Modeling temporal variations in interseismic subsidence rates recorded by corals in the Simeulue-Nias region, Sumatra: How can we explain them?

Abstract:

How large can temporal variations in interseismic deformation rates be? Meltzner et al. (2014, in review) reported that fossil coral microatolls on Simeulue and the Banyak Islands of Sumatra, Indonesia, show contemporaneous temporal variations in interseismic subsidence rates. For example, a fossil coral microatoll located at Bangkaru, in the Banyak Islands, reveals interseismic subsidence for a 10-year period between 1956 and 1966, followed by a reversal to interseismic uplift for ~ 15 years from 1966 to 1981, before reverting back to interseismic subsidence after 1981. These temporal variations can neither be accounted for by regional or local sea level changes, nor by postseismic deformation following major historic megathrust earthquakes. These coral observations span a period of ~ 260 years. Such an extensive temporal record of interseismic deformation at subduction zones is rare, and it offers us a unique opportunity to better understand what physical processes likely control the behavior of this portion of the megathrust.

In this study, we use elastic dislocation models to explore the extent to which the observed spatio-temporal variations can be consistently explained by tectonic processes. Possible processes include temporal changes in either the width of the locked fault zone or the interseismic coupling coefficient, and/or slow-slip event occurrence. Our models incorporate a realistic fault dip geometry and subduction rate. Our results suggest that a change in the width of the locked zone with time can explain temporal variations in interseismic subsidence rates on Simeulue Island. In addition, the reversal of the sense of vertical displacement at Bangkaru can be explained by the occurrence of a large, long, slow-slip event on the underlying megathrust.