S034-06
Low Interseismic Coupling of the Lesser Antilles Subduction Zone Inferred from Geodetic Observations

Thursday, 10 December 2020: 20:54
Virtual
Elenora van Rijsingen1, Eric Calais2,3, Romain Jolivet1,3, Jean-Bernard de Chabalier4, Jorge Jara2, Steeve J Symithe5, Richard E A Robertson6 and Graham Alexander Ryan6, (1)Ecole Normale Supérieure, PSL Research University, CNRS UMR 8538, Laboratoire de Géologie, Paris, France, (2)Ecole Normale Supérieure, PSL Université, CNRS UMR 8538, Laboratoire de Géologie, Paris, France, (3)Institut Universitaire de France, Paris, France, (4)Institut de Physique du Globe de Paris, Université de Paris, CNRS UMR 7154, Paris, France, (5)Université d'Etat d'Haiti, Faculté Des Sciences, URGéo, Port-au-Prince, Haiti, (6)University of the West Indies, Seismic Research Centre, St. Augustine, Trinidad and Tobago
Recorded Presentation
Abstract:
Geodetic measurements at subduction plate boundaries inform us about the degree of coupling between the downgoing and overriding plates, which results in seismic or aseismic behavior. Seismic regions, such as the northern Honshu subduction zone, build up elastic strain to be released in large earthquakes, while aseismic regions, such as the South Ecuador – North Peru segment, slip aseismically at a rate close or equal to the plate convergence rate without generating large events. Several subduction parameters have already been proposed to play a role in tuning the seismogenic behavior of the megathrust, but some subduction zones remain poorly understood, in particular those that are seismically quiet.

The Lesser Antilles subduction zone is one of these quiet regions, with no thrust events larger than MW 6.5 observed within the instrumental time interval. However, two large historical events in the 19th century, a M 7-8 in 1839 and M 7.5-8.5 in 1843, have been interpreted by some as interplate thrust events, although no direct evidence exists. If confirmed, this would be an indication that the Lesser Antilles subduction is seismic, and large interplate thrust earthquakes are to be expected in the future. Here we provide a new assessment of interseismic coupling for the Lesser Antilles subduction zone, based on updated GPS velocities and the latest models of slab geometry and elastic crustal structure. We use a Bayesian approach, allowing us to explore the entire range of plausible models and to provide realistic estimates of interseismic coupling.

We find low to very low coupling along the entire plate interface, including in the proposed rupture areas of the 1839 and 1843 events, where the sensitivity of our model to surface displacements is high. While a further understanding of the temporal variations in interseismic coupling is required, our results indicate that the Lesser Antilles subduction interface is uncoupled, which questions the idea that the 1839 and 1843 earthquakes were megathrust events. The mechanism responsible for the lack of current mechanical coupling remains elusive, but may be related to the highly hydrated and fractured incoming oceanic lithosphere.

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