The application of diatoms to reconstruct the history of subduction zone earthquakes and tsunamis

Earthquake and tsunami records on centennial and millennial temporal scales are necessary to understanding long-term subduction zone behavior and the occurrences of large, but infrequent events. Microfossils, such as diatoms, incorporated into coastal stratigraphy provide some of the most detailed reconstructions of the history of earthquakes and tsunamis. We explore qualitative and quantitative techniques that employ the relation between diatoms and salinity, tidal elevation, and life form to: (1) reconstruct records of vertical land-level change associated with large earthquakes; and (2) identify anomalous sand and silt beds deposited by tsunamis. A global database shows that diatoms 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. We use case studies from some of these locations to highlight advancements in the field and new capabilities that diatoms have enabled. Examples from the Pacific Northwest of North America illustrate the evolution of quantitative diatom-based reconstructions of earthquake-related land-level change. In Alaska and Japan, diatoms have documented land-level changes throughout the earthquake deformation cycle, including possible preseismic land-level change signals and postseismic deformation. Diatoms helped identify coseismic uplift along the central Chile subduction zone coast, and uplift and subsidence along the Alaska–Aleutian megathrust, expanding our knowledge of the variability of slip in megathrust ruptures. In tsunami studies, diatoms help determine the provenance of anomalous sands and silts found in low-energy coastal stratigraphic sequences. In Japan, allochthonous marine and brackish diatoms within sand deposits signaled repeated marine incursions into a coastal lake, helping identify a possible predecessor to the 2011 Tohoku tsunami. In the Pacific Northwest of North America and Chile, diatoms were used to estimate tsunami run-up beyond the landward limit of tsunami sedimentation. Examples from the North Sea, Thailand, and Japan show how the fragmentation and sorting of diatom valves may provide evidence of high-energy transport during the rapid, turbulent flow of a tsunami. To conclude, we emphasize the importance of studying the modern diatom response to changes in land level and/or tsunami inundation to improve diatom-based records of prehistoric earthquakes and tsunamis.