Strain partitioning during oblique plate convergence in northern Sumatra: Geodetic and seismologic constraints and numerical modeling

Global Positioning System (GPS) measurements along the subduction zone of n orthern Sumatra (2 degreesS to 3 degreesN) reveal that the strain associate d with the oblique convergence of the Australian plate with Eurasia is almo st fully partitioned between trench-normal contraction within the forearc a nd trench-parallel shear strain within a few tens of kilometers of the Suma tran fault. Kinematic analyses of interplate earthquake slip vectors provid e slip rates on the Sumatran fault within a few millimeters per year of GPS and geologic rates, giving us more confidence in the use of slip vectors f or inferring slip partitioning elsewhere. The inferred slip rate on the Sum atran fault is similar to1/3 less than the full margin parallel component o f plate motion. An across-forearc rotation in the slip vectors suggests tha t the missing are-parallel shear occurs seaward of the geodetic network, be tween the forearc islands and the trench. Simple finite element models are used to explore the conditions under which the change in the principal stra in rate directions between the forearc and the are region can occur. Modeli ng suggests that neither a preexisting strike-slip fault nor a zone of ther mally induced lithospheric weakness in the overriding plate is needed for s train partitioning to occur. In general, forearc slivers form over the regi on of interplate coupling and are driven along strike by the basal shear. A volcanic are can help the partitioning process by localizing the margin-pa rallel shear strain in the upper plate if its crust and mantle are weaker t han its surroundings. Interplate slip vectors and geodetic results from Sum atra together suggest that the highest coupling on the plate boundary occur s beneath and seaward of the forearc islands, consistent with inferences ab out the rupture zones of great nineteenth century earthquakes there, The Su matra example suggests that geodetic measurements of interseismic, margin-p arallel shear strain at oblique convergent margins can be used to map the l andward extent of the relatively high basal stress beneath the overriding p late if one can correct for strain localization caused by weak upper plate strike-slip faults.