HOLISTIC FAULT REACTIVATION SCENARIOS LINKED TO THE LONG EXPECTED NEW EDITION OF THE NICOYA BIG ONE EARTHQUAKE, COSTA RICA

Based on the recent short term recurrence of highly destructive earthquakes in NW Costa Rica (1900,1950,1990) several researchers have proposed a “big one“ Mw 7,6 event within the Nicoya peninsula in NW Costa Rica where the smooth sea bottom of the east pacific rise originated Cocos plate subducts the Caribbean plate at a rate of 11cm/year towards N30°E along the 5/m.y. old Middle America trench. Ongoing controversy (forecast wrt prediction) is based mainly on the lack of reliable instrumental data needed to constrain the seismic cycle and generate a plausible model from representative databases although no doubt exist about its highly destructive potential. Recent studies also assigns high seismic hazard to some other regions within the Caribbean plate-Panamá microplate, for example at southern Costa Rica-northern Panamá where the triple junction of the Nazca-Cocos-Caribbean imposes a complex and quite active seismotectonic setting.

To help decipher this context a fault reactivation scenario is herewith presented built from a joint application of the Coulomb Failure Stress ( CFS) and Slip Tendency (ST) methods. The former yields a 3D vision of the affected crustal volumes with increased seismic hazard induced by the expected earthquake at the megatrust Nicoya interface and the latter the geomechanical likelyhood of induced displacements on known or suspected and outcropping or blind regional faults. The results indicate that the area to be affected is about 9.000 km², with colateral damage to central Costa Rica with its dense concentration of social and economic infrastructure and the southern edge of Nicaragua. Vertical modeled displacements could be in the range of 0.22 m to 0.35 m.

The thrust, strike-slip and normal regional structures to be reactivated by the transfer of static Coulomb stresses are categorized and zoned while the ST confirms the same scenario. Incidence in the behavior of active volcanoes can not be ignored as demonstrated in the Pinatubo and Vesuvious volcanotectonic sequences. These results are a new insight, an original approach and a contribution to the evaluation of seismic hazards in Costa Rica and a suggested method to be applied elsewhere.