Gravitational body forces focus North American intraplate earthquakes

Abstract

Earthquakes far from tectonic plate boundaries generally exploit ancient faults, but not all intraplate faults are equally active. The North American Great Plains exemplify such intraplate earthquake localization, with both natural and induced seismicity generally clustered in discrete zones. Here we use seismic velocity, gravity and topography to generate a 3D lithospheric density model of the region; subsequent finite-element modelling shows that seismicity focuses in regions of high-gravity-derived deviatoric stress. Furthermore, predicted principal stress directions generally align with those observed independently in earthquake moment tensors and borehole breakouts. Body forces therefore appear to control the state of stress and thus the location and style of intraplate earthquakes in the central United States with no influence from mantle convection or crustal weakness necessary. These results show that mapping where gravitational body forces encourage seismicity is crucial to understanding and appraising intraplate seismic hazard.

Lithospheric density models reveal evidence for Cenozoic uplift of the Colorado Plateau and Great Plains by lower-crustal hydration

“…Finding, not surprisingly, a similar pattern in lower-crustal density, Levandowski et al (2017) suggested that the lowest densities may coincide with Proterozoic continental sutures (the Cheyenne belt-the suture between the Wyoming craton and Yavapai terranes-in SE Wyoming and the Yavapai-Mazatzal suture in SE Colorado). If so, it is reasonable to expect that fluids may preferentially exploit these preexisting, lithospheric-scale fracture/suture zones, and lower-crustal hydration may consequently be greatest there.…”

“…A more complete discussion of uncertainties in density models derived similarly to ours, and the sources thereof, is given by Levandowski et al (2017). Here, we present a brief discussion of the variability of densities in our accepted 3-D models and then turn our attention to systematic biases that may have been introduced by our assumptions and modeling procedure.…”

Multiscale variations of the crustal stress field throughout North America

“…nowledge of horizontal principal stress directions and relative stress magnitudes is fundamental to understanding the mechanical behavior of the Earth's crust, including the factors driving plate motion and seismicity. Recent studies have debated what proportion of intraplate deformation and topography is attributable to forces from plate boundary interactions, gravitational potential energy (GPE) within the lithosphere, and mantle flow [1][2][3][4][5] . Initial plate-scale stress mapping identified provinces of relatively constant stress separated by transition zones where stress orientations could rotate rapidly 6 .…”

Role of Large‐Scale Tectonic Forces in Intraplate Earthquakes of Central and Eastern North America

“…Although there is a general agreement that some of these earthquakes occur along favorably oriented ancient rifts (Schulte & Mooney, 2005;Stein et al, 1989;Sykes, 1978;Zoback & Richardson, 1996) and other major strength contrasts (Gallen & Thigpen, 2018;Mooney et al, 2012), there is no consensus on the sources of intraplate deformation and earthquakes. The intraplate events in North America have been variously attributed to glacial isostatic adjustment (GIA; Mazzotti et al, 2005;Muir-Wood, 2000), ridge push effects (Zoback & Zoback, 1981, 1980, dynamics of fault interaction (Liu & Stein, 2016), mantle weakening due to rifting or a plume event (Chen et al, 2016), gravitational body Bent (1996) forces (Levandowski et al, 2016(Levandowski et al, , 2017, or large-scale convection (Forte et al, 2007). The role of GIA in reactivating these faults by perturbing the background stress has been proposed in relation to the 1929 Grand Banks (M7.2) and the 1933 Baffin Bay (Mw7.4) events (Bent, 1995(Bent, , 2002.…”