Effects of Preexisting Structures on the Seismicity of the Charlevoix Seismic Zone

Abstract

The Charlevoix Seismic Zone (CSZ) is located along the early Paleozoic St. Lawrence rift zone in southeastern Quebec at the location of a major Devonian impact structure. The impact structure superimposed major, steeply dipping basement faults trending approximately N35 • E. Approximately 250 earthquakes are recorded each year and are concentrated within and beneath the impact structure. Most M4+ earthquakes associated with the rift faults occurred outside the impact structure. Apart from the unique distribution of earthquakes, stress inversion of focal mechanisms shows stress rotations within the CSZ, and in the CSZ relative to the stress orientation determined from borehole breakouts. The primary goal of this research is to investigate the combined effects of the preexisting structures and regional stresses on earthquake activity and stress rotations in the CSZ. We approach this using PyLith, a finite-element code for simulations of crustal deformation. Adopting the results from recent hypocenter relocation and 3-D tomography studies, we modify the locations and dips of the rift faults and assess the effect of the new fault geometries on stress distributions. We also discuss the effects of resolved velocity anomalies. We find that the observed stress rotation is due to the combined effect of the rift faults and the impact structure. One-dimensional velocity models of the CSZ with an embedded impact structure and a combination of 65 • -40 • -40 • and constant 70 • fault dip models with a very low friction coefficient of 0.3 and cohesion of 0 MPa can explain the observed seismicity and more than 50% of the stress rotations. Key Points:• The CSZ seismicity and stress orientations are the combined effects of the rift faults and the impact structure • Planar faults we considered can explain the observed seismicity but only 50% of the stress rotation • An impact structure 4 times less elastically stiff than the surrounding crust can explain the seismicity in the CSZ Supporting Information:• Supporting Information S1 Figure 2. (a) Model domain with the impact structure (gray) and crust (red). The three rift faults (black lines), dimensions, boundary conditions and the total amount of displacement are annotated. The outer 10-km-thick layer of crust (green) is added to contain fault edges within the domain, a requirement by PyLith. (b) The orientation of the model domain (red dotted box) and the associated coordinate axes (red arrows) relative to the geographic reference frame. The red dotted box is not the actual model domain but just a box that shows its "orientation." Cyan circles on the geographic map show the inner and outer impact structure boundaries and black solid lines trace rift faults (see Figure 1).for the region. We then try to correlate modeled stress distributions with the recently-relocated hypocenters (Powell & Lamontagne, 2017) and discuss implications for the nonuniform stress orientation around the impact structure that was previously recognized.

Crustal Velocity Variations and Constraints on Material Properties in the Charlevoix Seismic Zone, Eastern Canada

“…The geodynamic model of Fadugba et al. (2019) indicates that the St. Lawrence rift fault dips by up to 70° within the vicinity of the CSZ. Yu et al.…”

“…The trend of the hypocenters shown in Figure 6 indicates that at least the Gouffre River Fault, and possibly the St. Lawrence and Charlevoix faults (Figure 1), are high-angle normal faults dipping at ∼60°SE that likely developed during the breakup of supercontinent Rodinia (Kumarapeli & Saull, 1966) and have been reactivated in a reverse sense under the current stress regime (e.g., Mazzotti & Townend, 2010). The geodynamic model of Fadugba et al (2019) indicates that the St. Lawrence rift fault dips by up to 70° within the vicinity of the CSZ. Yu et al (2016) made a similar interpretation of high-angle dipping normal faults, namely, the Gouffre River, St. Lawrence, and Charlevoix faults, based on the distribution of relocated seismicity.…”

A New Madrid Seismic Zone Fault System Model From Relative Event Locations and Application of Optimal Anisotropic Dynamic Clustering

“…We used a program developed by Fadugba (2021) that is based on the cumulative tetrahedra volume method of Ouillon and Sornette (2011). A detailed description is presented in Fadugba (2021). Briefly, a randomized catalog of events for a particular portion of the NMSZ was generated.…”

Depth‐Dependent Crustal Stress Rotation and Strength Variation in the Charlevoix Seismic Zone (CSZ), Québec, Canada

“…Fadugba et al. (2019) attributed ∼50% of the stress rotation to the presence of the impact structure (elastic moduli and rock density variations) and weak paleorift faults. While the FMS‐inferred stress orientations by Mazzotti and Townend (2010) may be representative of the entire region, the limited number of solutions (60) from their study makes it difficult to estimate possible spatial variation of the stress orientation.…”

“…In particular, Mazzotti and Townend (2010) hypothesize that postglacial rebound in combination with a weak fault zone may explain the ∼30° clockwise rotation of the maximum horizontal stress from shallow borehole measurements to those inferred from CSZ earthquake FMSs. Fadugba et al (2019) attributed ∼50% of the stress rotation to the presence of the impact structure (elastic moduli and rock density variations) and weak paleorift faults. While the FMS-inferred stress orientations by Mazzotti and Townend (2010) may be representative of the entire region, the limited number of solutions (60) from their study makes it difficult to estimate possible spatial variation of the stress orientation.…”