Keynote Presentation: The Role of Rate and State Seismcity Models for the Discrimination and Characterization of Induced Seismicity

Abstract

The public perception for human-induced and triggered seismicity is often very high and public safety is a major issue for approving authorities. Even weak non-damaging events felt by population have led to major debates and in some cases fostered the closing down of geotechnical projects. Often, we cannot convincingly answer the question of the likelihood that damaging earthquakes may occur or not before the start or during the course of the engineering operations. Passive seismic techniques and advances in theoretical seismicity models are needed to improve the current situation. Seismological methods towards the characterisation of seismic sources are well advanced in mining applications (e.g. Grigoli et al., 2013, Sen et al., 2014, Maghsoudi et al., 2014). Micro-seismicity is also recognized as a valuable tool for completion, optimization, characterization and modelling of reservoirs or storage facilities (e.g. Cesca et al., 2014a). However, although the probabilistic description of seismicity is established in seismology since decades, a probabilistic approach is still not common practice for induced seismicity and reservoir studies in industry. A possible reason may be that seismicity models in seismology are often based on steady state or quasi static loading rate conditions, a situation rarely valid for engineering activities. Therefore, seismicity models considering the stress and pressure loading conditions of engineering activities need to be further developed andvalidated. The discrimination between natural and human related earthquakes is important for both issues. For instance, the nature of induced seismicity implies nearby geotechnical operations and engineering activity. However, the occurrence of a close-by earthquake does not always mean that the earthquake is human related, especially in regions with high tectonic activity. We need to establish community accepted methods for the discrimination of events. These should consider (geo-)physical and structural parameter from the natural background processes and the human related activities (e.g.Dahm et al., 2010b). Beside the discrimination by means of source parameter estimations (e.g. Cesca et al., 2013b, 2014b), probabilistic methods based on seismicity parameters are important (e.g. Dahm et al., 2012). A database of case studies, including both seismicity and production parameters, is important to validate such models and to establish common accepted procedures. Unfortunately, such databases are still not available or not accessible to a wider scientific community. The presentation reviews and summarizes the basics of seismicity models and their current role in natural earthquake and triggered / induced earthquake studies. Specific problems and questions for the different cases of induced / triggered seismicity and applications will be clarified. The behaviour and different aspects of a rate and state seismicity model are demonstrated. We discuss implementations of the rate and state model (Dietrich, 1994) for the characterization of triggered and induced seismicity, comprising the characterization of seismicity clouds related to hydro-fracture formation and the evaluation and discrimination of isolated significant earthquakes. Knowledge gaps and unsolved questions will be discussed.