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 ∼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. Plain Language Summary In subduction zones, the “locked zone” represents a shallow portion of the plate boundary where the two plates do not move past each other except for during large megathrust earthquakes. Deeper on the plate interface, the plates are still locked, but periodically slip past each other during slow earthquakes. These slow earthquakes, or slow slip events (SSEs), last on the order of week‐to‐years and can release the same amount of energy as a Mw 7 earthquake. Studying the location and frequency of SSEs in subduction zones is important for understanding the seismic cycle, as SSEs can influence the locked zone and the timing of megathrust earthquakes. Long‐term SSEs, which last on the order of months‐to‐years, are observed at numerous subduction zones around the world. However, these events have not yet been observed in the Cascadia subduction zone. Therefore, this study analyzes GNSS time series data to look for signs of long‐term SSEs in Cascadia. Our results reveal evidence for a long‐term SSE in southern Cascadia and potential smaller events along other parts of the margin. These findings are import