Neotectonic of the Cipreses Fault: An Active Thrust Fault in the Metropolitan Area of San Jose, Costa Rica

Date: 4/25/2019

Time: 06:00 PM

Room: Grand Ballroom

The Cipreses fault is located in the central part of Costa Rica, 5 km east of downtown San Jose, in the most populated region of the country. This fault is part of the Central Costa Rican Deformed Belt, a 100 km-wide deformed zone that has been interpreted as the result of the relative movement between the Caribbean Plate and the Panama Microplate produced by the subduction of the Cocos Ridge. In this paper, the Cipreses fault is analyzed based on geomorphological, geological, and seismological observations. The main geomorphological evidences found are: scarps, terraces, river incisions, and deflected and dammed streams. There are three main Pleistocene geologic units in the vicinity of the fault: tuff, breccia, and andesitic lavas. These units have been observed displaced by the fault in an outcrop. From the seismological analysis, we conclude that the instrumental seismicity is low, but at least one earthquake in 2010 (4.1 Mw) was originated in the Cipreses fault. The focal mechanism solution for this event indicates a thrust fault with a nodal plane oriented N30°E/35° in agreement with the fault plane found in the field and the geomorphological observations. Based on a conceptual model for the faulting, we interpret that the Cipreses fault corresponds to a master fault that propagates to the surface along two fault segments named Pinares and Guayabos. The displacements of these faults made two anticlinal folds, which are expressed at the surface as prominent scarps. Based on the found evidences we conclude that the Cipreses fault is an active thrust fault with a 14-km long surface expression and with a seismic potential of magnitude 5.9-6.5 Mw. The detailed study of active faults in Central Costa Rica may give insights into the understanding of the upper plate deformation related to the subduction of the Cocos Ridge and is also crucial for the appropriate determination of seismic hazards in the densely populated area of San Jose.

Presenting Author: Lepolt Linkimer

Authors