Tuesday, December 27, 2011

Cascadia's Fault

I recently read Cascadia's Fault: The Coming Earthquake and Tsunami that Could Devastate North America by Jerry Thompson (Counterpoint, 2012).

Thompson is a documentary filmmaker living outside of Vancouver in British Columbia who's produced and narrated several television documentaries on the Cascadia fault for the CBC.  Thompson's interest in the Cascadia subduction zone was due, in large part, to his home being located in an area that might be directly affected by a large earthquake along that fault zone.

I have to confess that I wanted to like this book, but I'm afraid I didn't.  First a little background...

I don't believe regular readers of this blog need much of an introduction to plate tectonics. The Earth's rigid outer shell, called the lithosphere, is split into a number of tectonic plates which all move relative to one another.  Virtually all of the continental United States and Canada, with the exception of a small sliver of southern California, is sitting on the North American Plate.

Just offshore of the Pacific Northwest (Northern California, Oregon, and Washington) is a small plate of oceanic crust called the Juan de Fuca Plate.

Since I love tangential stories, I'll share the following.  Juan de Fuca was a Greek navigator (Ioánnis Fokás) who sailed for Spain to look for the fabled Strait of Anián, a supposed Northwest Passage across the top of North America, and claimed to have discovered it in 1592.  The English captain, Charles William Barkley named the Juan de Fuca Strait in 1787 in his honor believing that's what Juan de Fuca described from his voyage almost 200 years earlier.

Anyway, here's a closer view of the region.  The north-south purple barbed line just off the coast of the Pacific Northwest is the Cascadia Subduction Zone - the trench down which the Juan de Fuca plate subducts as it pushes eastward, away from newly-forming ocean crust at the Juan de Fuca and Gorda Ridges, and as the North American Plate moves westward away from the distant mid-Atlantic Ridge on the opposite side of the continent.

Here's an oblique-view of the region showing the subduction of the Juan de Fuca Plate down the Cascadia Subduction Zone beneath North America.

Melting of the subducting plate provides magma for the Pacific Northwest chain of volcanoes comprising the Cascades Range.  Mount Lassen, Mount Shasta, Crater Lake, Mount Hood, Mount St Helens, Mount Rainier, and others have all erupted in historic times and will all erupt again (many people living in the shadows of these beautiful mountains are in a state of deep denial about that fact).

Subduction zones are also the locations of some of the largest and most destructive earthquakes on Earth.  So, while the San Andreas Fault is the one in the public's consciousness, the Cascadia Fault is potentially just as deadly (if not more so).

Let me just add that if Thompson, the author of Cascadia's Fault, had given a background like I did above, with drawings of the plates and subduction zone, I would have reviewed his book much higher than I will tomorrow, when I finish this discussion!

1 comment:

  1. Thank you.
    Do you think a 10.0 is possible if San Andreas south brakes then north San Andreas to Cascadia up to Canada setting off volcanoes along the way? Mega Tsunami follows and jets across the Pacific. I am curious about the Puerto Rican Trench and current state of activity. This is where I feel a sense of urgency. Your thoughts on this.