Collision versus sliver transport in the hanging wall at the Middle America subduction zone:: Constraints from background seismicity in central Costa Rica

[1] Earthquake focal mechanism solutions for 135 small-magnitude events are inverted for best fitting partial strain rate tensors that characterize contemporary strain in five areas that span the western margin of the Panama microplate in central Costa Rica. The results indicate the predominance of subhorizontal maximum stretching subparallel to the Middle America Trench (MAT) and provide constraints on the role of Cocos ridge collision at the MAT. The trajectory of maximum stretching changes similar to 25 degrees-45 degrees over several tens of kilometers from the Central Costa Rica Deformed Belt (CCRDB) where it is nearly E-W to the area inboard of the Cocos ridge where it is NW-SE. This change suggests that background seismogenic deformation reflects the transition from the trailing edge of a fore-arc sliver to an area of the upper plate affected by ridge collision. This diffuse deformation may be localized, in part, on conjugate strike-slip faults of the CCRDB.