Determination of Slow Slip Episodes and Strain Accumulation along the Cascadia Margin

Continuous GPS stations in the PANGA network clearly record subduction-related strain accumulation and slow slip episodes along the Cascadia convergent margin. Many of the slow slip episodes have been correlated in time and space with seismic non-volcanic tremor signals, leading to the discovery of Episodic Tremor and Slip (ETS) process. In this study, we use a hyperbolic tangent curve fitting technique for the identification of slow slip times and magnitudes within the GPS time series, independent of seismic tremor data. We demonstrate the effectiveness of this automated technique for both identification of slow slip observations and calculation of slow slip displacements. Out of the 44 visually-confirmed observations recorded at 10 representative stations across the subduction zone, no false negatives and one false positive were recorded by the automated process. Overall, the algorithm finds 163 slow slip observations that reveal geographic variations in both recurrence and magnitude. Recurrence patterns in individual GPS observations demonstrate coherence among neighboring stations over time and apparent along-strike segmentation of the subduction interface. The average observed displacement magnitude during a slow slip episode varies from 1.2 mm to 6.2 mm, while the annual rate of slow slip displacement varies from .53 mm to 5.0 mm. The curve fitting routine also reveals rates of short-term strain accumulation before and after slow slip episodes that are 30% larger than rates of slow slip displacement, suggesting either that a significant amount of slow slip occurs below our detection threshold or that some other process accounts for the strain release. Comparison of these rates with the long-term strain accumulation indicates that the long-term trends in displacement are 4- -5 times larger at 100 km from the trench but the long- and short-term rates are equal at about 270 km from the trench. The patterns in strain accumulation are consistent with strong coupling in the shallow seismogenic zone and weak coupling in the deeper transitional zone.