William Q. Sumner and M. Meghan Miller1
Continuous Global Positioning System (GPS) stations in the Pacific Northwest move northeast with respect to the North American plate as a result of coupling along the subducting Juan de Fuca plate. In early 2001 GPS stations from Puget Sound to northern Oregon reversed this northeast direction of motion for several weeks. The reversed motion shares some of the spatial and temporal signatures of aseismic creep events (or silent earthquakes) previously reported farther north along the Cascadia plate interface [1,2], although the periodic recurrence observed farther north has not been established here. The strongest aseismic motion was detected southwest of Seattle at the GPS stations SATS and RPT1. On February 28, 2001, during this period of aseismic slip, the Mw 6.8 Nisqually earthquake ruptured a fault within the Juan de Fuca plate. The rupture occurred at some 50 km depth; its epicenter lies between these two stations. The aseismic motion preceeding the earthquake is consistent with slip on the plate boundary. The displacements from the Nisqually earthquake itself are in good agreement with calculations using fault parameters from the inversion of seismic data.
1) Herb Dragert,, Kelin Wang, and Thomas S. James, Science, 292, 1525-1528, 2001
2) M. Meghan Miller, Tim Melbourne, Daniel J. Johnson, and William Q. Sumner, Science, 295, 2423, 2002
1 Department of Geological Sciences, Central Washington University, Ellensburg, WA 98926 sumner@geology.cwu.edu