The preservation potential of coastal coseismic and tsunami evidence observed following the 2012 Mw 7.8 Haida Gwaii thrust earthquake

We describe near-field coseismic and tsunami evidence collected following the 28 October 2012 M (sub w) 7.8 thrust earthquake that occurred offshore western Haida Gwaii, British Columbia, and discuss factors that influence its extent and preservation potential. Observations indicate minor geomorphic and sedimentological impacts on the rugged and unpopulated west coast of the islands, despite widespread coastal coseismic subsidence ( approximately 0.5 m), triggered landslides, and tsunami waves exceeding 3 m in runup (maximum 13 m) along approximately 230 km of coastline. Evidence left by the tsunami was minimal, likely because it occurred during a low tide that restricted its onshore reach, its flow depth, and its capacity to entrain and transport significant amounts of clastic sediment, sources of which are minimal or absent on the dominantly steep, rocky coastline. It is unlikely that subaerial evidence of coseismic subsidence and tsunami inundation will be recognizably recorded in the coastal stratigraphy of western Haida Gwaii, due to the relatively small magnitude of subsidence, a lack of suitable coastal environments such as tidal marshes to record paleoelevation differences, sedimentation rates that are too low to bury a paleoseismic or paleotsunami record, and long-term relative sea level fall leading to erosion and bioturbation. A higher preservation potential is likely for tsunami deposits in coastal lakes, ponds, and bogs, as well as coseismic slope failure deposits offshore, in lakes and in sheltered fjords. Our findings imply that large tsunamigenic earthquakes are likely undersampled in the paleoseismic record of Haida Gwaii and of other plate margins with similar characteristics.