GEOLOGICAL EVIDENCE FOR TSUNAMIS PRIOR TO THE 1700 CASCADIA EARTHQUAKE FROM VANCOUVER ISLAND, CANADA

Geological evidence for earthquakes and tsunamis are well documented along the southwest coast of Canada. Since the late 1990s there have been few new paleo-earthquake studies in this area in contrast to the west coast of the United States. New geological studies are needed to evaluate the timing, recurrence intervals, and rupture areas of megathrust earthquakes. Tsunami deposits associated with the 1964 Alaska, the 1700 Cascadia and older earthquakes from Tofino, Ucluelet and Port Alberni along the west coast of Vancouver Island are recorded. However, the ages of older tsunami deposits were not well constrained. In this study, we re-examined tsunami deposits at several sites to determine the timing of the older events.

Study efforts in 2015 and 2016 examined sand layers interbedded within peat and mud deposits at widely separated sites on Vancouver Island. We relied upon a combination of ²¹⁰Pb, ¹³⁷Cs and ¹⁴C dating to constrain the depositional ages of sand layers at Tofino, Ucluelet and Port Alberni. Plant microfossils and insects obtained directly above and below tsunami layer were used for radiocarbon dating. Radiocarbon ages indicate that the sand layer prior to the 1700 tsunami sediments was deposited between 550-300 calendar years before present.