Intraplate earthquakes, regional stress and fault mechanics in the Central and Eastern U.S. and Southeastern Canada

Abstract

Utilizing 75 high quality individual earthquake focal plane mechanisms and 10 formal stress inversions we investigate the consistency of regional stress orientations in the central and eastern United States and southeastern Canada, the variation of relative stress magnitudes across the region and the compatibility of slip on optimally-oriented nodal planes with frictional faulting theory. To map faulting styles and relative stress magnitudes across the region of study, we utilize the high quality focal plane mechanisms to calculate the AΦ parameter (following Angelier, 1979; Simpson, 1997) that ranges from 0 (uniform horizontal extension with SV >>SHmax=Shmin) to 1.5 (strike–slip faulting with SHmax>SV>Shmin) to 3 (uniform horizontal compression with SHmax=Shmin>SV). We find that horizontal stresses become increasingly more compressive with respect to the vertical stress from the south-central United States (characterized predominantly by strike–slip focal mechanisms) toward the northeastern U.S. and southeastern Canada (predominantly thrust mechanisms). In a manner similar to the study by M.L. Zoback (1992a), which used a much smaller data set, we utilize the Mohr–Coulomb criterion to calculate the difference in orientation between the theoretically-optimal orientation of a fault plane (for various coefficients of friction, μ) and the focal mechanism nodal planes assuming that pore pressure in the brittle crust is hydrostatic. For the 75 focal plane mechanisms utilized in our study, the preferred (better fitting) nodal planes deviate on average only 7° in strike and dip from the theoretically-optimal planes for μ=0.6. As such minor differences could represent small variations in the stress field (or uncertainties in the focal plane mechanisms), we conclude that nearly all earthquakes in the study region slip in a manner compatible with shear failure on pre-existing faults in the local stress field.