Deformation Analysis Of The Seismic And Post-Seismic Cheliff (Algeria) Geodetic Network Using 2d Elastic Finite Element Method (FEM)

The region of Cheliff, located at the North West of Algeria, is one of an exceptional interest for crustal motion study which is due to seismic activity. Usually, in the deformation analysis of geodetic networks, there are two conventional methods for estimating the movements of an area of ​​study: displacement vectors and strain tensors. However, the evaluation and representation of the deformation depend on the priori reference system and on the configuration of chosen elements that constitute the entire network. These constraints make difficult the interpretation of the results. Through this present paper, a solution based on the finite element method (FEM) is proposed to refine the estimation and the representation of the deformation of geodetic networks. In this context, a study of the deformation is carried out to analyze the horizontal motion of the Cheliff geodetic network due to the famous earthquake of October 10, 1980 (Ms = 7.3), based on two-dimensional elastic finite element model. This network was observed by classical triangulation in 1976 (by INCT) and in 1981 (by CRAAG). The different results are illustrated in terms of displacement vectors, strain and stress tensors where the estimated deformation is interpreted according to previous geophysical studies. They revealed a compressive phenomenon of Cheliff area, in the NNW-SSE orientation, due to the convergence of the African and Eurasian tectonic plates, and to a block rotation phenomenon, at the SE and NW parts of the fault, in a retrograde direction. The study was extended to the post-seismic geodetic network observed between June1990 and April 1992 by CRAAG. The network was established by distance measurements, with 12 monitoring points distributed along the reverse fault. The results obtained show a post seismic meaningful deformation, in the fault central segment, characterized by global NW-SE direction of strain tensors in agreement with ground data.