Past seismic slip-to-the-trench recorded in Central America megathrust

The 2011 Tōhoku-Oki earthquake revealed that co-seismic displacement along the plate boundary megathrust can propagate to the trench. Co-seismic slip to the trench amplifies hazards at subduction zones, so its historical occurrence should also be investigated globally. Here we combine structural and experimental analyses of core samples taken offshore from southeastern Costa Rica as part of the Integrated Ocean Drilling Program (IODP) Expedition 344, with three-dimensional seismic reflection images of the subduction zone. We document a geologic record of past co-seismic slip to the trench. The core passed through a less than 1.9-million-year-old megathrust frontal ramp that superimposes older Miocene biogenic oozes onto late Miocene–Pleistocene silty clays. This, together with our stratigraphic analyses and geophysical images, constrains the position of the basal decollement to lie within the biogenic oozes. Our friction experiments show that, when wet, silty clays and biogenic oozes are both slip-weakening at sub-seismic and seismic slip velocities. Oozes are stronger than silty clays at slip velocities of less than or equal to 0.01 m s–1, and wet oozes become as weak as silty clays only at a slip velocity of 1 m s–1. We therefore suggest that the geological structures found offshore from Costa Rica were deformed during seismic slip-to-the-trench events. During slower aseismic creep, deformation would have preferentially localized within the silty clays. Geodetic data, seafloor bathymetry and tsunami inversion modelling all indicate that the 2011 Mw 9 Tōhoku-Oki earthquake ruptured to the trench, with 50–80 m co-seismic slip occurring across the shallow portion of the megathrust1,2,3. These exceptional data sets showed, for the first time, that ruptures can propagate to the trench during subduction megathrust earthquakes. Previously, this domain had been considered to slip only aseismically4. This observation immediately raises follow-on questions: Is there evidence that co-seismic slip to the trench has occurred in other subduction zones? What is the potential for other megathrusts to co-seismically rupture to the trench? Following ocean drilling results in the Japan Trench5, investigation has focused on the smectite-rich, pelagic clays recovered from the shallow portions of the Tōhoku megathrust. Friction experiments showed that when the fault’s original fabric is preserved, the Tōhoku pelagic clays are cohesionless, reducing fracture energy and favouring earthquake rupture propagation6. The very small fracture energy and shear stress of pelagic clays when sheared at seismic slip velocities (~1 m s–1) can allow propagation of earthquake rupture from depth7,8, explaining slip to the trench during the 2011 Tōhoku-Oki earthquake8. On the ocean floor, deposition of pelagic sediments typically alternates between clays and biogenic oozes9,10, with the latter mostly subducting in the eastern central and south Pacific (Fig. 1). In contrast to pelagic clays, biogenic oozes have been proposed to inhibit both fault rupture propagation and displacement during earthquakes, and so prevent the occurrence of tsunamis9. Laboratory friction experiments have suggested, however, that biogenic oozes may play a key role in earthquake nucleation at depth11,12,13.