Collaborative Research: Sedimentary signature of shallow and tsunamigenic megathrust ruptures: Observations and physical models from recent catastrophic events

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The 2004 Sumatra and the 2011 Tohoku-Oki mega-earthquakes and tsunamis were catastrophic geologic events with major societal consequences. Both produced huge ruptures in areas of subducting tectonic plate boundaries that were not expected to have such large earthquakes. The large fault displacement reached the sea-floor of oceanic trenches, causing the huge tsunamis. Plate boundaries like these are the most consequential source of submarine earthquakes, especially tsunamigenic ones, and the only source of similarly large earthquakes. These two most recent ones were particularly significant in stimulating marine research and in providing further understanding of mega-earthquakes and associated tsunamis. Data on earlier large events are critical to addressing where and how often such events occur and what to expect from them, but few such events are available with instrumental data. The only recourse is reconstruction of older mega-earthquakes from their record in sediment deposits. Pioneering deep-water research in submarine earthquake geology is now using the sedimentary signature of the Tohoku-Oki earthquake to recognize signatures of previous large earthquakes. This research is advancing our understanding about the unique characteristics of sedimentation events associated with exceptionally large earthquakes and helping to distinguish them in the sedimentary record. Typical event deposits include laminated, chaotic, and homogeneous sediments (“homogenites”). This study will combine field observations, including deposits and documentation of widespread surface sediment mobilization, with a novel laboratory “sediment shaker”, to better understand the unique attributes of these rare but devastating events. The project supports the training of students and an early career scientist.The largest megathrust earthquakes at subduction margins remobilize sediment over ruptures 100s of km wide, leaving characteristic event-deposits in the stratigraphy. They generally include an acoustically transparent layer that is lithlogically, physically and geochemically homogeneous (“homogenite”), plus acoustically laminated layers with a wide range of sedimentary structures and composition (“turbidites”). The muddy homogenites from the 2011 Japan earthquake have been recently discovered to derive from widespread remobilization of surficial (few cm) sediment, while sand-rich turbidites are thought to derive from slope failures and diverse sources. This difference holds for other earthquakes and settings, suggesting distinct remobilization processes. This hypothesis will be tested by laboratory experiments that will explore whether these processes respond to different parts of the earthquake spectrum. Another question to be addressed is whether the exceptionally large and sustained long-period oscillations of the sea floor above M≥9.0 megathrust ruptures can entrain surficial sediment and thus account for the homogenites. Exceptional data sets from the 2011 Tohoku earthquake will be available to conduct the experiments. Through collaborations, samples from the Cascadia Margin, Lesser Antilles and Balearic Abyssal plain will be added. The comparisons of global observations from geophysical and core data with the physical experiments, will provide a major step forward in understanding the ways in which earthquakes remobilize sediments and advances the field of submarine paleoseismology to recognize megaquake event deposits.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.