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Earthquake
Facts and Earthquake Fantasy
Earthquakes are sudden rolling or shaking events caused
by movement under the Earth’s surface.
FACT: An earthquake is the ground shaking caused
by a sudden slip on a fault. Stresses in the earth's outer layer
push the sides of the fault together. Stress builds up and the rocks
slips suddenly, releasing energy in waves that travel through the
earth's crust and cause the shaking that we feel during an earthquake.
An earthquake occurs when plates grind and scrape against each other.
In California, for example, there are two plates the Pacific Plate
(which extends from western California to Japan, including much
of the Pacific Ocean floor) and the North American Plate (which
is most of the North American continent and parts of the Atlantic
Ocean). The Pacific Plate grinds northwestward past the North American
Plate along the San Andreas Fault at a rate of about two inches
per year. Parts of the San Andreas Fault system adapt to this movement
by constant "creep" resulting in many tiny shocks and
a few moderate earth tremors. In other parts, strain can build up
for hundreds of years, producing great earthquakes when it finally
releases. Large and small earthquakes can also occur on faults not
previously recognized; recent earthquakes in Alabama and Virginia
are good examples.
Can “Mega Quakes” really happen?
THEORETICALLY, YES. REALISTICALLY, NO. The magnitude
of an earthquake is related to the length of the fault on which
it occurs -- the longer the fault, the larger the earthquake. The
San Andreas Fault is only 800 miles long. To generate an earthquake
of 10.5 magnitude would require the rupture of a fault that is many
times the length of the San Andreas Fault. No fault long enough
to generate a magnitude 10.5 earthquake is known to exist. The largest
earthquake ever recorded was a magnitude 9.5 on May 22, 1960 in
Chile on a fault that is almost 1,000 miles long. The magnitude
scale is open-ended, meaning that science has not put a limit on
how strong an earthquake could be, and scientists can’t rule
out a “Mega Quake” because they’ve only been measuring
earthquakes for 100 years, a blink of an eye in geologic time. However,
scientists agree that “Mega Quakes” of magnitude 10
or more are implausible.
 Earthquakes
only occur on the West Coast.
FICTION: Earthquakes can strike any location at any time.
But history shows they occur in the same general patterns over time,
principally in three large zones of the earth. The world's greatest
earthquake zone, the circum-Pacific seismic belt, is found along
the rim of the Pacific Ocean, where about 81 percent of the world's
largest earthquakes occur. That belt extends from Chile, northward
along the South American coast through Central America, Mexico,
the West Coast of the United States, the southern part of Alaska,
through the Aleutian Islands to Japan, the Philippine Islands, New
Guinea, the island groups of the Southwest Pacific, and to New Zealand.
The second important belt, the Alpide, extends from Java to Sumatra
through the Himalayas, the Mediterranean, and out into the Atlantic.
This belt accounts for about 17 percent of the world's largest earthquakes,
including some of the most destructive. The third prominent belt
follows the submerged mid-Atlantic ridge. The remaining shocks are
scattered in various areas of the world. Earthquakes in these prominent
seismic zones are taken for granted, but damaging shocks occur occasionally
outside these areas. Examples in the United States are New Madrid,
Missouri, and Charleston, South Carolina. Many decades to centuries,
however, usually elapse between such destructive shocks.
The 1906 San Francisco earthquake was the deadliest ever.
FICTION: Though well known, the magnitude 7.8 San Francisco
earthquake and ensuring fire killed 3,000 and razed large sections
of the city. It was the most deadly in U.S. history, but that doesn’t
make it the worst the world has seen, by far. The deadliest earthquake
in recorded history struck Shensi province in China in 1556, killing
about 830,000 people. The 1976 magnitude 7.8 earthquake which struck
Tangshan, China killed somewhere between 250,000 and 800,000 people.
In 2003, the magnitude 6.5 earthquake in Bam, Iran killed more than
40,000 people. The earthquake in Chile on May 22, 1960, is the strongest
in the world with magnitude 9.5, and killed more than 4,000. For
the record, the largest U.S. earthquake occurred on March 28, 1964,
in Alaska. It was a magnitude 9.2 quake and took 131 lives.
California has the most earthquakes in the United States.
PARTIALLY FACT: Alaska registers the most earthquakes in
a given year, with California placing second. California, however,
has the most damaging earthquakes because of its greater population
and extensive infrastructure. Most of Alaska’s large earthquakes
occur in remote locations such as along the Aleutian Island chain.
Florida and North Dakota have the fewest earthquakes each year.
Earthquakes can occur just below the surface or deep below the surface.
