Faults and Non‐Double‐Couple Components for Induced Earthquakes

Abstract

Focal mechanisms of induced earthquakes reflect anthropogenic contributions to preexisting geological features and fault slippages. In this paper, we examine fault‐related (double‐couple (DC)) and possibly fluid‐related (non‐double‐couple (non‐DC)) mechanisms of induced earthquakes (M2–6) at regional scales. We systematically compare well‐resolved focal mechanisms of 33 events in the Western Canada Sedimentary Basin, among which 12 were induced by hydraulic fracturing and one by secondary recovery. Most of the seismicity is dominated by strike‐slip/thrust faulting regimes, whereas limited (but consistent) non‐DC components are obtained from injection‐induced seismicity in central Alberta. We interpret the persistent compensated‐linear‐vector‐dipole components (M2.1–3.8) as reflecting fracture growth and/or noncoplanar faults slippages during hydraulic‐fracturing stimulations. We further expand the moment tensor decomposition analysis to four representative classes of induced seismicity globally and find that the overall contribution of non‐DC components is comparable between induced and tectonic earthquakes.

Shear Velocity and Radial Anisotropy beneath Southwestern Canada: Evidence for Crustal Extension and Thick‐Skinned Tectonics

“…At crustal depths, azimuthal anisotropy and the associated stress directions have been inferred from borehole breakout (Reiter et al, 2014) and time‐dependent splitting parameters of direct S waves (Li et al, ). These crustal and sedimentary observations consistently exhibit a dominant NE‐SW trend, approximately aligned with the orientation of the maximum compression axis determined from focal mechanisms of recent induced earthquakes (Wang et al, ; ). In comparison with the relatively well‐constrained azimuthal anisotropy, regional‐scale crustal transverse isotropy with a radial symmetry axis (or “radial anisotropy”) is less well understood with the exception of a 4–5% average amplitude in the middle crust beneath the northern Canadian Cordillera (Dalton & Gaherty, ).…”

Spatiotemporal Variations in Crustal Seismic Anisotropy Surrounding Induced Earthquakes Near Fox Creek, Alberta

“…Recent studies show that most of the induced seismicity in central Alberta is dominated by strike‐slip/thrust faulting mechanisms, whereas limited nondouble‐couple components are related to fluid injection (Eaton et al, ; Wang et al, ). To assess the spatiotemporal changes in faulting characteristics in our study, the focal mechanism solutions were determined by fault‐related (double‐couple) mechanisms using P‐, SH‐, and SV‐wave first‐motion polarities and amplitude information (Snoke, ) from waveforms recorded at four stations.…”

“…One likely triggering mechanism is fluid migration along the fault damage zone (Galloway et al, ) via existing hydrological connections between the HF networks and macroscopic faults (Ellsworth, ; Grigoli et al, ; Langenbruch & Zoback, ). The strike‐slip focal mechanisms of the HF‐induced earthquakes (Figure ) in central‐southern Alberta have direct implications for the sizes and orientations of reactivated fault zones, and seismic moment tensors of regional earthquakes show approximately NE‐oriented P axes (Schultz et al, ; Wang et al, , ).…”

A Shallow Earthquake Swarm Close to Hydrocarbon Activities: Discriminating between Natural and Induced Causes for the 2018–2019 Surrey, United Kingdom, Earthquake Sequence

“…as is sometimes observed for induced earthquakes (Sileny, 2009;Wang et al, 2018), although such a discriminant is by no means definitive, because many cases of induced seismicity have double-couple source mechanisms. In cases where seismicity is induced by compaction in a conventional reservoir, Peer-reviewed post-print published in Seismological Research Letters on 28/08/2019 Link to publisher's version of article: http://dx.doi.org/10.1785/0220190125 21 we expect normal faulting at the edges of the reservoir, and reverse faulting within the overburden (Segall, 1989).…”