Linking Fast and Slow Earthquakes: The Role of Subducting Seamounts

Seamount subduction is thought to strongly influence the slip behavior of megathrust earthquakes, yet its role in hosting fast or slow earthquakes remains controversial. Seamounts alter fault stress, fluid flow, and upper plate structure, introducing heterogeneity to the subduction system.

Here we investigate how a subducting seamount affects the earthquake cycle by modulating normal stress along the megathrust. We combine  theoretical arguments based on energy balance (linear elastic fracture mechanics, LEFM) with numerical simulations. We use the efficient boundary element model FDRA, which simulates short-term earthquake cycles with rate-and-state friction. 

We first examine shallow seamounts and identify three regimes: (1) downdip-nucleated earthquakes that propagate through the seamount, (2) seamount-nucleated earthquakes that break the entire fault, and (3) alternating earthquake behavior, where the seamount hosts small, seamount-confined events as well as larger, system-wide ruptures. The transition between regimes (1) and (2) is controlled by the seamount’s stress perturbation and its distance from the loading boundary. Rupture arrest, and the transition to regime (3), are explained by energy balance at the crack tip (LEFM). Partial ruptures at the seamount occur when fracture toughness, enhanced by normal-stress heterogeneity, exceeds the stress intensity factor. At sufficiently high stress amplitudes, seamounts host slow slip events that are controlled by the nominal nucleation dimension and the seamount width

Next, we extend the model to the downdip brittle–ductile transition at 50 km depth to test the effect of deeper seamounts. Seismic cycles are dominated by downdip nucleation, with most ruptures arresting before the seamount, though some propagate through it. As a result, stress at the seamount is repeatedly reset by downdip ruptures, preventing seamount nucleation. A simple theoretical estimate shows that, without heterogeneity, homogeneous friction makes seamount nucleation unlikely due to a tradeoff between rupture propagation and nucleation: high fracture energy limits rupture propagation but also increases the seamount nucleation length.

Finally, we account for frictional heterogeneity by  implementing multiple hierarchical slip-weakening distance profiles and examine how this controls seismic behavior. Our results show that seamounts, even with small stress perturbations and at different depths, consistently promote slow slip in their stress shadow and increase earthquake activity at the stress shadow’s edges. Seamounts can act as rupture barriers, occasionally facilitate large system-wide ruptures, but they more commonly host smaller partial ruptures. 

Overall, our work demonstrates that seamounts can produce a rich variety of slip behavior, including slow slip, earthquake nucleation, partial and full ruptures, reconciling diverse observations across subduction zones worldwide. These regimes are well understood in terms of normal stress heterogeneity and prestress levels. A single seamount can produce different slip patterns throughout the seismic cycle, with important implications for seismic hazard.