Interplate coupling along the Caribbean coast of Colombia and its implications for seismic/tsunami hazards

Northwestern Colombia is located at a convergent plate boundary between the Caribbean plate and the North Andean Block; however, no megathrust earthquakes have been reported during last 500 years. In order to evaluate seismic potential in this area, we analyze GPS data during 2007-2018 from GeoRED - the nationwide GPS array in Colombia - to obtain interseismic 3-dimensional velocities. GPS velocity data indicate the northern part of the North Andean Block is differentiated as another block and we name it Macondo Block. The velocity data are then inverted to estimate interplate coupling on the subducting Caribbean plate interface. The result shows an isolated, fully locked patch south of the city of Cartagena extending from 9.0 to 20 km in depth on the subduction interface with an area of similar to 11,000 km(2). The estimated locked patch implies a potential of M-8 class earthquake with an average recurrence time of similar to 600 years, which is consistent with the absence of such an event in the available historical record. As another possibility, the observed surface deformation may be accommodated inelastically. A comparison of geological and geodetic strain rates in the convergence direction shows that the geological one is smaller by one or two orders of magnitude and does not support such an interpretation though the estimate itself still has a large uncertainty. For improving the evaluation of earthquake/tsunami potential, it is essential to conduct a careful geological investigation to identify recent evidence of crustal shortening or paleotsunami along northwestern Colombia.