Tectonic strain is partitioned between the megathrust and faults in the overriding plate at obliquely convergent plate boundaries (Fitch, 1972;McCaffrey, 1992). Strain in the overriding plate is accommodated by margin-parallel shear, often by margin-parallel strike-slip fault systems within or in proximity (10's of km) to active volcanic arcs. A classic example of this end member is the Indo-Eurasian plate boundary in Sumatra, whereby the Great Sumatran fault accommodates the ∼25 mm/yr of relative motion between the Sumatran forearc and Eurasian plate within 20 km of the volcanic arc (Genrich et al., 2000;Sieh & Natawidjaja, 2000). However, margin parallel shear in the overriding plate may not necessarily be accommodated by margin parallel strike-slip fault systems,