THE GREAT CASCADIA EARTHQUAKE OF 2XXX - Cascadia is America's own tectonic version of Sumatra, where the magnitude 9.3 earthquake 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. 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).

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The Great Cascadia

Earthquake 
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)

Member of Geological Society of America (GSA), and American Geophysical Union (AGU)

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)

ThoughtCo and Dotdash

ThoughtCo is a premier reference site focusing on expert-created education content. We are one of the top-10 information sites in the world as rated by comScore, a leading Internet measurement company. Every month, more than 13 million readers seek answers to their questions on ThoughtCo.

For more than 20 years, Dotdash brands have been helping people find answers, solve problems, and get inspired. We are one of the top-20 largest content publishers on the Internet according to comScore, and reach more than 30% of the U.S. population monthly. Our brands collectively have won more than 20 industry awards in the last year alone, and recently Dotdash was named Publisher of the Year by Digiday, a leading industry publication.

https://www.thoughtco.com/great-cascadia-earthquake-1441127

EARTHQUAKE AFTERSHOCKS - The definition that scientists use for an aftershock is any seismic event occurring within one rupture-zone length of a main shock and within the time it takes for seismicity to fall off to what it was before the main shock. Aftershocks aren't restricted to the spot where the main shock occurred, but can strike tens of kilometers away. Aftershocks don't necessarily get smaller as time passes. They get fewer, but sizable ones can happen long after most of the little ones have ended. Aftershocks are good ways to map the underground fault zone that ruptured in the main shock. Aftershocks are also interesting because they're fairly well behaved — meaning that they have a detectable pattern, unlike all other quakes.

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Earthquake Aftershocks

Aftershocks Are Not Afterthoughts

by Andrew Alden

Aftershocks, those who live through major earthquakes often say, are worse than the main shock in their own way.

At least the main shock took them by surprise and was over fairly soon, in less than a minute usually.

But with aftershocks, people are stressed already, dealing with disrupted lives and cities. They expect aftershocks at any minute, day or night.

When a building is damaged by the main shock, aftershocks can take it down — maybe when you're inside cleaning it up.

No wonder Susan Hough, the government seismologist who gets in the news whenever temblors do, calls aftershocks "ghosts of earthquakes past."

The Duration of Aftershocks

I can show you some aftershocks right now: just look at the map of recent earthquakes for the San Simeon area of California.

In any given week, there are aftershocks there from the 2003 San Simeon earthquake.

And east of Barstow you can still see a trickle of aftershocks from the October 1999 Hector Mine earthquake.

Indeed, some scientists argue that aftershocks may last for centuries in places, like continental interiors, where plate motions that build up stresses in the crust are very slow.

This makes intuitive sense, but careful studies using long historical catalogs will need to be done.

The Trouble with Aftershocks

Two things about aftershocks make them troublesome.

First, they aren't restricted to the spot where the main shock occurred, but can strike tens of kilometers away — and, say, if a magnitude 7 quake was centered out beyond the suburbs but one of its magnitude 5 aftershocks happened right underneath City Hall, the littler one might be the worse of the two.

This was the case with the Christchurch, New Zealand earthquake of September 2010 and its large aftershock five months later.

Second, aftershocks don't necessarily get smaller as time passes. They get fewer, but sizable ones can happen long after most of the little ones have ended.

In Southern California, this phenomenon aroused so much concern after the Northridge quake of 17 January 1994 that Hough wrote an op-ed piece for the Los Angeles Times on the subject three full years later.

Scientific Uses of Aftershocks

Aftershocks are scientifically interesting because they are good ways to map the underground fault zone that ruptured in the main shock. (Here's how they look for the cases of Northridge.)

In the case of the 28 September 2004 Parkfield quake, you can see that the first hour of aftershocks alone outlines the ruptured zone quite well.

Aftershocks are also interesting because they're fairly well behaved — meaning that they have a detectable pattern, unlike all other quakes.

The definition that scientists use for an aftershock is any seismic event occurring within one rupture-zone length of a main shock and within the time it takes for seismicity to fall off to what it was before the main shock.

This body of quakes fits three mathematical rules, more or less.

The first is the Gutenberg-Richter relation, which says that as you go down one magnitude unit in size, aftershocks increase in number by about ten times.

The second is called Bath's law, which says that the largest aftershock is, on average, 1.2 magnitude units smaller than the main shock.

And finally, Omori's law states that aftershock frequency decreases by roughly the reciprocal of time after the main shock.

These numbers differ a bit in different active regions depending on their geology, but they're close enough for government work as the saying goes.

So, seismologists can advise the authorities immediately after a large earthquake that a certain area can expect X probabilities of aftershocks of Y sizes for Z period of time.

The U.S. Geological Survey's STEP project produces a daily map of California with the current risk of strong aftershocks for the next 24 hours.

That's as good a forecast as we can make, and probably the best possible given that earthquakes are inherently unpredictable.

Aftershocks in the Quiet Zones

Still to be determined is how much Omori's law varies beyond active tectonic settings. Large earthquakes are rare away from plate boundary zones, but a 2000 paper in Seismological Research Letters by John Ebel showed that aftershocks of these intra-plate earthquakes could last for several centuries.

One of those was the 1663 Charlevoix, Quebec, earthquake; another was the 1356 earthquake in Basel, Switzerland. In the American Midwest, those would be prehistoric events.

In 2009 Seth Stein and Mian Liu argued in Nature that these quiet settings seem to slow everything down, with stress increasing slowly and aftershock sequences lasting longer.

They also noted that where the historical record is short, such as in the United States, it may be a mistake to judge the degree of earthquake hazard from events that are actually aftershocks rather than background seismicity.

This knowledge may not help you cope with your nerves if you live in an aftershock zone. But it does give you some guidelines as to how bad things will be.

And more concretely, it can help engineers judge how probable it is that your new building will be hit by significant aftershocks over the next few years and plan accordingly.

PS: Susan Hough and her colleague Lucy Jones wrote an article on this subject for Eos, the house journal for the American Geophysical Union, in November 1997.

The U.S. Geological Survey scientists closed by saying that "we would like to propose that the phrase 'just an aftershock' be hereafter banned from the English language."

Tell your neighbors.

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)

Member of Geological Society of America (GSA), and American Geophysical Union (AGU)

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)

ThoughtCo and Dotdash

ThoughtCo is a premier reference site focusing on expert-created education content. We are one of the top-10 information sites in the world as rated by comScore, a leading Internet measurement company. Every month, more than 13 million readers seek answers to their questions on ThoughtCo.

For more than 20 years, Dotdash brands have been helping people find answers, solve problems, and get inspired. We are one of the top-20 largest content publishers on the Internet according to comScore, and reach more than 30% of the U.S. population monthly. Our brands collectively have won more than 20 industry awards in the last year alone, and recently Dotdash was named Publisher of the Year by Digiday, a leading industry publication.

https://www.thoughtco.com/aftershocks-are-not-afterthoughts-1438993