TRIGGERED SEISMICITY EXPECTED IN HYDROELECTRIC RESERVOIRS CLOSE TO THE SUBDUCTION ZONE IN COSTA RICA

The tectonic stress field imparted by the subducting Cocos plate on the Caribbean plate along the central pacific region in southern Costa Rica and the changes in the associated fluid pressure and friction during impoundment and drawdown of hydropower reservoirs, determine the reactivation of faults and pre-existing discontinuities and the formation of new ones. The geomechanical attributes of the present day stress field, the rockmass properties and the ways in which they are combined according to the local and regional geological structure, lithology and rock matrix in association with the prevailing tectonic regimes, control the likelihood of fault reactivation. Some variables such as impoundment-drawdown rates and speeds can be modified, but it can not be done for the subduction related and background seismicity. In this context adequate modeling of the reactivation must take into consideration the stress and fluid pressures required to start the hydrofracturing, opening and sliping of faults networks.

In one of the analyzed cases in a reservoir with a maximum depth of 162 meters in Tertiary turbidites crosscrossed by NW dextral active faults and reverse NWW structures within a stress field with a subhorizontal σ₁ trending towards N22°E, the hydrofracturing must be at least 9. 02652e + 006 Pa to start and 4. 82652e + 006 Pa to open the existing faults. The Slip Tendency analysis indicates that only the reverse faults dipping less than 15 ° are prone to reactivate under the stated conditions while the dextral shears shall move when their strikes are in the range of N40°W to N10°E. The sensitivity of these variables in a scenario with a major seismic event has been shaped by a Coulomb Stress Failure modeling, which indicated that the stress transfer induced by a maximum credible Mw 6,5 earthquake with inverse geometry and 30% of dextral component would not increase the seismic hazard at any depth in the area of direct influence of the reservoir.