The October 1980 El-Asnam Earthquake in N.W. Algeria: Modelling Incoherence Functions of Spatially Variable Seismic Ground Motions

Abstract eng:
The main objective of this paper is to provide estimates of coherency functions of strong ground motion at engineering bedrock within the El-Asnam region (North-West Algeria). Due to lack of recorded array data, the so-called Empirical Green’s Function (EGF) method is used to simulate ground acceleration at different points in the engineering bedrock of the study area. The event being simulated is the 10 October 1980, Ms 7.2 El-Asnam Earthquake. Ground motion recorded at the Sogedia factory station during the 8 November 1980 aftershock with local magnitude 5.6 was used as empirical Green’s function. Lagged coherency functions estimated from simulated motions indicate significant loss of coherence with increasing frequency and separation distance. Since the EGF method, when used with Green’s function at a single station, can primarily model source effects, which are interpreted as the main reason behind the observed loss of coherency. The results also indicate that source effects are the main source of ground-motion incoherence in the near-source area, whereas it is negligible in the far field. In order to parametrize the estimated lagged coherency functions, they were compared with different models proposed in the literature. Curve fitting using nonlinear least square regression was performed to estimate the parameters of such models. It was found that the Hindy and Novak [1] model provides the best fit to the estimated lagged coherency functions. Along with the model parameters estimated in this study, this model, provides, at least, a first approximation, and possibly, an upper bound of ground motion incoherence in the study area for earthquake scenarios of the type presented in this study. .

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