Control of hazard due to seismicity induced by a hot fractured rock geothermal project

In 2003 hydraulic stimulations were carried out in a geothermal field in eastern El Salvador, Central America, as part of a project to explore the feasibility of commercial hot fractured rock energy generation. A key requisite for this environmentally-friendly energy source is that the fracturing of the hot rocks at depths of 1-2 km must not produce levels of ground shaking at the surface that would present a serious disturbance or threat to the local population. Thresholds of tolerable ground motion were inferred from guidelines and regulations on tolerable levels of vibration and from correlations between instrumental strong-motion parameters and intensity, considering the vulnerability of the exposed housing stock. The thresholds were defined in terms of peak ground velocity (PGV) and incorporated into a "traffic light" system that also took account of the frequency of occurrence of the induced earthquakes. The system was implemented through a dedicated seismograph array and locally derived predictive equations for PGV. The "traffic light" was used as a decision-making tool regarding the duration and intensity of pumping levels during the hydraulic stimulations. The system was supplemented by a small number of accelerographs and re-calibrated using records obtained during the rock fracturing.