CHARACTERIZATION OF THE AUGUST 2010 CASCADIA ETS EVENT USING GPS, STRAINMETER, AND TREMOR OBSERVATIONS

We investigate the relationship between slip and tremor during the August 2010 slow slip event in northern Cascadia. Episodic tremor and slip (ETS) events in northern Cascadia occur in a transitional regime between the updip seismogenically locked zone and the downdip continuously slipping zone. Geodetically detectable slow slip and nonvolcanic tremor appear to be broadly coincident. But the exact spatial and temporal relationships remain unclear at a finer scale. Typical GPS derived slip distributions tend to be spatially and temporally smoothed and offset slightly updip of tremor distributions. These discrepancies may be real, or they may be a consequence of the resolution of GPS data or an artifact of the inversion methodology. Borehole strainmeters provide an additional independent geodetic constraint for inferring slip characteristics, provide greater temporal resolution, and greater precision than GPS. However, various non-tectonic artifacts and other sources of error have made deriving reliable information from strainmeters during slip events difficult.

We attempt to better compare the pattern of slow slip and tremor by running a series of geodetic inversions and systematic forward models based on tremor distribution. Slip distributions are derived by inverting GPS and strainmeter observations using the Kalman-filter-based Extended Network Inversion Filter. To compare the tremor distribution to the geodetically derived slip we also construct slip distributions using tremor occurrences as a proxy for localized slip on the plate interface. The magnitude of slip per tremor occurrence is then scaled to best match the observed surface displacements. While the tremor-derived slip distribution provides a better fit to a few strainmeters than GPS-derived slip solutions, there are some systematic misfits at several GPS locations, particularly near the northern portion of the Olympic peninsula where there is a decrease in tremor activity. Preliminary results also show that by shifting the peak tremor-derived slip updip a few kilometers the overall misfit to the surface displacements can be decreased, supporting the conclusion that the peak tremor activity occurs slightly downdip of the peak slip.