USING CORAL GEODESY TO INVESTIGATE THE SEISMIC CYCLE IN THE BANYAK ISLANDS, INDONESIA

The location of the Banyak Islands above the down-dip limit of the 2005 Nias-Simeulue earthquake rupture provides a unique opportunity to study the seismic cycle. The synthesis of sea level histories based on Porites genus corals at three sites spanning a 40-km transect perpendicular to the Sunda trench allows a reconstruction of the interseismic, coseismic, and postseismic deformation that occurred before, during, and after the 2005 event. During the earthquake, the western island of Bangkaru uplifted while the eastern islands subsided, suggesting that the island group is located above the down-dip limit of slip. Early postseismic motions suggest that much of the coseismic subsidence in the Banyak Islands will be recovered quickly by postseismic uplift in the decades following the earthquake, with low rates of subsidence occurring in the latter part of the interseismic period. This deformation pattern implies that the Banyak Islands overlie the down-dip edge of a locked patch on the megathrust. This asperity probably slips only during large earthquakes whereas the adjacent down-dip region of the fault slips with initially accelerated postseismic rates, slowing and perhaps becoming completely locked late in the interseismic interval. However, the island of Bangkaru which experienced coseismic uplift has had little or no interseismic or postseismic subsidence, suggesting that an updip slip event is required for elastic recovery.