Long-term Fluid Injection Can Expedite Fault Development: Riedel Shear Structures Illuminated by Induced Earthquakes in Alberta, Canada
- 1School of Earth Sciences, Zhejiang University, China (hongyu.yu@zju.edu.cn)
- 2Geological survey of Canada, Sidney, British Colombia, Canada
- 3School of Earth and Ocean Sciences, University of Victoria, Victoria, British Colombia, Canada
- 4Geoscience BC, Vancouver, BC, Canada
Riedel shear structures (RSS) are often observed in the embryonic stage of strike-slip fault development. Typical RSS often involve a set of conjugated fault segments that interact with each other prior to the growth of a through-going fault, that is, the principal deformation zone (PDZ), sub-structures R, R’ and P. Ideally, sub-structures R and P should be symmetric to the PDZ while structures R and R′ are conjugated to each other. It is a critical concept linking the geomechanical behavior of individual earthquakes to structural geology at both local and regional scales. The depiction of RSS can help understand the geomechanical behavior of individual earthquakes. In the meantime, the influence of long-term fluid injections on the developing process of RSS, as manifested by the common occurrences of injection-induced earthquakes, has been rarely addressed.
Here we document a clear case in western Canada where the development of a local RSS system is expedited by 25 years of wastewater injection. RSS are manifested by an earthquake sequence consisting of 187 events (ML ranging 1.3–3.9) between 2018/01/01 and 2021/07/15 in an area without any previous seismic history.
Focal mechanisms of these events exhibit various faulting types with the majority (87%) being compatible with the background stress regime. The orientation of derived nodal planes and six fault segments depicted from the refined earthquake distribution collectively define the overall geometrical characteristics of RSS, which consists of four primary strike-slip structures striking 19º (R’), 79º (R), 94º (PDZ) and 109º (P), respectively. Mohr-Coulomb failure analysis further suggests a cumulative stress perturbation of up to 10.0 MPa, probably bringing the local structures very close to the critically stressed state.
Overall, our observations suggest that long-term fluid injection can expedite the development of local fault systems. Consequently, it is probably necessary to consider the full dimension of the local/regional RSS rather than the size of individual events in the assessment of the overall injection-induced seismic hazard.
How to cite: Yu, H., Kao, H., Wang, B., and Visser, R.: Long-term Fluid Injection Can Expedite Fault Development: Riedel Shear Structures Illuminated by Induced Earthquakes in Alberta, Canada, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4598, https://doi.org/10.5194/egusphere-egu23-4598, 2023.
