Influence of Farallon slab loading on intraplate seismicity in eastern North America

<p>Intraplate seismicity is an enigmatic phenomenon potentially driven by several geodynamic factors. The continental interior of eastern North America is an ideal region for studying the interplay between geodynamics and intraplate seismicity, as it has hosted many significant historical earthquakes, most notably the New Madrid sequence of 1811-1812, and is undergoing modern glacial isostatic adjustment (GIA) and long-wavelength subsidence due to the sinking of the Farallon slab. Seismicity in the region tends to concentrate within aulacogens or along paleo-rifted margins, which can serve as weak zones in the crust where stress accumulates. Within these zones, focal mechanism stress inversion shows significant rotational deviation of the maximum horizontal stress (S<sub>Hmax</sub>) direction from the regional NE-SW trend, which may be explained by long-wavelength stress perturbations in the presence of lithospheric weakness. We test the hypothesis that mantle-flow induced epeirogenic subsidence contributes to the intraplate seismicity of eastern North America via reactivation of pre-existing faults through development of high-resolution flow models using spherical finite-element calculations in CitcomS. To capture realistic temperature fields and incorporate the Farallon slab, we use a mineral-physics-based inversion of seismic velocities for the upper mantle and a numerically derived velocity anomaly to temperature anomaly scaling for the lower mantle using global and regional tomography. Existing studies have yet to fully incorporate the geologic structures and weak zones that have been suggested as necessary for intraplate seismicity to occur. As such, we utilize laterally variable temperature-dependent viscosities, upon which we superimpose low-viscosity crustal and lithospheric weak zones with distinct compositions and/or constitutive relationships. High resolutions allow us to resolve these weak zones at the scale of the aulacogens. CitcomS calculates the instantaneous flow field and associated stress tensor, with which we compute S<sub>Hmax</sub> and the Coulomb stress to investigate how the stress patterns change in the presence of the weak-zones due to the loading of the Farallon slab. We compare our stress orientation predictions to those of the World Stress Map. We find that low-viscosity mantle weak-zones loaded by a mass anomaly at depth can transmit elevated stresses to the base of the crust overlying the weak zones. We test multiple weak-zone scales, geometries, and strengths to determine the set of parameters that 1) best reproduces the observed stress field, and 2) sufficiently enhances Coulomb stresses to potentially reactivate faults. A more complete picture of the contribution of mantle flow to stresses and deformation at the regional and local level is critical to the development of better seismic risk assessment and to understanding the relative contributions from other drivers of stress perturbation, such as GIA and sea level loading, which have the potential to alter the spatial distribution of seismic hazard.</p>