Conclusions

LOADING FROM THE FARALLON SLAB

4.8 Conclusions

of Western Quebec slightly to the northeast, we would likely see similar patterns and similar stress magnitude perturbations as those of the current results.

While the substantial influence of the Farallon slab and local weak zones on the intraplate stress field is evident, there are still many local sources of stress perturba- tions and other factors driving intraplate seismicity to consider. One consideration is the actual viscosity of the weak zones. We only test the presence of weak zones by considering the end member cases of no weak zones versus weak zones with a viscosity of 1e–4 relative to the surrounding lithosphere. However, the viscosities of these different weak zones, if present, are likely to vary, possibly by orders of magnitude. There is also likely some trade-off between the impact of the viscosity reduction on the stress perturbations versus the impact of the weak zone depth.

Depth is better constrained than viscosity due to the availability of geophysical data, but this trade off still requires further investigation. The other major consideration and one of the most critical is that we do not account for the previous stress history of the fault. In our models, all faults are virtual faults, meaning no physical fault surface is included in the model; rather, the calculated stress tensors at a particular location are applied to a set of faults after the fact to determine there influence on the behavior of a fault of a particular orientation. Were faults present in the model itself, their presence would affect the stress distribution (Wu et al., 2021; Steffen et al., 2014b). Most importantly, the rupture history of the fault, which we do not explicitly model, will affect the local change in CFS and the likelihood and location of future earthquakes (Stein, 1999). This is especially true in light of the long-lived aftershock sequences and temporal clustering that are typical of intraplate settings (Stein and Liu, 2009; Dicaprio et al., 2008). The current models are limited in both the resolutions we are able to obtain and the purely viscous rheological formulation of CitcomS. Future models should seek to incorporate true faults, but would require higher lateral spatial resolutions more suited to a regional scale model and elasticity within the crust to properly capture brittle failure.

as much as∼20°in some places. This rotation is enhanced when lithospheric weak zones are included at the locations of ancient aulacogens and other pre-existing structures and improves the fit to the observed seismic𝑆_{𝐻 𝑚 𝑎𝑥} direction within most seismic zones. The presence of weak zones loaded by the Farallon slab at depth can also explain the pattern of clockwise rotation of the observed focal mechanism derived𝑆_{𝐻 𝑚 𝑎𝑥} direction relative to the regional borehole derived𝑆_{𝐻 𝑚 𝑎𝑥} as reported by Mazzotti and Townend (2010) in the New Madrid, Central Virginia, Charlevoix, and Lower Saint Lawrence Seismic Zones. However, larger stress perturbations are required to fully reproduce the observed degree of stress rotation in most of the seismic zones, except Charlevoix, suggesting the need for weaker weak zones or another source of stress such as glacial isostatic adjustment. The magnitude of the modeled stress perturbations are also dependent on the depth of the weak zones, which have a reduced influence on crustal stresses with greater depth. Thus, in order for pre-existing lithospheric weak zones to exert appreciable control on intraplate stress under the influence of mantle flow, they must be shallow/sub-crustal and in contact with the crust. However, even with shallower weak zones, many of the stress perturbations and rotations between the different models are quite small. Given the many mechanisms giving rise to intraplate stress and the uncertainty on geodynamic model inputs, including the tomography itself, the velocity to temperature conver- sion, the assumed viscosity law, and the slab parameterization, one may argue that the differences between the models are insignificant, particularly when compared to the spread of the𝑆_{𝐻 𝑚 𝑎𝑥} data within a single seismic zone. While future work should seek to better quantify the impact of the errors of the inputs, our models nevertheless demonstrate the substantial influence of the Farallon slab on intraplate stress.

Moreover, even small changes in stress are ultimately important because they place 𝑆_{𝐻 𝑚 𝑎𝑥} into a position that may be more favorably oriented to reactivate faults, depending on the fault geometry. Even without weak zones, influence of mantle flow from the Farallon slab is enough to explain fault instability at the locations of some significant historical earthquakes, such as the Reelfoot Fault in New Madrid

— the site of one of the 1811-1812 M 7-8 earthquakes — and the Timiskaming Fault in Western Quebec, on which the 1935 M 6.2 earthquake occurred. Across all seismic zones, the inclusion of weak zones brings the majority of faults closer to failure. However, fault orientation and fault weakness remain key controls on their reactivation potential. Weaker faults (𝜇 ≤ 0.4) in the NMSZ would not be enough to reactivate them unless optimally oriented, but weak faults (𝜇 ≤ 0.5) could explain fault reactivation in the WQSZ and CXSZ in the modeled stress field.

Many previous studies have argued that lithospheric mantle heterogeneity alone is the primary control on intraplate seismicity (Saxena et al., 2021; Levandowski et al., 2017; Zhan et al., 2016). Such heterogeneity in mantle viscosity controls the spatial variability of the velocity gradients by diverting flow to lower viscosity areas, so mantle heterogeneity remains an important factor, but the Farallon slab has the effect of augmenting the magnitude of those gradients, which ultimately helps move many faults closer to failure. Thus, while lithospheric mantle heterogeneity may govern the spatial distribution of intraplate stress, epeirogenic processes are largely responsible for the magnitudes of stress required to reactivate some intraplate faults and to explain continent-wide stress rotations.

C h a p t e r 5

GEODYNAMIC CONTROLS ON INTRAPLATE SEISMICITY IN