Seismic strain rates and distributed continental deformation in the northern Andes and three-dimensional seismotectonics of northwestern South America

Abstract

The seismic strain rates and configuration in the northern Andes are estimated by summing the components of the Centroid Moment Tensor (CMT) solutions for earthquakes with M w >5.0 and are compared with those predicted by various models of relative plate motions and GPS data. Moment tensor components are summed linearly in four different regions with similar seismotectonics to infer changes in the regional strain patterns along the northern Andes and to retrieve strain rates and relative plate velocities between this region and the stable South American plate. The Andean seismic strain rates are the result of east–southeast-directed contraction due to the collision of the Nazca and Caribbean plates with the South American plate and the collision of the Panama block against central Colombia, with a component of northeast-directed motion due to the obliquity of the eastern boundary of the northern Andes with respect to the boundary of the Nazca and Caribbean plates, with the South American plate along the Colombia trench and the Southern Caribbean Deformation zone, respectively, where the convergence is normal. The present motion of the Central Andes towards the northeast does not require the plate boundary between the Caribbean and South American plates to be along the Eastern Cordillera of Colombia and the Boconó Fault in Venezuela. The seismic moment rates for the northern Andes are small compared to the total expected seismic strain rates calculated from plate motion models and GPS data, suggesting that most of the strain is released aseismically and that is accommodated by folding, fault creeping, rigid block translation or other undetermined processes. Finally, a three-dimensional model of the CMT solutions and other seismicity in northwestern South America is presented, where major subducting slabs, including the Peru flat-slab and the Ecuador and Central Colombia segments of the Nazca and Caribbean plates, become more evident and easier to interpret. The Caribbean plate is interpreted to culminate at the location of the Bucaramanga “nest” offering an explanation to this swarm of seismicity in the northern Andes.