The precise way in which the traffic light system should work must be defined in advance, and responsibilities allocated among those involved. In addition to these preliminary clarifications and the ongoing analysis of new information about the subsoil, a “traffic light system” is generally used as a measure to control induced seismicity.
Traffic light systems aim to avoid the negative effects of induced seismicity, or to control them as far as possible. The traditional traffic light system is based on the close monitoring of induced seismicity and involves various measures depending on the seismic activity registered. A traditional system of this kind was applied in Basel in 2006 and subsequently found to be unsatisfactory, while an extended, adaptive traffic light system was used for the geothermal energy project in St. Gallen in 2013 based on the experience gained. The observed seismicity serves as an indicator of the necessary measures, and predictions are also made as to its possible course. This is based on the anticipated impact of planned pumping procedures, the permeability of the rock and other factors. The example in St. Gallen, however, also highlighted the limits of this adaptive system. Although a yellow alarm was triggered, which would have required immediate termination of pumping work, the work had to be continued due to rising gas.
A great deal of research and development will be required in this area over the coming years, and the findings will need to be tested in future demonstration and pilot systems. Based on the current state of science and technology, it is difficult to assess the extent to which we will be in a position to prevent (or control) induced earthquakes.