Abstract
Abstract Plate boundary forces acting within the Cocos Plate that is being subducted at a rate of 8.5–9.0 cm a −1 towards N32°E below the Caribbean Plate and the Panama microplate are found responsible for contemporaneous superimposed compressive, wrench and extensive fault patterns in Central Costa Rica. The stress inversion of fault-slip planes and focal mechanisms reveals a prevailing convergence-imposed N20°–45°E almost horizontal compression. Ellipsoid R values [ R =(σ 1 –σ 2 )/(σ 2 –σ 3 )] in the range of 0.3–0.05 and 0.8–0.93 are responsible for the permutation of σ 2 to σ 3 and σ 2 to σ 1 , respectively, and show typical Andersonian configurations with one stress axis vertical or close to it. Coulomb failure stress (CFS) analysis reveals that up to 5 bars (0.5 MPa) of tectonic loading are being imposed on east–west thrusts and on critically oriented conjugate NW- and NE-trending strike-slip faults. Non-optimally oriented structures are potential targets for reactivation even with 2 bars (0.2 MPa) of load. Triggering and interaction with volcanic activity is highly suspected in one documented recent case. When the regional fault population was tested for its slip tendency (τ / σ n ), a good correlation with CFS results was found.
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