...........................................................................................................................................
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)
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.
No comments:
Post a Comment