Seismotectonics of a newly formed transform zone near a hotspot: Earthquake mechanisms and regional stress in the South Iceland Seismic Zone

Abstract: We carry out mass inversion of focal mechanisms of earthquakes from 1991 to 2004 along the South Iceland Seismic Zone, within a quadrangle about 37 km wide and 65 km long, with 33,878 earthquakes illuminating a crust volume of nearly 35,000 km³. Our inversion aims at identifying the seismotectonic stress regimes that best account for the whole set of data, not at examining local variations and stress perturbations in space and time. Regardless of magnitude selection, the inversion reveals consistent orientations, for both a primary regime with NE-SW compression and NW-SE extension, accounting for about three-fourths of the data, and a secondary regime with NW-SE compression and NE-SW extension. The corresponding earthquakes are highly intricate in space and time, suggesting that the secondary regime, which shows larger dispersion and slightly lower magnitudes as compared with the primary regime, mainly results from the effects of elastic rebound, stress drop and fluid migration. Because the data acquisition and our inversion technique are independent, this analysis provides confirmation that even the negative magnitude earthquakes recorded by the Icelandic seismic network (SIL) are reliable. Our analysis reveals high levels of consistency between the seismotectonic stress pattern and the transform kinematics of the South Iceland Seismic Zone. The proximity of the Icelandic Mantle Plume, with a positive thermal anomaly, modified the rheology of the upper lithosphere and hence influenced structural development. Simple shear probably prevails within a 20 km wide corridor in the underlying viscous layer, inducing particular block faulting at the surface. Both the fault distribution and the effective block dimensions are consistent with a simple structural evolution model.