Temporal evolution of an episodic tremor and slip event along the northern Cascadia margin

Episodic tremor and slip (ETS) have now been observed along the northern Cascadia margin for over 15 years. Recent densification of GPS coverage and the introduction of Gladwin borehole strainmeters (BSMs) under the Plate Boundary Observatory have enabled the derivation of improved slip models and have allowed more detailed monitoring of the migration of the slip surface during prolonged ETS events. For this study we examine the along-strike migration of the May 2008 ETS as determined from the GPS and BSM observations. GPS sites overlying the 30 km depth contour of the subducting plate interface show that the slip initiates west of northern Puget Sound and propagates bidirectionally. Shear strain time series at regional BSM sites confirm this bidirectional expansion of the slip zone. In a simple model of along-strike propagation constrained by the GPS and BSM time series, the speed of the northwest propagation varies from 8, then to 2, and then to 15 km/d, but the speed of the south propagation stays at 6 km/d. The tremors observed for this ETS episode show a similar bidirectional migration pattern and similar changes in northwest migration velocity, but the migration front is slightly ahead of the slip propagation front. These results provide important information for understanding the mechanics of ETS.