TECTONIC MEGAOSCILLATORS: IMPLICATIONS FOR LARGE-SCALE STRESS TRANSFER IN THE LITHOSPHERE

Since the advent of continuous GPS geodesy a decade ago, it has become clear that active tectonic velocity fields are not constant, but rather exhibit an array of transient behaviors. Among the most conspicuous of these are the ~14 month oscillation of the Cascadian forearc region (amplitude ~5 mm), and similar but longer term motions in a number of other active forearcs, that are best explained by episodic strain release on a deep portion of the subduction megathrust. Large-scale transient deformation is also evident in continuous geodetic data from the northern Basin and Range province. Estimated site accelerations define two coherent domains between Reno and Salt Lake City, with an abrupt NNW-trending boundary between them in easternmost Nevada. Relative to the Colorado Plateau, sites west of the boundary have slowed at average annual rates of ~0.2 to 0.4 mm/yr, while those in easternmost Nevada and Utah have net accelerations of <0.1 mm/yr (Davis et al., in preparation). The western sites' annual velocity changes are ~5 to 10% of their total westward velocity with respect to North America, resulting in a zone of active contraction near the Nevada-Utah border.

A clue to the origin of these motions comes from the temporal and spatial correlation of a rapid transient (~20 mm/yr) of a site near Lake Tahoe with a deep crustal earthquake swarm in 2003. Unlike the other Nevada sites, this site did not slow down, acting as an “anchor point” on the edge of the Sierra Nevada. Then by virtue of the 2003 transient, it “caught up” with the eastward deviations of the other sites from their original trajectories (~5-8 mm total motion). By analogy with the transients associated with subduction megathrusts, an episodically creeping, subhorizontal “megaoscillator” ~500 km wide at or near the base of the crust may explain the motions, and explain why the strong seismic reflectivity of the Moho in Nevada does not persist into Utah. Decadal “shuttling” of Nevada implies that a relatively simple orogen-scale structure aseismically transfers stresses across 100s of kilometers of diffusely deforming lithosphere at human time scales, perhaps controlling fault interactions at length scales well beyond those of elastic or viscoelastic effects associated with earthquakes.