Strain accumulation associated with locked subduction megathrusts revealed by deep-ocean borehole observations

The cyclical storage and release of tectonic strain constitute the root cause of earthquake ruptures. Although onshore geodetic measurements have been widely used to assess subduction-fault locking, our understanding of strain accumulation, especially in offshore regions, remains incomplete. Here, we present evidence from deep-sea boreholes for interseismic and inter-slow-slip strain accumulation at Cascadia, Nankai, Costa Rica, and Barbados. Steadily rising formation fluid pressure (P_f) measured in fine-grained, low-permeability sediments is interpreted to reflect volumetric strain buildup and, by inference, increasing fault stress between slip events. We construct generic models to demonstrate the concept and site-specific models to illustrate margin-variable strain loading patterns. At northern Cascadia, the observed P_f trend (~0.6 kilopascals per year, reflecting a strain rate of ~0.15 × 10^-6 year^-1) can be explained by present-day full locking of the shallow megathrust. At Costa Rica, where the shallow megathrust hosts episodic slow slip events (SSEs), more rapid strain accumulation (P_f trend of ~3.0 kilopascals per year) is observed during intervals between strain-releasing shallow SSEs.