Distribution of slip at the northern Sumatran fault system

We model spatial variations in horizontal displacements of 117 geodetic sites measured during annual surveys in 1989-1996 with the Global Positioning System (GPS) as elastic strain across a locked strike-slip fault to infer the contemporary slip rate, locking depth, and location of the Sumatran fault (SF) in northern Sumatra (1 degreesS-3 degreesN). GPS-derived slip rate estimates increase slightly northward from 23+/-3 mm/yr at 0.8 degreesS to 26+/-2 mm/yr at 2.7 degreesN. They agree with geologic estimates north of the equator, but at 0.5 degreesS they are similar to 10 mm/yr higher. Strain appears to be distributed asymmetrically about the fault. South of 2 degreesN, about 5 mm/yr of shear is required within the offshore forearc, west of the fault, to achieve a closer agreement of fault locations inferred from GPS velocities with geologically identified traces of the SF. Locking depth estimates are of the order of 10-20 km. The western branch of the major fault bifurcation near 1 degreesN slips at a rate 5 times higher than the eastern branch. The two main strands of the fault at the northwestern tip of Sumatra (5.5 degreesN) appear to be nearly free of horizontal strain; significant slip must occur away from the two strands, probably farther east at two other geologically active branches. The Banda Aceh embayment is extruded to the northwest at a rate of 5+/-2 mm/yr. Within the estimated velocity uncertainties of several millimeters per year, fault-normal deformation along the SF is insignificant. Almost strain free, the northern part of the back are basin is part of the rigid Sunda Shelf, while the northern forearc is subjected to 8+/-5 x 10(-8)yr(-1) of extension nearly parallel to the arc.