Fracture prediction for the 1980 El Asnam, Algeria earthquake via elastic dislocation modeling

Abstract

Geomechanical models based on linear elasticity have been used to predict the mode and distribution of subseismic fractures around larger faults. These models can be tested against field observations of surface breaks (fractures) formed in the aftermath of large earthquakes. This paper presents forward models based on elastic dislocation theory of the deformation due to the M s 7.3 earthquake at El Asnam, Algeria in 1980. Using fault parameters from previous geodetic studies to define a set of larger faults, our models calculate the deformation field in the surrounding rock volume. We compare predicted strain and stress fields with the surface deformation measured in the area following the 1980 earthquake. Using a combination of the redistributed elastic dislocation stress due to slip on the major faults and a small component of overburden stress, the models successfully predict normal faults and tensile fractures in the hanging wall of the reverse fault system. Orientations of the predicted faults vary along strike of the fault system, being parallel to the main reverse fault in the south but oblique to it along the central segment, agreeing with observed surface breaks. The results of our forward modeling are not sensitive to the magnitude or direction of a regional tectonic stress. The predicted fractures are controlled by the near-field, short-term redistributed stresses due to coseismic slip. The agreement between modeled and measured deformation patterns adds confidence in the use of elastic dislocation theory to accurately predict small faults generated by coseismic slip on large faults.