Lateral viscosity variations within the lithosphere and their role in affecting the dynamics and seismicity of central and eastern North America

Abstract:

One of the big unknowns that controls the dynamics of the lithosphere is its
rheology. Many numerical studies have considered the lithosphere as a viscous
material to model its deformation. But what is even less known are the lateral
variations in viscosity and their role in affecting the deformation of
the lithosphere. In a recent study, we looked into understanding the causes
of intraplate earthquakes within central and eastern United States (CEUS) and
whether lateral viscosity variations within the lithosphere affect their occurrence.
We tested several lateral viscosity structures and found that the one with strong
cratons and old oceanic lithosphere (>10²⁵ Pas), weak plate boundary zones
(< 10²³ Pas), and moderately weak intraplate areas (5X10²⁴ Pas) predicted
strain rates that could match the velocities and strain rates inferred from GPS
observations in CEUS. Such a lateral viscosity distribution also predicted
deviatoric stresses whose most compressive principal axes coincided with the
P-axes of the intraplate earthquakes and stress inversions in the region.
We also found that even a slightly stronger (~ five times) Appalachian terrane
sandwiched between a strong North American craton and a strong western Atlantic
block overestimated the strain rates and velocities in CEUS. Moreover, we observed that
a high viscosity contrast between the craton and the weak Appalachian terrane
led to focusing of stresses in the New Madrid region, which could potentially
explain the high seismicity in the area.