Periodic slow earthquakes from the Cascadia subduction zone.

Continuous geodetic measurements from convergent margins have shown that deep transient creep events can release large amounts of strain energy without detectable seismic shaking, and they are thus known as slow or silent earthquakes. Because subduction zones generate the largest earthquakes, seismic hazard assessment relies on understanding the degree to which slow earthquakes reduce the energy released by infrequent large earthquakes. We present data that indicate the periodicity of slow earthquakes in Cascadia and the current onset of such an event. Along the Cascadia margin, great earthquakes (moment magnitude, Mw . 8) have occurred over its 1500-km length from central British Columbia to northern California as the Juan de Fuca plate subducts beneath the North American plate (1–4). Since 1992, continuous global positioning system (GPS) monitoring of changes in station position have been used to monitor surface deformation associated with tectonic events (5, 6). In the fore arc that lies between the offshore plate boundary and the Cascade volcanoes, current interseismic deformation is dominated by northeast-directed contraction, the result of shallow convergence of the Juan de Fuca plate beneath the North American plate. During 1999, a sudden shift from fore arc contraction to short-lived extension led to identification of a deep slow earthquake (7). Our analysis of GPS results from the Pacific Northwest Geodetic Array (PANGA) indicates that eight slow earthquakes have occurred since 1992. Both fore arc station positions in the global reference frame and precise baselines relative to Penticton, British Columbia (DRAO), show episodic, discrete extension events superimposed on interseismic crustal shortening (Fig. 1). DRAO lies inland and is largely unaffected by convergence-related contraction. The transient reversals from secular contraction are consistent with southwest-directed fore arc motion that is both spatially and temporally coherent. Each event starts over a 3-week period and lasts 2 to 4 weeks at any one station. Propagation of the slow earthquake across the affected region may last for 8 weeks. The slow earthquakes occur on average every 14.5 6 1 months over the past 10 years and typically begin in the Puget Lowlands near the arch in the downgoing Juan de Fuca plate. Observa-