Transition Regions Between Regions of Episodic Tremor and Slip in Cascadia

Abstract:

The Cascadia subduction zone has been of special interest since episodic tremor and slip (ETS) was first noticed in the Vancouver Islands. Since then it has been discovered that Cascadia has three broad (300–500 km), coherent zones with different recurrence intervals : Wrangellia zone with 14 ± 2 months, Silletzia zone with 19 ± 4 months, and Klamath zone with 10 ± 2 months. Wrangellia extends from northern Vancouver Island to ~47.5^oN, Silletzia covers central and northern Oregon and southern Washington, and Klamath extends from north California into Oregon. Wrangellia flood basalts are accreted remnants of an oceanic plateau where parts of the entire volcanic stratigraphy are exposed. Siletzia is a formation of early to middle Eocene epoch marine basalts and interbedded sediments in the forearc of the Cascadia subduction zone, which accreted to North America. The Klamath graben fault system is a group of north and northwest-trending normal faults that form a complex graben system that confines the Klamath Lake basin at the intersection of the northwestern Basin and Range and Cascade Mountains in southern Oregon.

From northern California to 50^oN latitude, we perform principal component analysis on Global Position System (GPS) data to look for signatures of slow-slip and quantify its recurrence interval. This approach allows us to mark the boundaries beyond which the effect of a given class of slow-slip is negligible and provide a higher spatial resolution on the different recurrence intervals present in Cascadia compared to previous studies. We also notice the presence of a signal with a period of nearly 6 years in all regions, which was removed to isolate the shorter period slow-slip signal. We then quantify the slip on the subducting slab for the slow-slip. Doing this over a decade and over different patches along-strike in Cascadia allows us to see what happens between episodes, and how that slip activity changes along the subduction zone. In our analysis a key transition zone is around 44^oN. In this region we notice patches where slow-slip is absent. While the varying geology of the different regions are correlated with broad ranges of recurrence intervals, we examine the possibility of along-strike variation in effective normal stress as a proxy for the effect of geology on the recurrence interval of ETS.