Detecting Seismic Signatures in the Rock Record at the Japan Trench

Abstract:

IODP Expedition 343 (JFAST) drilled through the plate boundary at the Japan Trench where potentially several megathrust earthquakes have occurred, most recently the 2011 M_w 9.1 Tohoku-oki earthquake. Here, we investigate structural features of the plate boundary at the JFAST site to determine their seismic history. Using trace element geochemistry as a tool to fingerprint sedimentary units, we develop a stratigraphy of the JFAST core that shows significant plate boundary complexity. To determine the seismic history of the faults identified in the core, we use a novel method to detect temperature rise along faults: the thermal destruction of organic molecules. We interpret the destruction of alkenones, a common biomarker found in marine sediments, along faults to be a result of coseismic heating. In order to constrain initial alkenone concentration, we use our trace element stratigraphy to correlate individual JFAST samples with their protolith in the DSDP Site 436 reference core. We find that at least three faults demonstrate significant destruction of alkenones, implying that these faults have experienced shallow seismic slip.

In the past, frictional work has been difficult to measure on faults due to a paucity of available coseismic temperature proxies preserved in the rock record. By coupling experimentally determined reaction kinetics of alkenone destruction with models of temperature rise during earthquakes, we convert the alkenone destruction measurements into estimates of maximum temperature rise on faults in the JFAST core. This novel measurement allows us to explore energy budgets at the subduction interface.