Induced seismicity

Principle 'Enhanced Geothermal Systems' (EGS) Source: Geothermal Explorers Ltd, 2005

Deep geothermal energy is produced using boreholes with depths of more than 400 m. During development and operation of such geothermal plants rupture processes of different size which are associated with induced seismicity can be generated in the deep underground. Cold fluid is injected into the borehole and heated fluid is pumped back so that the rock mass is subjected to injection pressures and thermal stress. Thereby cracks are generated or enlarged which can be detected as microseismic activity using seismic instruments. The increase in pore pressure on existing fault planes which are exposed to tectonic loading can also cause seismicity that is felt by local residents. A similar problem exists with technical processes which produce energy from the deep underground and at which pore pressure changes occur, e.g. hydraulic fracturing for the exploitation of shale gas, gas and oil production and injection of waste fluids or CO2.

For the planning and licensing, but also for the development and operation of geothermal plants the effects of the induced seismicity must be given in the form of possibly occurrent intensities and ground motions. To achieve this, methods for earthquake hazard estimation in the case of natural seismicity can be modified and applied. In an individual project of the first phase of the MAGS-Project ‚Microseismic Activity of Geothermal Systems’ a simple model was developed to determine the seismic hazard as the probability for the occurrence or exceedance of ground motions of a certain size. On this basis the procedures of codes and guidelines of civil engineering for the case of natural seismicity and engineering regulations for describing the effects of vibrations could be transferred to the case of induced seismicity. The main emphasis of the project was on the selection and test of a program for probabilistic hazard estimation, the derivation of the seismic source model and the determination of the input parameters. For the calculation of the hazard, special ground motion models for weak seismic events and small distances between source and site as well as catalogues of recorded or synthetic induced seismicity were needed.