In 1986, 1989 and in 1991, the PI conducted Global Positioning System (GPS) measurements in segments of Iceland, one of the few places on land where one can study active rifting and rift- transform interactions. From historic data extending back many centuries, a sequence of M>7 earthquakes is overdue in south Iceland and current deformation in the region must represent the final stage of the interseismic cycle. In 1988 a magnitude 5.8 earthquake occurred within the network, and in 1991 the volcano Hekla erupted displacing nearby control points by up to 6 cm. Over a broad region the data support a 2 cm/year shear signal applied to the South Iceland Seismic Zone, consistent with north Atlantic plate motions; however, the deformation is complex and is poorly resolved by earlier measurements. New measurements will provide constraints on the development of shear strain and rift widening in SW Iceland. The partition of the 2 cm/year budget between these two types of deformation may account for the observed irregular- ities in volcanism and earthquake sequences in SW Iceland.