FACT: Earthquakes occur in the crust or upper mantle, from the earth's
surface to about 500 miles below the surface. Seismologists use
earthquakes to study the interior of the earth and to pinpoint faults
and geologic structures such as the core-mantle boundary, subduction
zones, and the subsurface extent of the San Andreas Fault. Using
earthquakes and waves from earthquakes, scientist can see all of
the earth’s interior.
The ground can open up during an earthquake.
FICTION: A popular cinematic and literary device is a fault
that opens during an earthquake to swallow up an inconvenient character.
But unfortunately for principled writers, gaping faults exist only
in movies and novels. The ground moves across a fault during an
earthquake, not away from it. If the fault could open, there would
be no friction. Without friction, there would be no earthquake.
Shallow crevasses can form during earthquake induced landslides,
lateral spreads, or other types of ground failures. Faults, however,
do not gape open during an earthquake.
 California
will eventually fall into the ocean.
FICTION: The ocean is not a great hole into which California
can fall, but it is itself land at a somewhat lower elevation with
water above it. It’s absolutely impossible that California
will be swept out to sea. Instead, southwestern California is moving
horizontally northward towards Alaska as it slides past central
and eastern California. The dividing point is the San Andreas fault
system, which extends from the Salton Sea in the south to Cape Mendocino
in the north. This 800 mile long fault is the boundary between the
Pacific Plate and North American Plate. The Pacific Plate is moving
to the northwest with respect to the North American Plate at approximately
46 millimeters (two inches) per year (the rate your fingernails
grow). At this rate, Los Angeles and San Francisco will one day
(about 15 million years from now) be next-door neighbors, and in
an additional 70 million years, Los Angeles residents will find
themselves with an Alaska zip code!
An “Aftershock” can be greater than the initial
earthquake.
PARTIALLY FACT: “Foreshock" and "aftershock"
are relative terms. Foreshocks are earthquakes that precede larger
earthquakes in the same location. Aftershocks are smaller earthquakes
that occur in the same general area during the days to years following
a larger event or "main shock”. So if an “aftershock”
is bigger than its “main shock”, we change the names
and call the first one the foreshock and the large “aftershock”
becomes the main shock. Most aftershocks occur on the same fault
as the main shock, but other faults nearby and extensions of the
main shock fault will also produce aftershocks. Historically,
deep earthquakes are much less likely to be followed by aftershocks
than are shallow earthquakes.
Two major earthquakes occurred on the same day, so they
must be related.
NOT LIKELY: Often, people wonder if an earthquake in Alaska
may have triggered an earthquake in California; or if an earthquake
in Chile is related to an earthquake that occurred a week later
in Mexico. Over long distances, the answer is no. Even the Earth's
rocky crust is not rigid enough to transfer stress efficiently over
thousands of miles. There is evidence to suggest that earthquakes
in one area can trigger seismic activity within a few hundred miles,
including aftershocks clustered near the main shock. There is also
evidence that some major earthquakes manage to trigger seismicity
over much greater distances (thousands of miles), but these triggered
quakes are small and very short lived.
People can cause earthquakes.
PARTIALLY FACT: Earthquakes induced by human activity have
been documented in a few locations in the United States, Japan,
and Canada. The cause was injection of fluids into deep wells for
waste disposal and secondary recovery of oil, and the filling of
large reservoirs for water supplies. Most of these earthquakes were
minor. Deep mining can cause small to moderate quakes and nuclear
testing has caused small earthquakes in the immediate area surrounding
the test site, but other human activities have not been shown to
trigger subsequent earthquakes. Earthquakes are part of a global
tectonic process that generally occurs well beyond the influence
or control of humans. The focus (point of origin) of an earthquake
is typically tens to hundreds of miles underground, and the scale
and force necessary to produce earthquakes are well beyond our daily
lives.
 People
can stop earthquakes.
FICTION: We cannot prevent earthquakes from happening (or
stop them once they’ve started). However, we can significantly
mitigate their effects by characterizing the hazard (e.g., identifying
earthquake faults, unconsolidated sediment likely to amplify earthquake
waves, and unstable land prone to sliding or liquefying during strong
shaking), building safer structures, and preparing in advance by
taking preventative measures and knowing how to respond.
Nuclear explosions can start or stop earthquakes.
FICTION: Scientists agree that even large nuclear explosions
have little effect on seismicity outside the area of the blast itself.
The largest underground thermonuclear tests conducted by the United
States were detonated in Amchitka at the western end of the Aleutian
Islands, and the largest of these was the 5 megaton test code-named
Cannikin that occurred on November 6, 1971 that did not trigger
any earthquakes in the seismically active Aleutian Islands. On January
19, 1968, a thermonuclear test, code-named Faultless, took place
in central Nevada. The code-name turned out to be a poor choice
because a fresh fault rupture some 4,000 feet long was produced.
