Abstract

Although the 2000‐km‐long Sumatran fault is one of Earth’s longest and historically most active strike‐slip faults, previous large historical ruptures have gone unmapped, although some have been inferred from felt reports or from modeling of sparse geodetic data. Through field observation, we documented twin‐surface ruptures associated with two M>6 earthquakes in March 2007. The first rupture broke a 22 km part of the 60‐km long Sumani segment southeast from a releasing stepover; the second rupture occurred on a 22.5 km part of the 90‐km‐long Sianok segment northwest from the stepover. Dextral displacements on the two segments averaged 51 and 36 cm. The earlier earthquake doublet in 1926 also straddled this stepover but also did not seem to have ruptured the entire fault segments. Thus, using fault lengths between large stepovers to estimate earthquake magnitude will overestimate what is likely to occur.

Online Material: Photos of surface rupture and surface displacement measurements.

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