Megathrust earthquake swarms indicate frictional changes which delimit large earthquake ruptures

- Large earthquakes terminate in swarm regions 10× as often as rupturing through them.
- Swarms are associated with reduced plate coupling, indicating presence of slow slip.
- Swarms appear to be better proxies for segmentation than plate coupling alone.
- Stress heterogeneity can help promote swarm occurrence and halt large ruptures.

Using a recently constructed catalog of megathrust earthquake swarms, we present results that illustrate how fault slip for Mw⩾8.5 earthquakes with a publicly available finite fault rupture model is delimited along-strike by swarm regions. We further investigate this relationship for all Mw>7.5 subduction zone megathrust earthquakes that have occurred over the last 40 years by estimating the rupture area from early aftershocks. This extended comparison shows only 2 out of 42 earthquakes propagate through regions of the megathrust that have documented recurring earthquake swarm activity, while about 40% (17 out of 42) are terminate on at least one side near regions with swarm activity. In Japan, Chile, Sumatra, and Alaska, earthquake swarms correlate with regions of the plate interface that exhibit low interseismic strain accumulation. This suggests earthquake swarms are associated with reduced plate coupling, and possibly aseismic moment release; however earthquakes in Chile and Alaska ruptured through regions of weak coupling that do not show swarm activity. We propose instead that stress heterogeneity on the megathrust can best explain why large earthquake ruptures terminate in swarm regions but not in regions with weak coupling alone. Earthquake swarms appear to be a proxy for along-strike segmentation of subduction megathrusts, at least on the time scale of one seismic cycle.