Modern civilisation explores and penetrates the Earth’s crust, recovers from it and stores into it fluids and gases to a hitherto unprecedented degree. The contemporary crustal stress state is a key parameter for a wide range of technological problems such geothermal reservoir management as well as for the site selection process for a deep geological repository for radioactive waste. Furthermore, the stress evolution during the seismic cycle is one of the key processes that controls nucleation, rupture propagation and arrest of an earthquake. Thus, our ultimate goal is to quantify the in absolute 3D situ stress state and its spatio-temporal variability as well as the failure due to natural and induced processes.
A key challenge in this context is to derive from sparse and incomplete point-wise stress information derived from a wide range of stress indicator a 3D continuous description across different scales from boreholes to plate-wide regions. To achieve this we analyse stress data and use these to calibrate the initial stress conditions for 4D thermo-hydro-mechanical-dynamic (THM-D) models. Our long-term goal is to link the results of these deterministic models with the statistical methods to achieve in specific settings a physics-based probabilistic seismic hazard assessment.
Figure | From point data to 3D stress description
a. Stress data in Northern Switzerland (Heidbach et al. 2016). Lines show the orientation of maximum horizontal stress SHmax. Yellow square denotes the model area. Yellow square denotes the model area. Colours in the model volume show the differences between the maximum and the minimum horizontal stress SHmax-Shmin and the discretization into finite elements. White line denotes the cross section shown in Fig b
b.Lithology sequence that is implemented in the 3D geological model (Hergert et al. 2015) and model results in terms of the difference of horizontal stresses SHmax-Shmin.
c. Displayed are the magnitudes SV, Shmax and Shmin along a vertical line (white line in figure c) with SV the vertical stress. Red symbols denote data of Shmin magnitudes derived from hydraulic fracturing measurements.
A backbone of our research is the World Stress Map (WSM) Project (Heidbach et al. 2016). It is a global compilation of information on the crustal present-day stress field and a collaborative project between academia and industry that aims to characterize the crustal stress pattern and to understand the stress sources. All stress information is analysed and compiled in a standardized format and quality-ranked for reliability and comparability on a global scale. The project is service oriented and provides besides the public database a number software tools that help to generate stress maps, investigate the stress pattern as well as to analyse and visualize the results from 3D models. We analyse the spatial variability of the crustal stress field and established a modelling workflow to derive from point-wise data a 3D continuous description of the crustal stress field (Reiter & Heidbach 2014; Ziegler et al. 2016a). In the ongoing phase IV (2017-2023) of the WSM project we focus on compiling systematically also stress magnitude data and to extend the WSM service to a 3D stress tensor predictions with uncertainties in areas where sparse or even no data is available.
WSM- World Stress Map Project Phase IV (2017-2023)
WSM 3D - From sparse point data to 3D description (proposal in preparation)
A key challenge for the assessment of the stability of a deep geological repository (DGR) for high-level nuclear waste and gas storage sites is the quantification of the 3D in-situ stress field. This determines the distance to failure of pre-existing faults or the generation of new fractures. This distance is a critical to predict and quantify the future impact due to natural loads (e.g. earthquakes, glaciation, erosion) and man-made induced changes (e.g. excavation, heat generation). A pre-requisite for this prediction is the 3D continuous description of the initial stress field from borehole to regional scale, i.e. from meter to 10s of kilometre (Hergert et al. 2015; Heidbach et al. 2013a). Our earlier and ongoing research projects are located in Sweden and Switzerland with the Swedish Radiation Safety Authority (SSM) and the National Cooperative for the Disposal of Radioactive Waste (NAGRA), respectively (Yoon et al. 2017; Heidbach et al. 2013b). In these countries the DGR selection and characterization process is already quite mature. In Germany this process has re-started recently and we just started basic research projects in this new research field that will last for decades.
iCross - Uncertainties of geomechancial models and impact of earthquakes (Beginn Sommer 2018)
SpannEnD- Stress field of Germany from borehole to regional scale (2017-2021)
SUBI - Safety of underground gas storage during cyclic loading (2017-2020)
ThermoQuakes - Thermo-mechanical modelling of seismicity related to nuclear waste in hard rock (2015-2017)
In the past years induced seismicity evolved to be a critical issue for the public acceptance of geothermal sites in central Europe. The same holds on for hydrocarbon production as seen in the Groningen gas field (The Netherlands) and in the North German Basin (Grünthal 2014). In contrast to natural seismic hazard induced seismicity can be mitigated by smart reservoir engineering (Gaucher et al. 2015; Zang et al. 2013). This, however, needs the basic understanding of the thermo-hydro-mechanical (THM) processes that can push the stress conditions into a critical state. We investigate in this context basic physical processes by means of THM models with a particular emphasis to the stress changes on pre-existing faults (Yoon et al. 2015). Furthermore, we aim at linking our forward simulation results to the classical probabilistic seismic hazard assessment to estimate in scenarios the increase of the seismicity rates and its physical controls (Hakimhashemi et al. 2014). Ultimately, our results should lead to best-practice recommendations for the reservoir management (Müller et al. 2018).
GAB – Collaboration with the Geothermal Bavarian Alliance
IMAGE– EU project on Integrated Methods for Advanced Geothermal Exploration (2014-2017)
GEISER – EU project on Geothermal Engineering Integrating Mitigation of Induced Seismicity in Reservoirs (2010-2013)
In the coming decades, extreme events which previously had little impact will be affecting urban agglomerations of several millions of people. There is an urgent need to develop up-to-date, authoritative and robust building codes. However, in specific settings such as low strain areas and where faults are beneath or next to large urban agglomerations, the established methods of probabilistic seismic hazard assessment have to be extended. Our long-term goal is to link the results of deterministic forward models with statistical methods to achieve in specific settings a physics-based probabilistic seismic hazard assessment. For urban areas such as Istanbul we aim to develop an earthquake scenario simulator to test the consequences for the city (Hergert & Heidbach 2010). Furthermore, for low strain areas, where seismic catalogues are sparse, we develop and test an alternative approach to translate physical processes of stress accumulation into seismicity rates.
ESM - Earth System Modelling: Seismic hazard in low strain areas (2017-2020)
SHAC - Seismic hazard in Chile (2016-2019)
ILP Task Force III - The seismic cycle at continental transforms (2016-2020)
MEMO - Marmara Sea Earthquake Modelling (proposal in preparation)
stress2grid: Matlab script to estimate with different methods the mean orientation of maximum horizontal stress SHmax on regular grids using the WSM database release 2016 (Ziegler & Heidbach 2017)
GeoStress: Add-on for the professional analysis and visualization software Tecplot 360 EX. It provides features such as estimation of slip tendency, coulomb failure stress, fracture potential from 4D finite element model results (Stromeyer & Heidbach 2017)