Fluids perturbations induce earthquakes in natural seismic swarms or during anthropic activities in geological reservoirs. Despite their crucial importance to anticipate and mitigate seismic hazards, the fluid-earthquake processes at depth are still an open question, because of 1) their complex intrications with blind aseismic deformations and 2) their various interpretations among different geological contexts.
In light of recently proposed models, this project therefore aims to bring new constrains on the driving processes of both natural and artificially induced seismicity. It will focus on refining seismological observations and interpretations on both cases, in order to propose and validate models through coupled seismo-hydro-mechanical modeling. Finally, the differences in scale and contexts will be used to evaluate the universality of the processes.
Context and objectives
It is well-known that fluids induce seismicity at crustal depth, both in natural (volcanoes, active fault zones) and in reservoir exploitation contexts. In recent years, anthropogenic activities in storage reservoirs led to earthquakes with magnitude up to ~6, i.e. large enough to produce damages, in areas where natural seismic activity is usually low. For example, waste-water disposal made that Oklahoma became in 2016 the US state with the highest seismic rate, ahead of California. Induced seismicity is also the main drawback of geothermal energy, with for example the recent seismic activity near Strasbourg. At the same time, natural seismic swarms are bursts of seismicity clustered in time and space, without clear mainshock. Their duration (from days to months) requires a driving mechanism, which is generally poorly known. They can stop on their own, or evolve toward large, potentially damaging, earthquakes, such as the precursory phase of the l’Aquila earthquake. In both natural and induced seismicity, a better understanding of the processes occurring at depth is of primary importance to anticipate how the swarm may evolve and to mitigate the seismic risk associated with reservoir exploitation or tectonic loading.
In both natural and injection-induced swarms, a better understanding of the processes occurring at depth is of primary importance to anticipate how the swarms may evolve and to mitigate the seismic risk associated with reservoir exploitation or tectonic loading. This project therefore aims to respond to the following question: can we infer common physical mechanisms in the triggering and driving of natural and injection-induced seismicity? To do that, the main objective is to understand the complex interactions existing between seismicity, fluid pressure and deformation, i.e. how they trigger each other.
WP1- Improved observations to constrain fluid and aseismic deformation processes across scales: We will revisit existing data with in-depth and innovative analysis on three well-instrumented geological objects, from three different contexts and scales, which all show a rich seismicity. First, in the fast-extensional (~15mm/an) Corinth gulf (Greece), for more than 20 years, the dense instrumentation (CRL, Corinth Rift laboratory, EPOS-NFO) has allowed a multi-parametric monitoring of the tectonic activity, mainly including seismic swarms with intertwined fluid and aseismic deformation processes. Second, since 1993, more than 10 fluid injection tests in the deep-geothermal pilot-site of Soultz-sous-Forêts (France) revealed fluid-aseismic couplings. Finally, recent experiments of injection-induced seismicity, at a decametric scale in underground laboratories (LSBB, France; Mont Terri, Switzerland), allow for a precise quantification of the aseismic processes. The differences in geological contexts, scales and measured data will allow different but complementary observations.
WP2- Seismo-hydro-mechanical modelling of fluid flow and fault slip: Acting on our improved knowledge of natural and induced seismicity (WP1), we will develop and make extensive use of fully coupled numerical modelling to simulate fluid pressure diffusion, fault slip and earthquakes. These models will be tightly based on natural fault observations to mimic the seismological and deformation observations from WP1. Numerical results will then feed catalogs as input for WP3. Within this project, two complementary approaches will be used: (1) realistic 3D fault network with hydromechanical coupling; (2) a single fault model to analyze seismic cycles with complex interactions between fluid diffusion and aseismic slip.
WP3- Statistical analysis of observations from different contexts: Numerous cases of induced seismicity are now well documented, at scales from the laboratory, to the decametric experiments and the reservoirs. Precise catalogs of seismicity are also available for numerous natural swarms within different geological contexts, with variable size and duration. Despite a-priori different processes for volcanic swarms due to high temperature and complex fluid properties, we will also attempt to include them in our analysis. The aim of this WP is to gather catalogs to infer statistically common processes and metrics through scales and objects. Additionally, synthetic catalogs from WP2 will help to constrain the analysis.
- Géoazur, Université Côte d'Azur (L. De Barros, PI)
- ITES, Université de Strasbourg (O. Lengliné)
- ISTerre, Université de Grenoble-Alpes (A. Helmstteter)
- Mines Paris Tech (P. Dublanchet)
- ENS ULM (P. Briole)