Tectonic uplift and subsidence inferred from coral archives of relative sea level in Balaoan, La Union, Philippines

TitleTectonic uplift and subsidence inferred from coral archives of relative sea level in Balaoan, La Union, Philippines
Publication TypeConference Paper
Year of Publication2020
AuthorsSarkawi GM, Meltzner AJ, Gopal A, Lim JTY, Mitchell AP, Sarmiento LF, Weil-Accardo J, Maxwell KValdez, Komori J, Li X, Ke L, Wang X, Hu H-M, Shen C-C, Gong S-Y, Lu Y, Ramos NTuazon
Conference NameAmerican Geophysical Union Fall Meeting
Date Published12/2020
Conference LocationOnline
Abstract

Oblique convergence between the South China Sea tectonic plate and the Philippine Mobile Belt results in complex deformation in western Luzon Island, Philippines, accommodated through the Manila subduction zone and a set of upper-plate faults. Proxy records of relative sea level (RSL) over time along the west coast of Luzon can be used to discern the drivers of local and regional RSL and infer vertical deformation and accumulated strain on faults. Glacial isostatic adjustment is expected to contribute to RSL at rates below 1 mm/yr over the past ~1500 yr in the region, but tectonic deformation in coastal regions may drive RSL change at significantly higher rates.

To ascertain the earthquake potential, we use fossil and living coral microatolls to reconstruct RSL. We identified several fields of coral microatolls at a site in La Union province, Philippines, with radiocarbon and U-Th dates indicating at least three generations from 1100 cal yr BP to present. The surface morphology of the coral microatolls indicates a gradual RSL rise over the microatolls’ lifetime punctuated by occasional small diedowns. Further analysis of microatoll cross sections and a comparison of successive generations of microatolls reveal instances of decimeters of RSL rise and fall, consistent with land-level changes inferred to be coseismic subsidence and uplift. The younger fossil generation grew in the historical period, and the historical record may provide additional insight into inferred tectonic deformation. Through elastic dislocation modeling, we attempt to attribute RSL changes to their most likely mechanisms, in particular associating tectonic deformation with specific causative faults. These models infer subsidence events from the Manila Trench while the permanently sustained uplift in the region is accommodated by upper-plate faults. By comparing RSL histories elsewhere along the west coast of Luzon coast, we are attempting to identify the key drivers of RSL change in La Union.