Two large clusters of earthquake activity in June of 1999 and January of 2000 have dominated recent seismicity along the Endeavour Segment of the Juan de Fuca Ridge. The impacts of the June 1999 sequence on the hydrologic system, which include changes in vent temperature and chemistry within the Main Endeavour Vent Field, have been well documented previously. Analysis of seismic and hydroacoustic data indicates that both sequences exhibit a swarm-like behavior, characterized by the absence of a dominant main shock event. The epicentral locations of events within the two swarms overlap spatially, with centroid positions near 47degrees49' and 47degrees46'N latitude. During the June 1999 swarm, the initial activity spans the along-axis region where a shallow axial magma chamber reflector was later imaged. The epicenters then migrate similar to12 km to the south at a rate of 0.3 m/s, consistent with lateral dike propagation. A distinct subcluster of events also occurred in the vicinity of Surveyor Volcano on the overlapping portion of the Cobb Segment. Given its distance from the main swarm, this activity may represent a triggered response to dynamic shaking. The January 2000 swarm has a more limited along-axis extent, relative to the June 1999 swarm, with no indication of lateral migration. Much of this activity is concentrated in a region predicted to have undergone extension due to dike propagation in 1999. Although it contains fewer total events and is of shorter duration, relative to the June 1999 swarm, the January 2000 activity exhibits a higher peak rate of seismicity and greater mean event magnitude. As in situ temperature monitoring was not in place during January 2000 and vent fluids were not sampled until June 2000, the impacts of this swarm on the hydrothermal system are unknown. The southernmost tip of the Endeavour Segment also is found to be a region of repeating swarm activity. Although morphologic evidence indicates the Cobb Segment has been propagating northward recently, this seismic activity suggests that the western limb of the Endeavour-Cobb overlap zone remains active.
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