Tectonic uplift and subsidence inferred from coral microatoll archives of relative sea level in La Union province, northwestern Luzon, Philippines

What are the main drivers of relative sea level (RSL) change over the past 1100 yr along the west coast of Luzon Island, Philippines? Glacial isostatic adjustment is expected to contribute at rates below 1 mm/yr, but tectonic deformation in coastal regions may drive RSL change at significantly higher rates. Oblique convergence between the South China Sea tectonic plate and the Philippine Mobile Belt results in a complex pattern of deformation in western Luzon Island. This deformation is accommodated through the Manila subduction zone and a series of upper-plate faults on- and offshore. Proxy records of RSL over time at multiple locations can be used to discern the drivers of local and regional RSL and to infer vertical deformation and accumulated strain along the Manila Trench and upper-plate faults.

To ascertain the earthquake potential in this region, 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 dates indicating at least three generations from 1100 cal yr BP to present. Coral microatoll analyses show characteristic morphology of fossil microatolls with a low center and higher outer rings, indicating a gradual RSL rise over the microatoll's lifetime punctuated by occasional small diedowns. However, details revealed in microatoll cross sections and through a comparison of successive generations of microatolls are consistent with larger, abrupt changes in land level, which we infer to be coseismic. By comparing RSL histories along the western coast of Luzon and through elastic dislocation modeling efforts, we will attempt to identify the contributions to RSL change from different mechanisms in the region, in particular attributing tectonic deformation to specific causative faults.