Modelling of the ground acceleration in the near source range: the case of 1976 Friuli earthquake (M=6.5), Northern Italy

A mixed statisticaldeterministic model of earthquake rupture is developed for evaluating the strong ground motion in the near source range receiver distance comparable to the fault length). The source parametrization is based on the ksquare model and the propagation is computed by asymptotic Green’s functions. The method is applied to the case of 1976, Friuli earthquake (M= 6.5) in northern Italy which occurred on a lowdip thrusting fault. Acceleration records at 29 stations are computed for 100 simulations of rupture histories. The mean value map of peak ground accelerations shows clearly a maximum to the south due to the inner geometry and directivity of the source. The variation of the estimated PGA versus the epicentral distance is strongly dependent on azimuth and is not decreasing monotonically. The comparison of these curves with those predicted by empirical acceleration– distance relationships shows discrepancies in the near source distance range. This study shows the importance of considering the complexity of the source rupture process for strong motion estimate in the near source range.