POLLEN IN THE SERVICE OF PALEOSEISMOLOGY ON THE PACIFIC COAST OF CANADA

The Cascadia Subduction Zone on the west coast of the United States and Canada has experienced large earthquakes and associated tsunamis during the late Holocene. Together with geological approaches of stratigraphy and sedimentology, a number of paleobiological techniques have been used by various researchers to help estimate the timing and effects of both great (> magnitude 8) and smaller seismic events. Tree rings, diatoms, and foraminifera from buried peats and other sediments have been used to reconstruct changes in salinities and relative sea-levels, which can thus be used to estimate coseismic subsidence and rebound in coastal marshes. Pollen analysis of surface sediments from modern plant communities of known elevation relative to sea level have also been used to estimate coseismic subsidence.

The last great subduction earthquake in Cascadia occurred on January 26, 1700. This event left evidence of subsidence and a tsunami-generated sand sheet at Tofino, western Vancouver Island. Elevational studies of modern tidal marsh communites were combined with pollen analyses of modern and buried plant horizons. Pollen analysis showed that a high marsh community was suddenly replaced by halophytic low marsh vegetation, indicating sudden subsidence of approximately 60 cm, corresponding well to an estimate based on fossil foraminifera.

Palynological studies on the inner British Columbia coast have also helped to document past earthquake activity. At Mud Bay, Surrey, the Serpentine Fen area has a detailed record of paleoseismic events. Past and current work has produced unmistakeable evidence of seismic shaking, leading to clastic dykes injected into overlying sediments, fossil sand blows onto buried soil surfaces, and pollen evidence of sudden vegetation changes to more marine conditions, indicating subsidence. Studies are continuing on detailed pollen and plant macrofossil analysis of Serpentine Fen localities to help assess the future risk of seismic damage to this populated region.