Discriminating slow slip earthquakes and sediment gravity flows from oceanographic signals: the Alaska Amphibious Community Seismic Experiment

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Subduction zones represent the most serious seismic hazard to United States coastal population centers. The episodic but unpredictable earthquakes from these subduction zones generate undersea landslides called turbidites that produce dangerous tsunami waves that can inundate coastal cities, producing epic damage and large-scale fatalities. The 2011 Tohoku earthquake, turbidite and resulting tsunami that claimed 15,899 deaths in Japan is a recent example of this hazard, where the damage to infrastructure continues to the present day. The 1964 Magnitude 9.2 Alaska margin earthquake and tsunami was the largest seismic event to hit the United States in modern times, causing over $2.3 billion dollars in damage and pointing clearly to the potential risks of subduction zone earthquakes and tsunamis to U.S. coastal cities. The recently-conducted National Scientific Foundation Community Seismic Experiment deployed ocean bottom instruments in a 14-month off-shore experiment on the Alaska margin in order to assess this risk. An additional suite of sensors and data loggers was added to this experiment by the University of Washington, specifically to evaluate the risk from turbidite currents initiated by Alaskan margin earthquakes. Within this current project, these new data will be integrated from all of the community experiment sensors in order to (1) determine the minimum magnitude earthquake that can trigger both turbidity currents and tsunamis, (2) produce numerical models to eliminate the production of false positive alarms that are generated from non-seismic Alaskan winter storms and ocean currents, and (3) help assess the potential for the Alaska Subduction Zone margin to produce the large tsunamis that potentially threaten the west coast of the United States and Canada. The project includes support for a UW graduate student whose PhD thesis includes training in evaluating subduction zone seismic hazards, a numerical modeler who will construct an Aleutian Island regional ocean model to help evaluate false positives produced by storms and local ocean current eddies and an expert in identifying marine landslides on subduction zone margins.The recent NSF Alaska Amphibious Community Seismic Experiment deployed 74 Ocean Bottom Seismometer instruments in the 14-month off-shore experiment on the Alaska subduction zone margin. In addition, the University of Washington added an additional 105 sensors and data loggers to these seismometers, specifically to identify turbidity currents generated by both local and distant seismic activity and by non-seismic initiation due to winter storms and vigorous ocean eddy circulation. This study will leverage the availability of the Community Experiment seismic data, bottom pressure and temperature data, and the generation of a regional ocean modeling system ocean circulation model. The primary goal is to develop improved techniques to correct bottom pressure time series for oceanographic phenomena in order to optimize the identification of long-duration seismic events and sediment gravity flows. Specifically, the study will integrate the multiple observational proxies which include bottom temperature, pressures at a stable reference site, network-averaged temperatures and pressures, atmospheric pressures and sea surface heights, together with the generation of a numerical model to improve oceanographic bottom pressure predictions and corrections. The seafloor instrument pressure data will also be used to investigate the effectiveness of differencing along-isobaths rather than the traditional approach of using a deep-water single-station reference dataset. The combined Community Experiment data will also be analyzed for short-term temperature signals diagnostic of turbidity currents and more subtle signals of precursory flows and enhanced pressures from long-lived suspended sediments. If turbidity flows are identified on the Alaskan margin, seismic data and models will be used to assess the triggering mechanisms and slope stability based on swath bathymetry maps, and sediment properties obtained from archive drilling and piston cores.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.