Modelling instantaneous dynamic triggering in a 3-D fault system: application to the 2000 June South Iceland seismic sequence

摘要

We present a model of seismogenesis on an extended 3-D fault, subject to the external perturbationsof coseismic stress changes due to an earthquake occurring on another fault (the causativefault). As an application, we consider the spatio-temporal stress redistribution produced on theHvalhn´ukur fault by the MS 6.6 2000 June 17 mainshock in the South Iceland Seismic Zone(SISZ). The latter is located nearly 64 kmfrom the causative fault and failed 26 s after the mainshock with an estimated magnitudeMw = 5.25 ± 0.25, providing an example of instantaneousdynamic triggering. The stress perturbations are computed by means of a discrete wavenumberand reflectivity code. The response of the perturbed fault is then analysed solving the truly3-D, fully dynamic (or spontaneous) problem accounting for crustal stratification. In a previousstudy, the response of the Hvalhn´ukur fault was analysed by using a spring–slider fault model(SS fault model), comparing the estimated perturbed failure time with the observed origintime. In addition to the perturbed failure time, this model can provide numerical estimates ofmany other dynamic features of the triggered event, which can be compared with availableobservations—the rupture history of the whole fault plane, its final extent and the seismic momentof the induced event.We show the key differences existing between a mass–spring modeland this extended fault model; in particular, we show the essential role of the load exerted bythe neighbouring slipping points of the fault. By considering both rate- and state-dependentlaws and non-linear slip-dependent law, we show how the dynamics of the 26 s fault stronglydepend on the assumed constitutive law and initial stress conditions. In the case of rate- andstate-dependent friction laws, assuming an initial effective normal stress distribution that issuitable for the SISZ and consistent with previously stated conditions of instantaneous dynamictriggering of the Hvalhn´ukur fault, we obtain results in general agreement with observations.