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Figure 2. Sequence and magnitude of earthquakes in the swarm. Filled or open circles indicate events large enough to be located with enough precision to distinguish between the groupings on the two different transform faults.
In a 4.5 hour period, more than 60 events in an earthquake swarm on the western boundary of the Easter microplate were detected by an array of ocean bottom seismometers and GSN station RPN on Easter Island. The larger events of the swarm were recorded by many GSN stations; the surface wave radiation patterns indicate that these events were strike-slip earthquakes located on two transform faults separated by about 25 km. Slip on the faults was closely coupled, with activity alternating back and forth randomly between the two transforms. Coupled seismic activity is usually attributed to triggering by static stress changes or dynamic stresses in propagating shear waves generated by another earthquake, but these earthquakes are too small for either mechanism to be plausible. The swarm may have been the seismic manifestation of a larger, primarily aseismic, slip event or slow earthquake involving both transforms, perhaps triggered by dike injection on the Easter-Pacific spreading center.
Forsyth, D.W., Y. Yang, M.-D. Mangriotis, and Y. Shen, Coupled seismic slip on adjacent oceanic transform faults, Geophys. Res. Lett., 30(12), 1618, doi:10.1029/2002GL016454, 2003.
This work was supported by National Science Foundation grants OCE-98912208 and CE-9896393.
Date Taken: January 1, 2006 Location: Donald W. Forsyth
