Filling the Gap in Cascadia: The Emergence of Low-Amplitude Long-Term Slow Slip

Long-term slow slip events have been observed at several subduction zones around the globe, where they play an integral part in strain release along megathrust faults. Nevertheless, evidence for long-term slow slip has remained elusive in the Cascadia subduction zone. Here we conduct a systematic analysis of 13 years of GNSS time series data from 2006 to 2019 and present evidence of at least one low-amplitude long-term slow slip event on the Cascadia subduction zone, with the possibility of others that are less resolved. Starting in mid-2012, a 1.5-year transient is observed in southern Cascadia, with a group of coastal GNSS stations moving similar to 2 mm to the west. The data are modeled as a Mw 6.4 slow slip event occurring at 15-35 km depth on the plate interface, just updip of previously recognized short-term slow slip and tremor. The event shares many characteristics with similar long-term transient events on the Nankai subduction zone. However, the total fault slip amplitude is an order-of-magnitude smaller in Cascadia when compared to large events elsewhere, making long-term slow slip detection challenging in Cascadia. While there are other westward long-duration transients in the refined data set, the surface displacements are below the level of the noise or are limited spatially to a few neighboring stations, making interpretation unclear.

Automated summary

Evidence of at least one low-amplitude long-term slow slip event has been found in the Cascadia subduction zone, with the possibility of others that are less resolved. While there are other westward long-duration transients in the refined data set, the surface displacements are below the level of the noise or are limited spatially to a few neighboring stations, making interpretation unclear.