Coral C-13/C-12 records of vertical seafloor displacement during megathrust earthquakes west of Sumatra

TitleCoral C-13/C-12 records of vertical seafloor displacement during megathrust earthquakes west of Sumatra
Publication TypeJournal Article
Year of Publication2015
AuthorsGagan MK, Sosdian SM, Scott-Gagan H, Sieh K, Hantoro WS, Natawidjaja DH, Briggs RW, Suwargadi BW, Rifai H
JournalEarth and Planetary Science Letters

The recent surge of megathrust earthquakes and tsunami disasters has highlighted the need for a comprehensive understanding of earthquake cycles along convergent plate boundaries. Space geodesy has been used to document recent crustal deformation patterns with unprecedented precision, however the production of long paleogeodetic records of vertical seafloor motion is still a major challenge. Here we show that carbon isotope ratios (image) in the skeletons of massive Porites   corals from west Sumatra record abrupt changes in light exposure resulting from coseismic seafloor displacements. Validation of the method is based on the coral image response to uplift (and subsidence) produced by the March 2005 image 8.6 Nias–Simeulue earthquake, and uplift further south around Sipora Island during a image megathrust earthquake in February 1797. At Nias, the average step-change in coral image was image for coseismic displacements of +1.8 m and −0.4 m in 2005. At Sipora, a distinct change in Porites microatoll growth morphology marks coseismic uplift of 0.7 m in 1797. In this shallow water setting, with a steep light attenuation gradient, the step-change in microatoll image is image, nearly four times greater than for the Nias Porites  . Considering the natural variability in coral skeletal image, we show that the lower detection limit of the method is around 0.2 m of vertical seafloor motion. Analysis of vertical displacement for well-documented earthquakes suggests this sensitivity equates to shallow events exceeding image in central megathrust and back-arc thrust fault settings. Our findings indicate that the coral image paleogeodesy technique could be applied to convergent tectonic margins throughout the tropical western Pacific and eastern Indian oceans, which host prolific coral reefs, and some of the world's greatest earthquake catastrophes. While our focus here is the link between coral image, light exposure and coseismic crustal deformation, the same principles could be used to characterize interseismic strain during earthquake cycles over the last several millennia.