Secondary acceleration of slip fronts driven by slow slip event coalescence in subduction zones.

The coalescence of slow slip events (SSEs) in subduction zones has been proposed as a potential precursor to large earthquakes, yet the physical conditions under which SSE fronts coalesce remain poorly understood. Here, we investigate coalescing SSEs along the Cascadia subduction zone. Using Global Navigation Satellite System (GNSS) data, we invert for the spatiotemporal evolution of the slip rate of SSEs from 2012 to 2023. We identify a coalescing event in 2021, which occurred during a phase of SSEs moment rate decrease, contrary to the previously documented 2013 coalescence. Coalescence triggered a secondary increase in slip rate and a rupture expansion in the 2021 event. To explore the mechanisms driving coalescence, we perform numerical simulations based on rate-and-state friction. Our results show that heterogeneity in frictional parameters and effective normal stress influences the occurrence rate and slip rate evolution of coalescing SSEs by modulating their propagation speeds and interaction probabilities. Although coalescing events lack distinct moment-duration or moment-area scaling trends, they are part of the broader class of migrating SSEs, which are associated with a b-value change in the magnitude-frequency distribution. These findings improve our understanding of SSE coalescence, which can potentially influence the timing and extent of future earthquakes.