In 2003 hydraulic stimulations were carried out in El Salvador's Berlin geothermal 
field as part of a project to explore the feasibility of commercial hot fractured 
rock energy generation. A key requisite was that the induced seismicity associated 
with the reservoir stimulation at depths of 1–2 km should not produce levels of 
ground shaking at the surface that would present a threat or serious disturbance to 
those living in and around the field. A pump control system - which we refer to as 
the traffic light system - was implemented which utilised real time seismic 
monitoring data and ground motion estimates calibrated using accelerograph 
measurements. 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. These thresholds were defined in terms of peak ground 
velocity (PGV) and incorporated into the traffic light system through locally 
derived predictive equations for PGV in terms of event magnitude. The system also 
took into account the frequency of occurrence of the induced earthquakes. During the 
stimulation activities we encountered a much lower level of induced seismicity than 
anticipated such that the boundaries of the traffic light system were not tested but 
we argue that an approach such as this is a viable method for ensuring that 
hydraulic stimulation is conducted in an environmentally responsible manner. The 
major shortcoming of this type of approach is that it does not address the issue of 
seismicity occurring after the end of the pumping operation. This presentation is 
based on the material reported in the paper "Control of hazard due to seismicity 
induced by a hot fractured rock geothermal project", by Julian J. Bommer, Stephen 
Oates, Jose Mauricio Cepeda, Conrad Lindholm, Juliet Bird, Rodolfo Torres, Griselda 
Marroquin and Jose Rivas, which is about to appear in the journal, Engineering 
Geology.

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