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localized to DRAO itself are not the source of the transient reversals. The 14.5-month aver- age recurrence is similar to the period of theChandler resonance (8). Nonetheless, thisresonance cannot explain the observed base- line changes or the sawtooth pattern in thetime series.
M. Meghan Miller,* Tim Melbourne, Daniel J. Johnson, earthquake that began on 7 February 2002 at Friday Harbor, Washington (Fig. 1; inset).
Discrete offsets in the north and east compo-nents fit the pattern observed at nearby sta- fected region may last for 8 weeks. The slow tions in previous events and are visible at this earthquakes occur on average every 14.5 Ϯ 1 new station in the daily positions. The offset months over the past 10 years and typically and partial recovery in the north component amounts of strain energy without detectable begin in the Puget Lowlands near the arch in reflect propagation of a slow earthquake un- seismic shaking, and they are thus known as the downgoing Juan de Fuca plate. Observa- slow or silent earthquakes. Because subduc- tion zones generate the largest earthquakes, quakes invites an assessment of the role of seismic hazard assessment relies on under- transient, aseismic creep events in the de- forming Earth. Over the time scale of re- quakes reduce the energy released by infre- currence intervals for the plate interface, quent large earthquakes. We present data that indicate the periodicity of slow earthquakes in Cascadia and the current onset of such an field determined over the past 10 years by GPS contains a rich record comprising per- manent deformation overlain by an elastic occurred over its 1500-km length from cen- strain field, which is ultimately recovered tral British Columbia to northern California as the Juan de Fuca plate subducts beneath the North American plate (1- 4 ). Since 1992, regular and cyclical nature of the transient continuous global positioning system (GPS) events indicates that they are a fundamen- monitoring of changes in station position have been used to monitor surface deforma- tion associated with tectonic events (5, 6 ). Inthe fore arc that lies between the offshore References and Notes
plate boundary and the Cascade volcanoes, 1. J. Adams, Tectonics 9, 569 (1990).
2. K. Satake, K. Shimizaki, Y. Tsuji, K. Ueda, Nature 379,
current interseismic deformation is dominat- ed by northeast-directed contraction, the re- 3. B. F. Atwater, E. Hemphill-Haley, Recurrence intervals sult of shallow convergence of the Juan de for great earthquakes of the past 3,500 years at Fig. 1. Time series for GPS stations along the
northeastern Willapa Bay, U.S. Geol. Surv. Prof. Pap. Fuca plate beneath the North American plate.
Cascadia margin. Baseline lengths are from 108 (1997).
During 1999, a sudden shift from fore arc 4. D. K. Yamaguchi, B. F. Atwater, D. E. Bunker, B. E.
weekly station positions with respect to Pen- contraction to short-lived extension led to Benson, M. S. Reid, Nature 389, 922 (1997).
ticton (DRAO). (Inset) Time series shows daily
5. H. Dragert, R. D. Hyndman, Geophys. Res. Lett. 22,
positions for Friday Harbor (SC02) for 2002 in the global reference, illustrating the onset of 6. M. M. Miller et al., Tectonics 20, 161 (2001).
7. H. Dragert, K. Wang, T. S. James, Science 292, 1525
8. L. Xie, S. R. Dickman, Geophys. J. Intl. 121, 117
indicates that eight slow earthquakes have occurred since 1992. Both fore arc station for each event. Yet the last three events have 9. This material is based on work supported by the NSF under grants EAR-0002066, EAR-000003483, positions in the global reference frame and well-mapped displacement fields that differ EAR-9616540, and EAR-9973191 and through sup- precise baselines relative to Penticton, British across the network, implying differences in port of the University NAVSTAR Consortium Boul- the fault area that failed. Modeling suggests der Facility; by NASA contract NAG5-7672; and by extension events superimposed on interseis- creep of a few cm along the plate interface at the U.S. Geological Survey through annual awards under the National Earthquake Hazard Reduction mic crustal shortening (Fig. 1). DRAO lies Program to member institutions of the PANGA inland and is largely unaffected by conver- release approximates a M ϭ 6.7 earthquake, consortium. The Western Canada Deformation Ar- gence-related contraction. The transient re- consistent with previous analysis (7 ).
ray operated by the Geological Survey of Canada is versals from secular contraction are consis- No corresponding periodicity characteriz- tent with southwest-directed fore arc motion es the DRAO time series in the global refer- that is both spatially and temporally coherent.
Department of Geological Sciences, Central Washing- Each event starts over a 3-week period and other PANGA back arc stations, those that lie ton University, Ellensburg, WA 98926, USA.
lasts 2 to 4 weeks at any one station. Propa- east of the volcanic front. Thus, tectonic, *To whom correspondence should be addressed. E- gation of the slow earthquake across the af- tropospheric, or monument instability signals www.sciencemag.org SCIENCE VOL 295 29 MARCH 2002

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