National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 2011

A seismic swarm and regional hydrothermal and hydrological perturbations: the northern Endeavour segment, February 2005

Hooft, E., H. Patel, W. Wilcock, K. Becker, D. Butterfield, E. Davis, R. Dziak, M. Lilley, P. McGill, D. Toomey, and D. Stakes

Geochem. Geophys. Geosyst., 11(12), Q12015, doi: 10.1029/2010GC003264 (2010)

Seismic swarms at mid-ocean ridges frequently accompany the tectonic and magmatic events associated with plate spreading. The February 2005 swarm occurred near an overlapping spreading center (OSC) at the northern end of the Endeavour segment and is the first swarm on the Juan de Fuca Ridge to be recorded on a local seafloor seismic network. The swarm included several earthquakes, which were recorded at regional distances, and resulted in triggered seismicity and a hydrothermal response within the Endeavour vent fields as well as regional-scale hydrologic pressure perturbations. Here we report on over 6000 earthquakes that were recorded during this seismic sequence. The spatial and temporal pattern of the seismic sequence is complex consisting of increased seismicity around the entire OSC during yeardays 52–55, a small short swarm during days 56-57, and the main swarm during days 58–64. The short day 56–57 swarm and the initial phase of the main day 58–64 swarm are both located at the northern end of the Endeavour ridge axis. After 11 hours, the activity of the main day 58–64 swarm jumps to the southwest end of the Endeavour Valley. Small magnitude events are dominant (b-values are 1.5 to ~2) and seismicity rates wax and wane over several days indicating that a magmatic process drives the activity. Most of the moment release, including six strike-slip events, occurs at the southwest end of the Endeavour Valley, where the swarm epicenters generally migrate south. This seismic sequence triggers regional responses. The start of the main day 58–64 swarm on the northern Endeavour ridge initiates a hydrologic pressure response at four sealed seafloor boreholes ~25–105 km away. This response also has a diffusive component: 2.5 days after the swarm onset there is an increase in vent field seismicity at the center of the Endeavour segment, which correlates with a modest temperature increase at a diffuse vent in the Mothra field. We infer that the main process driving the February 2005 seismic sequence is a largely-aseismic magmatic intrusion at the northern tip of the Endeavour axis. Resulting stress changes cause tectonic deformation, as evidenced by the large strike-slip events, at the opposing limb of the Endeavour OSC in the southwest Endeavour Valley. This may be accompanied by southward dike propagating from the West Valley segment consistent with the eventual decapitation of the northern Endeavour ridge axis by the West Valley segment. We infer that the delay in the seismic and hydrothermal vent field perturbations is driven by a hydrologic pressure pulse that diffuses away from the main magma intrusion.

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