Predicting Fractures Using an Elastic Dislocation Model of the 1980 El Asnam, Algeria, Earthquake

We present a forward elastic dislocation model of the predicted fractures due to the Ms 7.3 earthquake at El Asnam, Algeria in October 1980. Using a new fault model based on parameters from previous seismological studies that fits the benchmarked geodetic data, we compare the predicted distribution, mode and orientation of fractures with the observed surface deformation in the area following the 1980 earthquake. Geodetic data are limited to measurements of the vertical component of surface deformation and by using the surface fractures caused by the coseismic stress perturbation to constrain our models we can indirectly sample all three components of the near-surface displacement field. In addition, the surface fracture observations are spatially more widely distributed than the geodetic measurements. By combining the redistributed elastic dislocation stress due to slip on the major faults and the overburden stress, the models successfully predict normal faults and tensile fractures in the hangingwall of the reverse fault system. With a pre-1980 oblique sinistral-reverse slip increment, the model can reproduce the observed oblique NNE striking normal faults along the north-central section of the main reverse fault. Long-term regional stresses cannot be sustained by fractured and weathered near-surface rocks, consistent with the results of our forward modelling which are not sensitive to the magnitude or direction of a regional tectonic stress. The predicted fractures are predominantly controlled by the near-field, short-term redistributed stresses due to coseismic slip. The agreement between modelled 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.