Similar Seismic Ruptures and Interseismic Strain Rate Variations on the Nias–Simeulue Patch of the Sunda Megathrust

Abstract:

Fossil coral microatolls from fringing reefs above the great (M_W 8.6) megathrust rupture of 2005 record an extensive prior uplift, during the historically reported great earthquake of 1861. Such evidence spans nearly the entire 400-km strike length of the 2005 rupture, which we have shown previously to be bounded by two persistent barriers to seismic rupture. Moreover, at those sites where we have constrained the 1861 uplift, it is comparable to uplift in 2005. Thus the 1861 and 2005 ruptures appear to be nearly identical in both extent and magnitude. At one of these sites a still earlier uplift, about AD 1422, also appears to mimic the amount of uplift in 2005. The high degree of similarity among ruptures of this Nias–Simeulue section of the Sunda megathrust is unique across the 2000-km long Sumatran section of the Sunda megathrust offshore. At one site on the northwestern tip of Nias, reefs also rose during an earthquake in AD 1843, known historically for its damaging tsunami along the eastern coast of the island.

Another intriguing aspect of the coral records is that they demonstrate pronounced changes in rates of deformation between earthquakes. On southern Simeulue, interseismic rates of subsidence were low between 1750 and 1820 but abruptly increased three to four decades before the 1861 rupture. This may indicate that strong or deep locking of the megathrust began only a few decades before the great earthquake. More recently, Bangkaru island, near the hinge line between uplift and subsidence in 2005, switched from ~2.5 mm/yr of steady subsidence between 1956 and 1966, to ~8 mm/yr of steady uplift until 1981, and then reverted to ~2.5 mm/yr of steady subsidence until the coseismic uplift of 2005. These observations might be explained by a 15-year slow slip event under Bangkaru.