Seismic Strain rate along the Middle America Trench reveals significant differences between Cocos-North America and Cocos-Caribbean convergence

Abstract

SUMMARY Centroid-moment tensor solutions for thrust-faulting earthquakes in the last 27 yr are used to obtain and compare seismic strain rate associated to the subduction process along the Cocos‐North America and along the Cocos‐Caribbean convergent margins. In both cases, the eigenvectors of the strain-rate tensor align along radial and tangential directions with respect to their respective Euler Pole. Seismic strain rate along the Cocos‐North America plate boundary is about 2.8 × 10 −7 yr −1 , a value close to those previously found along the Japan and Aegean convergent margins. Strain rate along the Cocos‐Caribbean margin is one order of magnitude smaller, namely 3.7 × 10 −8 yr −1 . The relative plate velocity is also calculated using seismic moment tensors. We find a relative plate convergence of 1.13 ◦ Myr −1 for the Cocos‐North America plate margin, and 0.13 ◦ Myr −1 for Cocos‐Caribbean. This means that 80 per cent of the plate motion along the Cocos‐North America plate margin is expressed as thrust-faulting earthquakes, as compared to the velocity given by global plate motion models. In contrast, only about 10 per cent of the Cocos‐Caribbean relative motion is translated into thrusting events. Similar results are obtained when a 102-yr earthquake database is used. We suggest that differences in strain rate and percentage of plate motion are due to differences in the behaviour of the overriding plate and/or the Central America forearc sliver being detached from the Caribbean Plate and being incorporated into the Cocos Plate.