Neotectonics and the 2004 and 2005 earthquake sequences at Sumatra

Coseismic and interseismic deformation of the outer-arc islands of Nias and Simeulue associated with the great Indonesian earthquakes of 2004.12.26 and 2005.3.28 is generally ascribed to elastic stress storage and release on the interface between the Indian and Eurasian plates at the Java trench. However, the nature of the associated shallow seismicity and the pattern of Holocene deformation of the islands suggest that there has also been substantial activity on imbricate faults within the sediment prism behind the trench. The distinction between the two kinds of seismicity is of practical as well as theoretical significance because, in contrast with the megathrust, outer-arc faulting is not subject to appreciable postseismic relaxation and is, moreover, well placed to generate tsunami.