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General Subsidence of the Lesser Antilles Over a Decoupled Subduction Megathrust

Description: 

In the Lesser Antilles, apparently contradictory observations led to different conclusions over the degree of locking of the subduction megathrust, hence on the evaluation of the seismogenic potential of the subduction zone. While two large earthquakes hit the area during the 19th century, existing block models suggested the megathrust is largely uncoupled on average, raising the question of the nature of the earthquakes or of the permanence of kinematic coupling over long periods of time. We re-evaluate in a probabilistic framework the distribution of coupling along the megathrust using velocities derived from GNSS measurements. Our approach allows to consider the formal measurement uncertainties as well as the uncertainties associated with errors in basic assumptions to derive surface predictions. Our model confirms that the megathrust is most likely uncoupled, with a reasonable level of certainty. We compare predictions from this coupling model to vertical displacements averaged over each individual island of the Lesser Antilles arc. While no coupling should result in no net vertical motion over the interseismic period, we observe a general subsidence of the entire arc. We relate such subsidence, ongoing since approximately 125,000 years, to geodynamic processes at the regional scale unrelated to the state of coupling along the megathrust. Our study provides observations of vertical motion over a range of time scales for an end-member, fully uncoupled subduction megathrust, to be put in perspective with equivalent observations over coupling megathrust such as in Chile, Japan or the Himalayas.

Session: Active Faults in the Caribbean and Central America

Type: Oral

Date: 4/20/2023

Presentation Time: 02:30 PM (local time)

Presenting Author: Eric Calais

Student Presenter: No

Invited Presentation: 

Authors