Coastal erosion and recovery from a Cascadia subduction zone earthquake and tsunami

Tsunamis generated by great earthquakes threaten coastal infrastructure, development, and human life. Earlier work has documented the inland extent and frequency of past tsunamis, but little is known about the magnitude of material eroded during prehistoric tsunamis and how erosion and coastal recovery are recorded in coastal stratigraphy. In this study we use high-resolution ground-penetrating radar (GPR) to image and quantify coastal erosion experienced during a late Holocene (~ 900 cal BP) Cascadia subduction zone earthquake and tsunami along the northern California coast. The GPR profiles illustrate three stratigraphic signatures created during co-seismic subsidence and tsunami erosion and coastal recovery. The first is erosional truncation of the underlying seaward-dipping reflections created by pre-tsunami normal beach progradation. The second is a series of landward-dipping, flat-lying, and channelized reflections marking the filling of erosional topography and coastal reworking of the irregular shoreline following inundation and erosion. The third is an abrupt landward termination of the stratigraphic unit marking coastal straightening and post-tsunami rejuvenation of normal coastal progradation. Erosion from the ~ 900 cal BP earthquake and tsunami extended > 110 m inland of the contemporary shoreline and removed/reworked 225,000 ± 28,000 m3 of sand from a 1.7-km stretch of the coast, far exceeding anything experienced during historical El Niños along the Pacific Coast of North America. This study provides the first quantitative estimate of the amount of coastal erosion from a pre-historic earthquake and tsunami and outlines a strategy for estimating erosion during similar events elsewhere.