QuickTime animation developed by Jenda Johnson to illustrate how sudden release of stored elastic energy (elastic rebound) in a subduction zone causes the leading edge of the over-riding plate to jump seaward and uplift while the near-shore land area subsides. The sudden displacement of the ocean floor generates a tsunami. The tsunami that arrives onshore near the subduction zone is the “local tsunami” that arrives 20 – 30 minutes after the displacement of the ocean floor by the earthqauke. The tsunami that travels into the open ocean will arrive hours later on distant shores. Notice that the near-shore area uplifts as elastic energy is slowly stored by deformation of the plates that are locked by friction along the plate interface. When the earthquake releases the stored energy, the near-shore area suddenly drops by a meter or more. This causes near-shore areas that were near sealevel before the earthquake to drop into the intertidal zone. This “co-seismic subsidence” kills trees in near-shore forests and results in a ghost forest of dead trees in a tidal marsh.
This PDF provides a brief (12 page) introduction to regional plate tectonics, earthquakes, and volcanoes of the Pacific Northwest. The focus is mainly on the subduction zone (coast to Cascade Mountains) because that is where most of the earthquakes and volcanoes occur, and where tsunamis can be generated. This information was gathered and repurposed by Jenda Johnson and Bob Butler from the Pacific Northwest Seismograph Network (www.pnsn.org/), the U.S. Geological Survey (www.usgs.gov), and from two books: Orphan Tsunami of 1700, by Brian Awtater and others, and At Risk: Earthquakes and Tsunamis on the West Coast by John Clague and others.