Styles of surface rupture accompanying the June 17 and 21, 2000 earthquakes in the South Iceland Seismic Zone

Abstract

Two large M s=6.6 earthquakes occurred on June 17 and 21, 2000 in the South Iceland Seismic Zone. The roughly E–W trending zone is undergoing left-lateral shear. However, most known surface rupture has been along north-striking, right-lateral strike-slip faults. Rupture associated with the June 2000 events follows a similar pattern. Although the two earthquakes had similar magnitude, fault plane solutions, and overall rupture lengths of 15 to 20 km, the pattern of rupture from each was notably different in character. The June 17 event ruptured along a series of NNE-trending, left-stepping segments, giving the fault as a whole an almost due north trend. At the largest scale rupture is relatively straight and continuous. At the smallest scale, rupture style seems to vary with small-scale topography and ground texture. Rupture from the June 21 event is more complex and can be divided into five discrete segments. To the north, deformation is distributed across two zones of left-stepping fractures, along which widening and subsidence have occurred. The central segment consists of right-stepping fractures defining a 2 km long, ENE trending zone. Sense of shear is clearly left-lateral strike slip. The two southernmost segments define an NNE trend. In several places along the rupture zones of both earthquakes it can be verified that the ruptures occurred along pre-existing faults. The observed faulting structures are similar to those of earlier earthquakes in South Iceland, both with regard to style and spatial arrangement. However, our observations suggest that some of the historical earthquakes may have been larger than the June 2000 events.