Decadal to Centennial Vertical Paleogeodetic Record of the Seismic Cycle in the Western Solomons from Coral Paleogeodesy and Stable Isotopes

The extremely shallow location of the seismogenic megathrust in the western Solomons and the existence of significant island land area on the upper plate overlying the seismogenic zone enables us to use corals to obtain vertical motion history closer to the trench and lower plate than anywhere else in the world. In addition, coral paleogeodesy on Porites microatolls acting as long-term vertical positioning station may provide a relative sea level (RSL) change record spanning hundreds of years. Our goal is to develop a centennial record of sea level change and vertical tectonics from multiple Porites microatolls. By isolating the RSL record common to each microatolls, we can then derive a vertical tectonic record by removing the RSL variations from the raw time series recorded by the microatolls.  To achieve that goal, we present recent work combining coral paleogedesy, annual δ13C record and modeling of coral morphology over the last 80 years in the western Solomons. The steps to obtain a long-term record of sea level change and vertical tectonics on samples of a ~80 year old Porites head collected in 2013 after the 2007 Mw 8.1 earthquake. We sampled the coral over 2 to 3 annual bands every ~2 months at various depths and times, performed a stable isotope analysis on each sample, cross-correlated each record and plotted the variation in δ13C versus water depth. Linear regressions show that the variation in accumulated δ13C as a function of water depth relative to the coral’s top water depth is 41 cm/‰ with a R² coefficient of 0.98. We the sampled bimonthly stable isotopes along 80 annual bands. The span of each year is determined from correlating the annual banding and the seasonal cycles in δ13C and δ18O. Applying the linear relationship to the δ13C generates a raw record of relative sea level change. We then use the monthly tide gauge record in Honiara (Guadalcanal) to remove the effects of regional sea level change to the RSL time series obtain from the coral. The result is a record of the vertical tectonic motion of part of the Western Solomon before and after the Mw8.1 2007 earthquake. We analyze the results in terms of the yearly vertical record of the seismic cycle. Current geodetic records at subduction zones constrain at most deformation during one earthquake cycle while multiple earthquake cycles are needed to robustly constrain the physical state of a megathrust.  We hope to be able to extend the coral paleogeodetic record in the Weatern Solomons over several hundred years over multiple seismic cycles.  This would represent a critical data gap that hampers our understanding of subduction physics and our ability to forecast earthquakes.