The largest earthquakes in the world occur at subduction zones.  A subduction zone is where one continental plate is being pushed underneath another.  Only a subduction zone can generate an earthquake greater than magnitude 8.5 (a “great earthquake”)

The Cascadia Subduction Zone “megathrust fault” is one of these.  It is about 600 miles long, stretching from northern Vancouver Island to Cape Mendocino, California.  In this case the Juan de Fuca tectonic plate is being pushed underneath the North American Plate.

Typical of subduction zones, there is an arc of volcanoes about 100 miles from the plate boundary. These volcanoes occur because subducted volatiles (water vapor and Carbon Dioxide) build pressure within the mantle, and being less dense, eventually make their way to the surface within magma, often violently.  Such explosive volcanoes as St. Helens, Krakatoa, and Vesuvius are all subduction volcanoes.

The reason that subduction zone earthquakes can be more violent and powerful than slip-fault quakes is the mechanism is quite different than a sliding motion.  Look at the above image.  The lithosphere on the right can be bent downwards with the lithosphere that is sliding under it.  Eventually when the system builds enough tension, it can slip and the lithosphere at the right can suddenly spring upwards.

See the next three images for details on how that happens

Scientists had long believed that the Cascadia Fault did not generate large earthquakes, because in all recorded history (an entire 140 years for the Pacific Northwest), it never had.  Turns out that many of these geologists were not thinking in geological time periods.

Below, a map of the “Ring of Fire”, a boundary of volcanism and earthquakes roughly at the boundaries of the Pacific tectonic plate.

The argument geologists were making was that because there was no evidence of great earthquakes in the pacific northwest, this particular subduction zone must be sliding gently without locking up.  Other geologists were not convinced, because the Cascadia subduction zone looked similar to those of Alaska and Chile.

Brian Atwater of the U.S. Geological Survey, at the University of Washington in Seattle was one of the skeptics.  He learned that the North American Plate was rising 3 millimeters per year with respect to the Pacific Plate (just as diagrammed above in the middle picture).  This equates to 3 meters lift every 1,000 years.

If the fault had snapped at some point in the past, he figured, there would be evidence of land that was suddenly submerged when the humped up land snapped back.  He dug samples out of Neah Bay, and made an awesome discovery.  He found a layer of Seaside Arrowgrass, buried in sand and peat.  The grass had been submerged and covered with sand so suddenly that the grass didn’t even decompose!  This finally provided hard evidence for a great earthquake and a mighty tsunami.

The following year, Atwater returned with fellow geologist David Yamaguchi, and they discovered “ghost forests“, entire stands of trees which have been suddenly killed by salt water.  These trees were once above sea level, but when the North American Plate snapped back down, they are now below sea-level.

Below, the Neskowin Ghost Forest offshore of Northern Oregon.

Clearly the land had dropped into the ocean in the not too distant past, killing these trees.  Tree ring dating of the western red cedar refined the kill date to 1699-1700, shortly before written records began in the Northwest.  The Japanese however, kept records.  In 1700, on the eastern seaboard of Japan, a 16 foot high tsunami arrived, but it was an “orphan tsunami“, because it came without an associated earthquake.  This was the smoking gun. Cascadia is a nasty fault that *does* generate great earthquakes and major tsunamis.

Below is a dig showing how the tsunami of 1700 brought ocean sand inland and buried what was then the topsoil.

In fact the Cascadia fault has a lot in common with the faults that produced the Sumatra quake in 2004 and the Japan quake in 2011 that brought us the Fukushima disaster.  More recent analysis reveals that the fault has slipped 41 times in the last 10,000 years.  Nineteen of those were massive end-to-end ruptures of the fault.

Why no earthquakes since the one in 1700?  Because the plates are locked and building tension.  Odds are about 37% for a major quake within 50 years, with a 10% chance of a massive end-to-end rupture.  We might have new “ghost forests” before too long…

Now let’s look at a different map of the “Ring of Fire”.

Finally, check out this video!  Very informative…

MegaQuake Could Hit North America – BBC (Full Documentary)
https://www.youtube.com/watch?v=vEgLjgnv_3c

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