A 750-
mile long
subduction fault along the west coast of
North America, stretching from
Vancouver,
BC to
Cape Mendocino,
CA. Called a
subduction "zone" because, unlike the famous
San Andreas Fault or others where the fault is
linear as plates slide past each other, the subduction fault is along a
plane as one
tectonic plate sinks beneath another. In the case of the
Cascadia subduction zone, it's the remnants of the ancient
Farallon Plate subducting underneath the
North American Plate: the Juan de Fuca Plate in the north and the
Gorda Plate in the south. See the excellent
ASCII map at the
Juan de Fuca Plate node.
Diagram of the subduction zone:
/\
__/\__/ \_________
~~~~~~~~~~~~~/
____________/ N. AMER. PLATE
\____
\___
\__
JUAN DE FUCA _ \_
PLATE /|\ \_
| \
| \
Subduction Zone------> \
\
The subduction zone causes a continuous upward
thrust along the coast, forming the
Coast Range mountains. Also, the heat generated at the lower, farther inland parts of the zone causes rising
magma, creating the
volcanoes of the great
Cascade Moutains.
While it was originally thought that the locked zone of the Cascadia Subduction Zone—the plane along which the plates interface—was relatively narrow, thus meaning the anticipated major "megathrust" earthquakes would tend to be smaller than in other subduction zones around the world. However, recent studies1 indicate that the zone extends farther inland than originally thought. Additionally, the zone appears to be completely locked, rather than the plates slowly sliding past each other, resulting in immense build-up of strain. This indicates that the next "big one" quake to be produced by the fault is more likely to be in the magnitude range of ~9.0 rather than ~8.0-8.5 originally thought. The last quake to strike the fault was on January 26, 1700 (as determined by a combination of carbon-dating and Japanese tsunami records), meaning that the next catastrophic earthquake will likely strike the region sometime between right about now and the year 2200.
(1) http://www.oregonlive.com/news/99/05/st050408.html