EVIDENCE FOR PERIODIC SILENT EARTHQUAKES ALONG THE CASCADIA PLATE INTERFACE FROM THE PACIFIC NORTHWEST GEODETIC ARRAY

Strain accumulation and release down-dip of the seismogenic plate contact in subduction zones are not well understood. On the basis of thermal and constitutive modeling, these interfaces are thought to transition with depth from runaway rupture to stable sliding, between which lies a meta-stable regime where runaway fault rupture does not self-nucleate. Whether the deep interface steadily slips over the long term at some fraction of the convergence rate or is only triggered to rupture by seismogenic slip up-dip has historically proven difficult to identify. PANGA characterizes active deformation in the Pacific Northwest using an array of continuous GPS stations that monitor a range of temporal and spatial scales. This analysis forms the basis for mapping three transient deformation events since 1998 in a complete four-dimensional sense; baseline results from as early as 1992 suggest at least an additional four such events. For the three most recent events, including the 1999.6 discovery event of Dragert et al. [2001], time series analysis allows mapping of the creep front migration in space and time. Despite apparently characteristic offset at the station ALBH, these three events differ in their geographic extent and propagation sequence. An analysis of the displacement fields for each event is consistent with several centimeters of movement along the subduction zone interface between the Juan de Fuca and North American plates. Changes in the length of GPS baselines between Penticton, B.C., in the Cascadia back-arc and stations above the transition zone and inner part of the locked zone of the Cascadia megathrust suggest an additional four transient events; the DRAO-ALBH baseline from the Western Canada Deformation Array contains the longest record. This series of events implies an average periodicity of 1.2 +/- 0.1 years since 1992. Strain release along the deeper plate contact within the Cascadia subduction zone in the Pacific Northwest is not continuous, but occurs as periodic and characteristic slow earthquakes, with at least seven events identifiable since 1993. These slow events typically last two to four weeks, occur with an average period of 14.5 months, and appear to release all moment accumulated since the previous slow event. The cyclical nature of these events appears to be a fundamental mode of strain release.