Surface water load and earthquake stress interactions near the Pirrís reservoir, Costa Rica

Abstract

This study analyzes the relationships between the stress changes produced by the water load variations due to the filling operations at the Pirrís Reservoir, Costa Rica, and the stress changes generated by three relevant earthquakes after the first impoundment. The earthquakes differ in their magnitude, distance from reservoir and depth. Two of them correspond to a shallow crustal faulting, and the third earthquake corresponds to a subduction thrust-fault. The first filling operations began in March 2011 and the daily time history of water levels was available until December 2017. The seismicity has been recorded by a dense local network of seismological stations operating since January 2008. Effects of both types of sources (water loads and earthquakes) were combined to obtain the total Coulomb Failure Stress in the reservoir area, analyzing which of them was dominant in each case and time. We then analyzed the effects of the pore pressure diffusion due to the water loads on the nearby seismicity including two of the studied earthquakes. Results lead to different conclusions depending on the characteristics of each earthquake. An important result is observed after the occurrence of the subduction earthquake, where the coseismic effects become dominant along the entire study area, eclipsing the effect of water loading at all depths. This shows that in a regime stressed by a regional earthquake, albeit of its moderate magnitude, and regional distance, the effect of water loads is barely perceptible. In the absence of tectonic activity, however, the impact of water loading is notable.