San Francisco after the earthquake
courtesy of University of Nebraska Press

fence offset by 8.5 ft

(from UC Berkeley Earthquake Engineering Research Center)

Giant mega-thrust earthquakes are the outcome of a catastrophic sliding of the Juan de Fuca Plate under the North American Plate.

In the time period between mega-thrust earthquakes, the Juan de Fuca plate is trying to slide beneath the North American plate.

This causes the rocks along the plate edges to be compressed, deformed, and uplifted. Knowing where and how fast the rocks are being deformed enables the estimation of the size of the fault that may slip in the next mega-thrust earthquake.
How do scientists measure tectonic plate movements?

Currently the Juan de Fuca and North American plates are stuck together in the 'locked zone' (1). The seaward edge of the continent is dragged down as the plate subducts, causing an uplifted area on the landward side (2).

Over time the stress builds up and the locked zone 'ruptures' or becomes unstuck, releasing 10 - 20 meters of slip and causing a 'great earthquake' (3). The abrupt uplift of the seaward edge generates a tsunami, and a collapse or subsidence of the uplifted area (4).
The last known 'great' earthquake in the Cascadia Subduction Zone occurred in January 1700.

When the land subsides in a coastal region (1), freshwater lowlands fall below sea level and are flooded with sea water which kills the vegetation (2). The drowned trees are left standing.

As tidal sediment and vegetation accumulates, rebuilding the the land surface, new marshes are reestablished, yet the drowned trees remain (3).