PALEOGEODESY FROM CORALS IN MEGATHRUST TECTONIC SETTINGS

Coral microatolls contain unusually precise records of land-level changes along subduction zones at interseismic (decades), coseismic, and longer-term (centuries to millennia) time scales. Several species of the genus Porites are especially well-suited for paleogeodesy studies because their annual growth rings closely track sea-level changes to within centimeters and they are usually widely dispersed along tropical fringing reefs. The annual to centennial centimeter-scale sea-level record contained in broadly-distributed coral microatolls enhances the more precise, but inevitably more sparsely distributed, GPS geodetic measurements.

In recent years, studies of Porites microatolls dated by precise U-Th methods have clarified several fundamental questions about megathrust behavior. Models of coseismic rupture, interseismic locking, and Holocene forearc deformation have benefitted from analyses of coral data, particularly in Sumatra and the Solomon Islands. For example, the distributions of coseismic coral uplift and subsidence near Sumatra in 2004, 2005, and 2007, and the Solomon Islands in 2007, place tight limits on megathrust coseismic slip distributions that are significant improvements over models derived only from teleseismic data or inferred from structural or geophysical proxies for slip. The manner in which rupture patches overlap and the controls (or lack thereof) on ruptures by upper- and lower-plate structures have been illuminated by coral geodesy and paleogeodesy. The existence of interseismically highly-coupled patches and correlative coseismic slip patches over several seismic cycles in Sumatra, as recorded by corals, suggests that lateral variations in slip behaviors persist over several earthquake cycles. Over thousands of years, the low elevations of mid- and late-Holocene microatolls along the outer arc of Sumatra demonstrate that forearc deformation is primarily elastic, and that interseismic strain is released primarily along the megathrust.