On Association of Seismic Lineaments and Stress Patterns of the Southeastern United States: Criteria of Failure

It is generally agreed that shallow focus, low magnitude, crustal earthquakes are caused by a sudden frictional movement on preexisting faults, and that the orientation of the faults may be delineated by locations of past earthquakes. The spatial distribution of a large number of epicenters in the Southeastern region of the United States may be interpreted in terms of many linear patterns. These patterns indicate weak fracture zones which are conveniently called seismic lineaments. Orientations of the seismic lineaments are mostly NE-SW, NW-SE and N-S within the Piedmont and Coastal Plain Provinces, but they are nearly N-S, and NE-SW within the Blue Ridge, and Valley and Ridge Provinces. Azimuthal distribution of the seismic lineaments shows a dominant N30°E direction while azimuthal distribution of the available crustal stress orientations for this region indicates a dominant N60°E direction. Based on Anderson theory of faulting for this dominant orientation of the seismic fracture patterns and the dominant direction of the stress pattern, the frictional angle of crustal rock, Φ, is estimated to be about 30°. From a linear frictional law for failure and the two dimensional stress system the maximum shear and principal stresses are calculated. Results indicate that sliding on a weak fracture plane can occur for a wide range of angle P between the sliding plane and the principal stress axes, mean stresses, cohesive shear strengths, and rock frictional angles Φ = 30±10°. The maximum shear and principal stresses on the sliding planes are calculated for a range of parameters, as a function of β. The plots of the maximum shear and principal stresses indicate a very broad U shaped curve, where the minimum values vary from 0.68 to 2.3 Kbars for the maximum shear and from 3.23 to 17.84 Kbars for the maximum principal stresses respectively. or cohesive shear strength of 0.5 Kbars and frictional angle Φ = 30°, the maximum shear stress of 0.68 Kbars and maximum principal stress of 3.23 Kbars occur at P = 60° corresponding to the N30°E direction of the observed seismic lineaments