The ongoing sequence of earthquake disasters along the Sumatran subduction zone has shocked the world since it began with the giant Sumatra-Andaman earthquake on 26 December 2004. The Sumatra subduction megathrust has produced five catastrophic earthquakes exceeding moment magnitude (Mw) 8.5 since 1797, yet the nature of any recurrent interseismic precursors that herald these events remains obscure. Development of palaeoseismic records for subduction zone earthquakes is challenging because the seismic sources are underwater and the great-earthquake cycle spans hundreds of years (Sieh et al., 2008), thus unusually stable and long-lived natural recorders are required.
Here we show that carbon isotope ratios (δ13C) in massive Porites corals positioned above the Sumatra megathrust are sensitive to vertical crustal motions during earthquakes (Fig. 1). It has been known for some time that water column light intensity, coral symbiont photosynthesis, and coral skeletal δ13C are inextricably linked. In the first instance, we built on this concept by documenting the response of skeletal δ13C to co-seismic uplift for a Porites coral from Sipora Island that was raised 0.7 m during the Mw 8.7-8.9 earthquake in February 1797 (Zachariasen et al., 1999). The abrupt 1.6‰ increase in skeletal δ13C marks uplift of the coral into shallower, brighter water (Fig. 2).
The 28 March 2005 Mw 8.7 Nias-Simeulue earthquake provided a rare opportunity to see if δ13C in Porites corals is sensitive to both co-seismic uplift and subsidence. Vertical crustal deformation around the island of Nias ranged from +2.9 m above the rupture to -1.1 m landward from the trench (Briggs et al., 2006). In May 2009 we collected underwater drill-cores from Porites corals along the coast of Nias that continued to grow under altered light exposure after the earthquake.
Results for sites that rose 1.8 m and subsided 0.4 m are shown in Fig. 2. All six coral records from the +1.8 m reef show a significant increase in δ13C after the earthquake, with an initial δ13C shift of 0.7‰. The coral δ13C response to 0.4 m subsidence is smaller, but three of five corals drilled show a clear 0.3‰ decrease in δ13C. In both cases, coral δ13C variability during the 10 years leading-in to the quake is significantly smaller than the shift in δ13C due to co-seismic changes in ambient light intensity. Together, the records show that δ13C in the skeletons of massive Porites is sensitive to vertical crustal motion.
Given this encouraging result, we are analysing δ13C in long vertical cores extracted from fossil Porites corals to see if skeletal δ13C also responds to crustal deformation brought about by decades to centuries of interseismic strain accumulation. If this aspect of the work is successful, we will then be positioned to reconstruct co-seismic, post-seismic, and interseismic crustal strain above the Sumatra megathrust over the last ~6,000 years, thus allowing us to document the tectonic patterns of many great-earthquake cycles in the past.
Briggs, R.W., Sieh, K., Meltzner, A.J., Natawidjaja, D., Galetzka, J., Suwargadi, B., Hsu, Y.-j., Simons, M., Hananto, N., Suprihanto, I., Prayudi, D., Avouac, J.-P., Prawirodildjo, L., Bock, Y. (2006), Deformation and slip along the Sunda megathrust in the great 2005 Nias-Simeulue earthquake, Science, 311: 1897-1901.
Sieh, K., Natawidjaja, D.H., Meltzner, A.J., Shen, C.-C., Cheng, H., Li, K.-S., Suwargadi, B.W., Galetzka, J., Philibosian, B., Edwards, R.L. (2008), Earthquake supercycles inferred from sea-level changes recorded in the corals of West Sumatra, Science, 322: 1674-1677.
Zachariasen, J., Sieh, K., Taylor, F.W., Edwards, R.L., Hantoro, W.S. (1999), Submergence and uplift associated with the giant 1833 Sumatran subduction earthquake: Evidence from coral microatolls, J. Geophys. Res., 104: 895-919.