Anti-correlation of coseismic and permanent vertical deformation above the Sunda megathrust on Nias island, Sumatra

The relationship of coseismic elastic deformation to long-term permanent deformation above megathrusts is important for neotectonic and seismic hazard studies. On Nias island, an island on the forearc high above the Sunda megathrust, we are able to investigate this problem. We are comparing coseismic deformation associated with the 2005 Mw 8.7 Nias-Simeulue rupture to the long-term uplift pattern of Nias. Fossil coral microatoll elevations and U/Th dates from fringing Holocene reef platforms allow us to quantify permanent vertical deformation along much of the Nias coastline. A reasonable hypothesis might be that some increment of the 2005 uplift pattern will be retained as permanent uplift of Nias. Instead, we find that the long-term, permanent uplift of Nias is basically anti-correlated with the 2005 coseismic uplift signal. In 2005, the island tilted toward the Sumatran mainland, with coseismic uplift of almost 3 meters on the west coast and near zero on the east coast. The long-term pattern we observe is nearly the opposite: Tilt is trenchward and uplift rates are an order of magnitude greater on the east coast than on the west. The belt of highest 2005 coseismic uplift along the west coast has experienced very little net uplift, and possibly even net subsidence, in the latest Holocene. Vertical coseismic deformation along the east coast of Nias ranged from tens of centimeters of uplift to tens of centimeters of subsidence in 2005, yet mid-Holocene terraces there sit 5 meters or more above sea level. Also, along the southern part of the east coast, minor faults offset the Holocene reef platform. This long-term deformation is the opposite of what would be expected if megathrust rupture were playing a major role in the long-term uplift of the island. Thus, it appears that interseismic deformation of the wedge is elastic and wholly recovered by megathrust rupture, and that permanent deformation is primarily along secondary structures that did not rupture in 2005.