monterrey Bay Aquatic Research Institution, 7700 Sandholdt Road, Moss Landing, CA 95039, United States
AB: From 2003-2006, the W.M. Keck Foundation supported the operation of an ocean-bottom seismometer network along the central portion of the Endeavour Segment of the Juan de Fuca Ridge as part of a multidisciplinary prototype NEPTUNE experiment to investigate linkages between seismic deformation, perturbations to hydrothermal fluxes, and microbial productivity along oceanic plate boundaries. The network consisted of one Guralp CMG-1T broadband seismometer and seven short-period seismometers, and was deployed beneath the seafloor using remotely operated vehicles to improve coupling to the seafloor and minimize low-frequency noise from ocean currents. Eight student analysts processed data from 2003-2004 and manually located ~13,000 earthquakes. Another ~6,400 earthquakes have also been located manually for a seismic swarm occurring near the northern end of the Endeavour Segment between late-February and early-March, 2005. In order to efficiently obtain a complete catalog of earthquake locations for the remainder of the deployment period, we have developed an automated method. Our algorithm involves three steps: triggering on potential arrivals using a ratio of short- and long-term root-mean-square values, associating triggers into events if they occur within 2.5 sec of each other and exhibit spectral characteristics of an earthquake, and locating earthquakes based on automatically picking triggers. We require that each hypocenter be located with a minimum of six picks across four stations with at least two P- and two S-wave picks. Comparisons for test periods in 2003- 2004 show that the catalog generated by the algorithm has a comparable level of completeness to that of the student analysts. To date, we have obtained preliminary automatic locations for 396 days of data from August 2004-September 2005. The catalog includes hypocentral parameters for ~22,000 earthquakes, including over 2,000 events that occur underneath the hydrothermal systems. In 2003-2004 most of the earthquakes in the network were located between the Main and High Rise vent fields, but in October 2004 the locus of the most intense vent-field seismicity shifts further north beneath the High Rise and Salty Dawg vent fields. A regional swarm beginning February 6, 2005, which was located ~15km to the north of the network, triggered a marked spike in seismicity beneath the High Rise and Salty Dawg fields. A larger regional swarm to the north of the network that began on February 27, 2005 that has been analyzed previously also triggered increased seismicity in the network, but with most of the earthquakes located between the Main and High-Rise vent fields. Following these swarms, average levels of segment- and vent-field scale seismicity decrease markedly to 20% and 15% of pre-swarm levels, respectively, and these low-levels of seismicity persist until the end of the analyzed interval. Thus, two regional swarms, that are close in both space and time, perturbed seismogenic processes beneath the Endeavour hydrothermal vent fields in significantly different ways. We interpret the subsequent period of seismic quiescence to be a consequence of swarms relieving regional stresses along the entire Endeavour Segment, and modulating the hydrothermal and magmatic processes that generate stresses beneath the hydrothermal vent fields.
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 7220 Oceanic crust
DE: 7245 Mid-ocean ridges
DE: 8178 Tectonics and magmatism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2008 Fall Meeting