HOLOCENEEARTHQUAKERECORDS FROM THECASCADIASUBDUCTIONZONE ANDNORTHERNSANANDREASFAULTBASED ONPRECISEDATING OFOFFSHORETURBIDITES

The detailed radiocarbon agevs.calibrated (cal) age studies of tree rings reported in this Calibration Issue provide a unique data set for precise14C age calibration of materials formed in isotopic equilibrium with atmospheric CO2. The situation is more complex for organisms formed in other reservoirs, such as lakes and oceans. Here the initial specific14C activity may differ from that of the contemporaneous atmosphere. The measured remaining14C activity of samples formed in such reservoirs not only reflects14C decay (related to sample age) but also the reservoir14C activity. As the measured sample14C activity figures into the calculation of a conventional14C age (Stuiver & Polach 1977), apparent14C age differences occur when contemporaneously grown samples of different reservoirs are dated.

Intertidal mud has buried extensive, well-vegetated lowlands in westernmost Washington at least six times in the past 7000 years. Each burial was probably occasioned by rapid tectonic subsidence in the range of 0.5 to 2.0 meters. Anomalous sheets of sand atop at least three of the buried lowlands suggest that tsunamis resulted from the same events that caused the subsidence. These events may have been great earthquakes from the subduction zone between the Juan de Fuca and North America plates.