Plate motions have built the topography that has induced the weather that has brought the fire that has prepared the topography for city-wrecking flows of rock debris. These events, driven by unrelenting tectonic force, often occur at time intervals beyond human experience. For example, there are significant seismic hazards due to plate convergence along the western boundary of the North American plate. There is no record of a thrust earthquake at the Cascadia margin over the last 200 years for which there are reliable historic records. This observation does not mean that the Cascadia margin is seismically inactive.
Earthquakes do occur, however, within the subducting oceanic plate.
When compared with upper plate events, the frequency of the lower plate events is less but with a greater magnitude. Upper plate events all have magnitudes less than 6. At present, the highest rate of seismicity is associated with deformation in the upper plate in the vicinity of Puget Sound. Others have argued that the large volume of sediment being accreted at the plate boundary and the high temperatures associated with subduction of a young oceanic plate may significantly diminish or negate the probability of a large or great thrust earthquake. Paleoseismological evidence, including rapidly buried coastal marshes, tsunami deposits, sands blows, rockfall deposits, and distinctive turbidite horizons in lake sediments and also in the Cascadia trench basin, continue to provide a unique and very important view of the long term seismological history of the Cascadia margin.