Three-dimensional mechanical models for the June 2000 earthquake sequence in the south Iceland seismic zone

Abstract

A sequence of earthquakes, including two M w = 6.5 events, occurred in June 2000 in an atypical transform zone located in the southwest of Iceland. The inter-, co-, and post-seismic time intervals of the associated deformation cycle have been well recorded by previous geodetic studies, including GPS and InSAR. Using a numerical code based on a finite-element method, we analyze the influence of structural and rheologic heterogeneities on the earthquake deformation cycle within the south Iceland seismic zone (SISZ). The co-seismic deformation is especially sensitive to the increase of lithospheric stiffness with depth, as shown by a joint inversion of the geodetic data. The estimated distribution of co-seismic slip for the June 21 mainshock is deeper when estimated in a realistic layered lithospheric model than in a homogenous half-space. The geometry of the rheologic layers also plays an important role during the post-seismic interval, when several inter-related processes (poro-elastic effects around the fault plane, afterslip in it, and/or visco-elastic relaxation in the lower crust and/or upper mantle) appear to occur on overlapping time and length scales. We also consider the seismicity, both in its contribution to stress transfer and in its tendency to migrate from east to west. At least six processes may be involved in changing the stress distribution: (1) propagation of seismic waves, (2) changes of static stress caused by major co-seismic slip, (3) cascading seismicity, (4) fluctuations in hydrological conditions, (5) ductile flow of subcrustal rocks, and (6) inter-seismic strain accumulation. All six of these phenomena occurred in the SISZ before, during and after the June 2000 earthquake sequence. By using three-dimensional finite-element models with realistic geometric and rheologic configurations to match the observations, we test the influence of the increase of rigidity with depth and the variable crustal thickness on processes (2), (3), (5) and (6). Inter-seismic strain (6) accumulating in an elastic upper crust that thickens eastward can produce an asymmetric stress distribution. This process may explain the tendency of subsequent earthquakes to migrate from east to west across the south Iceland seismic zone within a single sequence. Yet it cannot explain their timing or the location of the first event. Our modeling with realistic geometric and rheologic configurations suggests that the June 17 earthquake triggered the June 21 event by a combination of several time-dependent post-seismic processes.