Constraining Aseismic Slip in the Cascadia Subduction Zone with Plate Boundary Observatory Borehole Strainmeters in Northern Oregon

Abstract:

Plate Boundary Observatory borehole strainmeters (BSMs) on the northern Oregon coast are well-positioned to record strain from episodic slip in the partially-locked "transition" zone of the Cascadia subduction interface and to constrain the depth limits of such slip. At this latitude, prior analyses of tide-gauge, leveling, and GPS data place the landward limit of the locked seismogenic zone 30 km or more offshore, with partial locking extending as far as 50 km inland. Aseismic slip events accompanied by tectonic tremor have recurred here 16 to 24 months apart, based on seismic and GPS data since 1990 and 1998, respectively. Each of the four vigorous tremor episodes since the BSMs were installed in February, 2006 has been recorded by coastal strainmeters. For coastal BSM B024, gauge extensions up to 300 ppm during tremor events in June, 2011 and February, 2013, are the largest signals since 2010 after corrections for borehole trends, seasonal variations, and atmospheric pressure. Forward models matching coastal GPS offsets and strain time series for B024 can be constructed by superimposing rectangular dislocations surrounding each day’s tremor epicenters (assumed to be on the plate interface). The up-dip limit of tremor varies along strike, but slip up-dip of the tremor is not needed to match the strain data. Net east-west contraction at B024 for both these episodes requires most, if not all, slip to take place landward of the up-dip limit of tremor, about 30 km inland. Coastal BSMs could help characterize aseismic slip in the offshore part of the transition zone. Strain excursions of equal size, but opposite sign, would result from a hypothetical event with a downdip limit of slip 20 km offshore that is otherwise similar to the 2011 event. Yet since 2010, no such strain signal has been observed. Any slip since 2010 in the offshore transition zone may have been continuous, or composed of smaller, more frequent, episodes less discernible in BSM data.