20th-Century Strain Accumulation on the Lesser Antilles Megathrust Based on Coral Microatolls

Abstract:

The Lesser Antilles subduction zone forms the eastern boundary of the Caribbean plate. The seismic potential of the megathrust remains poorly known, despite the hazard it poses to numerous island populations and its proximity to the Americas. As it has not produced any large earthquakes in modern times, the megathrust has often been assumed to be aseismic. However, historical records of great earthquakes in the 19^th century and earlier, which were almost certainly megathrust ruptures, demonstrate that the subduction is not entirely aseismic. Recent occurrences of giant earthquakes in areas where such events were previously thought to be impossible have inspired the geoscience community to re-evaluate the seismic potential of other “low-hazard” subduction zones, such as the Lesser Antilles. Using the method of coral microatoll paleogeodesy developed in Sumatra, we examine 20^th-century vertical deformation on the forearc islands of the Lesser Antilles and model the underlying strain accumulation on the megathrust. Our data indicate that the eastern coasts of the forearc islands have been subsiding relative to the arc islands, suggesting that on the time scale of the 20^th century, a portion of the megathrust just east of the forearc islands has been locked. Our findings are in contrast to recent GPS-based models that suggest little or no strain accumulation anywhere along the Lesser Antilles megathrust. This discrepancy is potentially explained by the different time scales of measurement, as recent studies elsewhere have indicated that interseismic coupling patterns may vary on decadal time scales and that century-scale or longer records are required to accurately assess seismic potential. The accumulated strain we have detected will likely be released in future earthquakes, uplifting the previously subsiding areas as well as contributing to seismic and potentially to tsunami hazard in the region.