Evidence of instantaneous dynamic triggering during the seismic sequence of year 2000 in south Iceland

Abstract

We analyze the coseismic stress perturbation during the 17 June 2000 south Iceland seismic sequence; the main shock (Ms 6.6) was followed by three large events within a few tens of seconds (8, 26, and 30 s) located within 80 km. The aim of this paper is to investigate short-term fault interaction and instantaneous triggering. This happens when a fault perturbed by a stress change fails before the end of the transient stress perturbation. We compute the shear, normal, and Coulomb stress changes as functions of time in a stratified elastic half-space by using discrete wave number and reflectivity methods. We calculate dynamic stresses caused by the main shock at the hypocenters of these three subsequent events. Our numerical results show that the onset of the last two events is slightly delayed with respect to the arrival time of the second positive peak of Coulomb stress variation, while the first event occurred after the first positive stress peak. We have also analyzed the response of a spring slider system representing a fault governed by a rate- and state-dependent friction law, perturbed by shear and normal stress variations caused by the main shock. The fault response to the computed stress perturbations is always clock advanced. We have found suitable constitutive parameters of the modeled fault that allow the instantaneous dynamic triggering of these three earthquakes. If the initial sliding velocity is comparable with the tectonic loading velocity, we obtained failure times close to the observed origin times for low values of the initial effective normal stress.