T31D-02
Dispatches from the Trench: Insights into the Complex Relationship Between the Short-Term Elastic Earthquake Cycle and Longer-Term Permanent Tectonic Deformation from the Coral Record at Ranongga, Western Solomons
Wednesday, 16 December 2015: 08:15
306 (Moscone South)
Kaustubh Thirumalai, University of Texas, Institute for Geophysics, Austin, TX, United States, Frederick W Taylor, Institute for Geophysics, Austin, TX, United States, Chuan-Chou Shen, NTU National Taiwan University, Taipei, Taiwan, Luc L Lavier, Jackson School of Geosciences, Austin, TX, United States, Cliff Frohlich, Univ Texas, Austin, TX, United States, Laura M Wallace, University of Texas at Austin, Institute for Geophysics, Austin, TX, United States, Chung-Che WU, Department of Geoscience, National Taiwan University, Taipei, Taiwan, Hailong Sun, Chinese Academy of Sciences, The State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Guiyang, China and Alison K Papabatu, Dept. Mines, Energy, Water Resources, Minsitry of Natural Resources, Honiara, Solomon Islands
Abstract:
Key insights into the amounts of plate convergence that have been accommodated as coseismic slip vs. other mechanisms over Holocene time scales are inferred from the timing and amounts of uplift associated with past megathrust earthquakes. A Mw 8.1 earthquake in 2007 helped us calibrate the relationship between megathrust rupture and the geography and amount of vertical displacement by measuring the amounts of coseismic uplift of shallow-living corals. We discovered, mapped and used 230Th ages of ancient in-situ corals extending 20 m higher than those killed by the 2007 uplift to build an uplift history on Ranongga and adjacent islands to reconstruct a series of forearc paleouplifts overlying the seismogenic zone extending from ~4 – 40 km from the trench. Our results are surprising: from mid-Holocene to present, we find distinct spatio-temporal heterogeneity in uplift across different sites that are located close together and find that there are far too few events (by ~50%) in our coral record to accommodate the current Australian-Pacific convergence rate of ~97mm/yr. These findings lead us to suggest that 1. uplift imposed during the 2007 Mw 8.1 event may be retained in some locations but removed in others prior to the next megathrust rupture; and 2. apart from large coseismic megathrust rupture, additional mechanisms such as smaller ruptures, slow slip events or aseismic creep, and/or splay faulting must help to accommodate at least half of the interplate convergence. Thus, we propose that the Western Solomon thrust be visualized as a fault zone, and not simply a fault plane: every time the system accommodates slip, a new set of cards is dealt. Though such a situation may or may not be unique to this region, coral paleogeodesy can provide detailed histories of vertical deformation at locations accruing uplift that can provide a foundation for hypotheses concerning the accommodation of interplate slip.
