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Improving Subduction Zone Hazards Assessments Using the Coastal Stratigraphic Record


Earthquake and tsunami records on centennial and millennial temporal scales are necessary to understanding subduction zone hazards and the occurrences of large, but infrequent events. Subduction zone paleoseismology combines the methods of coastal stratigraphy, sedimentology, micropaleontology, geophysical and sediment transport modeling, and sea-level research to produce some of the most detailed long-term histories of coseismic vertical deformation and tsunami inundation along subduction zone coastlines. Microfossil-based (e.g., diatoms, foraminifera) techniques that employ the relation between microfossils and salinity, tidal elevation, and life form to quantify coseismic land-level change across sharp stratigraphic contacts and identify anomalous sand beds deposited by tsunamis are particularly valuable to subduction zone paleoseismic studies. Microfossil-based techniques have been successfully employed in the reconstruction of earthquake and tsunami histories in Chile, the Indian Ocean, Japan, New Zealand, the North Sea, the Pacific Northwest of North America, and the South Pacific. In Alaska and Chile, microfossils have documented both uplift and subsidence at proposed subduction zone segment boundaries, expanding our knowledge of the variability of slip in megathrust ruptures. In tsunami studies in Alaska, Chile, and Japan allochthonous marine and brackish microfossils within anomalous sand deposits signaled previously undocumented high-energy marine incursions into coastal lowlands. At the Cascadia subduction zone, a marsh monitoring experiment emphasized the importance of studying the modern diatom response to changing environmental conditions to refine estimates of past coseismic deformation. Finally, paleoseismic studies have better informed our modeling of teleseismic tsunamis that pose a flooding hazard to near- and far-field coastlines. Forward modeling of teleseismic tsunamis originating along the Aleutian megathrust combined with probabilistic sea-level rise projections for southern California illustrate the increased flooding threat to highly populated areas from far-field tsunamis as sea level rise accelerates over the next 100 years, emphasizing the need for interdisciplinary approaches to future coastal hazards assessments.