Stress State, Fault Criticality and Fluids

holcim Schweiz_Eidgen_1d_cmyk unifr

Faults are an important structural feature in geology. They are discrete zones with a finite width dissecting sedimentary beds and/or non-stratified basement-type rocks. Earthquakes initiate predominantly on pre-existing faults in response to, and interaction with, the regional and local stress fields. Faults also provide the primary pathways for fluid flow, and the flow properties can change dramatically concomitant with earthquake slip [Miller and Nur, 2000]. This feature of a switch from low permeability to high permeability at the onset of slip is the underpinning of enhanced deep geothermal systems, and since the long-term permeability structure determines the subsequent flow properties in the reservoir, it also determines the economic viability of the resource. The degree of fluid overpressure needed to trigger microseismicity (and enhance the permeability) depends primarily on the existing state of stress into which fluids are injected and the strength of faults. If the crust is already critically stressed, then minor perturbations from fluid injection should trigger microseismicity, while if not critically stressed, substantial overpressure is needed to enhance the permeability structure. However, the state of stress and its proximity to failure is completely unknown in the Molasse Basin, and constraining or quantifying the proximity to failure is essential for mechanistic assessments of risk, hazards, and deep heat extraction projects viability.

Numerous local and regional fault systems control the tectonic setting and evolution in the Swiss alpine foreland, particularly at the transition between the Molasse Basin and the Jura fold-and-thrust belt. Two families of faults are recognized: faults related to the development of folds (in the Molasse Basin and in the Jura Mountains) having a general NE-SW orientation; and strike-slip faults arranged in NNE-SSW and NW-SE conjugate systems. The seismicity record indicates that these faults are active, particularly for vertical strike-slip faults. With respect to future exploration of natural resources such as hydrocarbons, geothermal energy and CO2 sequestration, our understanding of the stress state around fault systems in the subsurface, and their link with fluid circulation, plays a key role. However, our present knowledge of the stress state and the genetic link with the faults systems and fluid flow in the Alpine foreland of Switzerland is insufficient.

We propose an integrated suite of field, laboratory, and numerical modeling studies to address essential and still unanswered questions:

  • What is the fault anatomy, and its 3D structure in the regional tectonic framework. What is the influence of inherited features in the cover and in the basement s.l.?
  • How are the fault anatomy and the kinematics evolving over time and how are they influenced by stress and fluids?
  • How do fault properties scale to fault anatomy?
  • What are the physical properties of the fault system in the investigated natural laboratory?
  • How can the physical properties be implemented into numerical models?
  • What is the state of stress in the investigation area and how are stress and strain partitioned along major strike-slip faults
  • How do faults interact in space and time and at what scales?
  • How does stress influence the flow properties along the investigated fault system?
  • Are all faults in the Alpine foreland critically stressed?
  • What is the influence of fluid circulation on the fault criticality and how does it influence seismic behavior. What are the rupture surface dimensions involved?
  • Are rainfall and changes in the regional/local aquifers controlling seismicity and fault criticality?