Subduction transition and relation to upper plate faults revealed by the 2019 Mw 6.0 and 6.2 Costa Rica-Panama border earthquakes

We study the May 12, 2019, Mw 6.0 (hereafter Laurel earthquake) and a June 26, 2019, Mw 6.2 (hereafter Manaca earthquake) earthquakes located in a complex strike-slip to subduction transition zone in the Costa Rica-Panama border using a dense local network of broadband and strong-motion instruments, concluding that the Laurel mainshock data (hypocenter, centroid, and focal mechanism), concurrently with its aftershock distribution, indicate an almost vertical north-south oriented, dextral strike-slip fault. This fault ruptured at depths between 10 and 30 km in the lower crust of Panama Microplate and the geometry implies a shallow influence of the Panama fracture zone. Meanwhile, for the Manaca earthquake, the aftershocks are concentrated on a 25 km long NW-SE trend, with a depth between 20 and 45 km. One of the nodal planes of the focal mechanism of the Manaca earthquake, as well as the centroid location coincides with the aftershock locations trend, supporting the hypothesis that a nearby steeply dipping sinistral strike-slip fault originated this event. Earthquake locations and focal mechanism analyses of collected data from global and local earthquake catalogs suggest that the strong coupling caused by the subduction of the Cocos Ridge and Nazca plate is largely accommodated by outer arc crustal block migration with a combination of NW-SE sinistral and N-S dextral strike-slip faults deforming the overriding Panama Microplate. Sinistral motion of crustal blocks, on the Costa Rica side, are consistent with observed motions along with the Azuero-Sona fault system and Coiba Faults on the Panamanian side.