Persistent termini of 2004- and 2005-like ruptures of the Sunda megathrust

Abstract

To gain insight into the longevity of subduction zone segmentation, we use coral

microatolls to examine an 1100-year record of large earthquakes across the boundary

of the great 2004 and 2005 Sunda megathrust ruptures. Simeulue, a 100-km-long island

off the west coast of northern Sumatra, Indonesia, straddles this boundary: northern

Simeulue was uplifted in the 2004 earthquake, whereas southern Simeulue rose in 2005.

Northern Simeulue corals reveal that predecessors of the 2004 earthquake occurred

in the 10th century AD, in AD 1394±2, and in AD 1450±3. Corals from southern

Simeulue indicate that none of the major uplifts inferred on northern Simeulue in the past

1100 years extended to southern Simeulue. The two largest uplifts recognized at a

south-central Simeulue site—around AD 1422 and in 2005—involved little or no uplift

of northern Simeulue. The distribution of uplift and strong shaking during a historical

earthquake in 1861 suggests the 1861 rupture area was also restricted to south of central

Simeulue, as in 2005. The strikingly different histories of the two adjacent patches

demonstrate that this boundary has persisted as an impediment to rupture through

at least seven earthquakes in the past 1100 years. This implies that the rupture lengths,

and hence sizes, of at least some future great earthquakes and tsunamis can be forecast.

These microatolls also provide insight into megathrust behavior between earthquakes,

revealing sudden and substantial changes in interseismic strain accumulation rates.