Frédérique Leclerc, Earth Observatory of Singapore
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Where convergence is slow and seismicity rate is low, convergent margins' seismic hazard is usually not well understood, due to poor constraints on the way convergence is accommodated. In such cases, geodesic measurements often fail to characterize the megathrust's seismic behavior. Therefore, long-term upper-plate deformation constitutes valuable information that reveals how the convergence is accommodated and what processes are taking place along the active margin.
The slow and apparently uncoupled Lesser Antilles (L.A.) subduction zone and the SE Indonesian arc-continent collision margin are such plate boundaries - relatively seismically quiet during the historical period, and where first-order questions are still unresolved: how is the convergence accommodated? What processes is taking place along the margin? What and where are the active tectonic structures? What are their geometry and kinematics?
Both margins are located at latitudes where reefs are growing and record long-term vertical coastal movements. By studying the stratigraphy and morphology of emerged but also submerged reef complexes, through marine geophysical data and field investigation, as well as by modeling the reef deposition, we can determine the vertical movements of the coasts over several 100 kyr. Both along- and across a trench, the geometry and kinematics of upper-plate deformation allow us to determine and interpret active structures and megathrust-related processes that accommodate the convergence. In the L.A., we will further compare the long-term deformation to the short-term (seismic cycle related) deformation to question our present understanding of the seismic behavior of the subduction zone.