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The Great Cascadia
Earthquake
of 2xxx
of 2xxx
by Andrew Alden
Cascadia
is America's own tectonic version of Sumatra, where the magnitude 9.3
earthquake and tsunami of 2004 occurred.
Stretching
off the Pacific shore from northern California some 1300 kilometers to the tip
of Vancouver Island, the Cascadia subduction zone appears capable of its own
magnitude 9 earthquake.
What
do we know about its behavior and its history? What would that great Cascadia
earthquake be like?
Subduction
Zone Earthquakes, Cascadia and Elsewhere
Subduction
zones are places where one lithospheric plate
plunges beneath another (see "Subduction in
a Nutshell").
They
create three kinds of earthquakes: those within the upper plate, those within
the lower plate, and those between the plates.
The
first two categories can include large, damaging quakes of magnitude (M) 7,
comparable to the Northridge 1994 and Kobe 1995 events.
They
can damage whole cities and counties. But the third category is what concerns
disaster officials.
These
great subduction events, M 8 and M 9, can release hundreds of times more energy
and damage wide regions inhabited by millions of people. They are what
everybody means by "the Big One."
Earthquakes
get their energy from strain (distortion) built up in rocks from the stress
forces along a fault (see "Earthquakes
in a Nutshell").
Great
subduction events are so large because the fault involved has a very large
surface area on which rocks gather strain.
Knowing
this, we can easily find where the world's M 9 earthquakes happen by locating
the longest subduction zones: southern Mexico and Central America, South America's
Pacific coast, Iran and the Himalaya, western Indonesia, eastern Asia from New
Guinea to Kamchatka, the Tonga Trench, the Aleutian Island chain and Alaska
Peninsula, and Cascadia.
Magnitude-9
quakes differ from smaller ones in two distinct ways: they last longer and they
have more low-frequency energy.
They
don't shake any harder, but the greater length of shaking causes more
destruction. And the low frequencies are more effective at causing landslides,
damaging large structures and exciting water bodies.
Their
power to move water accounts for the fearsome threat of tsunamis, both in the
shaken region and on coastlines near and far (see more on tsunamis).
After
the strain energy is released in great earthquakes, whole coastlines may
subside as the crust relaxes.
Offshore,
the ocean floor may rise. Volcanoes may respond with their own activity.
Low-lying
lands may turn to mush from seismic liquefaction and
widespread landslides may be triggered, sometimes creeping along for years
afterward. These things may leave clues for future geologists.
Cascadia's
Earthquake History
Studies
of past subduction earthquakes are inexact things, based on finding their
geologic signs: sudden changes of elevation that drown coastal forests,
disturbances in ancient tree rings, buried beds of beach sand washed far inland
and so on.
Twenty-five
years of research has determined that Big Ones affect Cascadia, or large parts
of it, every few centuries. Times between events range from 200 to about 1000
years, and the average is around 500 years.
The
most recent Big One is rather well dated, although no one in Cascadia at the
time could write.
It
occurred around 9 p.m. on 26 January 1700. We know this because the tsunami it
generated struck the shores of Japan the next day, where the authorities
recorded the signs and damages.
In
Cascadia, tree rings, oral traditions of the local people and geologic
evidence support this story.
The Coming
Big One
We've
seen enough recent M 9 earthquakes to have a good idea of what the next one
will do to Cascadia: they struck inhabited regions in 1960 (Chile), 1964
(Alaska), 2004 (Sumatra) and 2010 (Chile again).
The
Cascadia Region Earthquake Workgroup (CREW) recently prepared a 24-page booklet, including photos
from historic quakes, to bring the dreadful scenario to life:
· Strong
shaking will last for 4 minutes, killing and injuring thousands.
· A
tsunami up to 10 meters high will wash over the coast within minutes.
· Much
of coastal Route 101 will be impassable due to wave and landslide damage.
· Parts
of the coast will be cut off from inland cities when the roads are buried.
Roads through the Cascades may likewise be blocked.
· For
rescue, first aid, and immediate relief most places will be on their own.
· Utilities
and transportation in the I-5/Highway 99 corridor will be disrupted for months.
· Cities
may have "significant fatalities" as tall buildings collapse.
· Aftershocks
will continue for years, some of them large earthquakes in themselves.
From
Seattle on down, Cascadian governments are preparing for this event. (In this
effort they have much to learn from Japan's Tokai
Earthquake program.)
The
work ahead is enormous and will never be finished, but all of it will count:
public education, setting up tsunami evacuation routes, strengthening buildings
and building codes, conducting drills and more.
The
CREW pamphlet, Cascadia Subduction Zone Earthquakes: A magnitude 9.0 earthquake scenario,
has more.
Andrew Alden
Professional geologist, writer, photographer,
and geological tour guide
Thirty-seven years of experience writing
about geological subjects
Six years as a research guide with U.S. Geological Survey (USGS)
Experience
Andrew Alden is a former writer for
ThoughtCo who contributed hundreds of articles for more than 17 years. Andrew
works as a geologist, writer, editor, and photographer. He has written on
geological subjects since 1981 and participates actively in his field. For
example, Andrew spent six years as a research guide with the U.S. Geological Survey,
leading excursions on both land land and at sea. And since 1992, he has hosted
the earthquakes conference for the online discussion platform, The Well,
which began as a dialogue between the writers and readers of the Whole Earth
Review.
In addition, Andrew is a longtime member of
the member of the Geological Society of America — an international
society that serves members in academia, government, and industry; and
the American Geophysical
Union — a community of earth and space scientists that
advances the power of science to ensure a sustainable future.
Andrew lives in Oakland, California; and
though he writes about the whole planet and beyond, Andrew finds his own
city full of interest too and blogs about its
geology.
Education
Andrew Alden holds a bachelor's
(B.A.) degree in Earth Science from the University of New Hampshire,
College of Engineering and Physical Sciences, in Durham, N.H.
Awards and Publications
Andrew Alden on Earthquakes (The Well Group, Inc.,
2011)
Assessment of River — Floodplain Aquifer Interactions (Environmental
and Engineering Geoscience, 1997)
Andrew
Alden on Hosting (The Well Group, Inc., 1995)
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