Seismograph records showed that the seismic waves produced by the
fault movement were much less energetic than those produced directly
by the nuclear explosion. Locally, there were some minor earthquakes
surrounding the blasts that released small amounts of energy. Scientists
looked at the rate of earthquake occurrence in northern California,
not far from the test site, at the times of the tests and found
nothing to connect the testing with earthquakes in the area.
You can prevent large earthquakes by making lots of small
ones, or by "lubricating" the fault with water
FICTION: Seismologists have observed that for every magnitude
6 earthquake there are about 10 of magnitude 5, 100 of magnitude
4, 1,000 of magnitude 3, and so forth as the events get smaller
and smaller. This sounds like a lot of small earthquakes, but there
are never enough small ones to eliminate the occasional large event.
It would take 32 magnitude 5's, 1000 magnitude 4's, and 32,000 magnitude
3's to equal the energy of one magnitude 6 event. So, even though
we always record many more small events than large ones, there are
far too few to eliminate the need for the occasional large earthquake.
As for "lubricating" faults with water or some other substance,
if anything, this would have the opposite effect. Injecting high-
pressure fluids deep into the ground is known to be able to trigger
earthquakes—to cause them to occur sooner than would have
been the case without the injection. This would be a dangerous pursuit
in any populated area, as one might trigger a damaging earthquake.
 We
can predict earthquakes.
FICTION: Earthquake prediction is the holy grail for earthquake
scientists, but there currently is no accepted method to accomplish
the goal of predicting the time, place and magnitude of an impending
quake. Research into earthquake prediction continues. However, the
USGS approach has been to focus on providing long-range forecasts
of the likelihood locations and impacts of damaging earthquakes.
For example, scientists estimate that over the next 30 years the
probability of a major earthquake occurring in the San Francisco
Bay area is 62%, and 60% in Southern California. Scientists are
also able to predict the type of ground motion to expect based on
the geology and the history of earthquake activity of the region.
Engineers and building code developers use these models of site
response to improve the safety of structures, thereby reducing the
ultimate earthquake risk.
Animals can predict earthquakes.
FICTION: Changes in animal behavior cannot be used to predict
earthquakes. Even though there have been documented cases of unusual
animal behavior prior to earthquakes, a reproducible connection
between a specific behavior and the occurrence of an earthquake
has not been made. Because of their finely tuned senses, animals
can often feel the earthquake at its earliest stages before the
humans around it can. This feeds the myth that the animal knew the
earthquake was coming. But animals also change their behavior for
many reasons, and given that an earthquake can shake millions of
people, it is likely that a few of their pets will, by chance, be
acting strangely before an earthquake.
Some people can sense that an earthquake is about to happen.
MAYBE: There is no scientific explanation for the symptoms
some people claim to have preceding an earthquake, and more often
than not there is no earthquake following the symptoms.
It’s been raining a lot, or very hot--it must be
earthquake weather!
FICTION: Many people believe that earthquakes are more
common in certain kinds of weather. In fact, no correlation with
weather has been found. Earthquakes begin many kilometers (miles)
below the region affected by surface weather. People tend to notice
earthquakes that fit the pattern and forget the ones that don't.
Also, every region of the world has a story about earthquake weather,
but the type of weather is whatever they had for their most memorable
earthquake.
 The
Golden Gate Bridge, Seattle Space Needle and other buildings will
all eventually fall during an earthquake.
NOT LIKELY: Architects and engineers are using knowledge
learned from past earthquakes to make roads, bridges, and buildings
safer in the event of major earthquakes. Local officials are also
enacting new building codes to ensure new buildings are built with
earthquake safety in mind. This includes both improving the design
of new buildings and bridges as well as strengthening older units
to incorporate the latest advances in seismic and structural engineering.
Landmark buildings and infrastructure, as well as houses, apartments,
hospitals, schools and other public and private facilities in earthquake
prone areas can be at risk. But the best building codes in the world
do nothing for buildings built before that code was enacted. While
the codes have been updated, the older buildings are still in place.
Fixing problems in older buildings—also known as retrofitting—is
the responsibility of the building's owner.
Earthquakes don’t kill people, buildings and their
contents do.
FACT: The greatest risk in an earthquake is the severity
of the shaking it causes to manmade and natural structures and the
contents within these that may fail or fall and injure or kill people.
There have been large earthquakes with very little damage because
they caused little shaking and/or buildings were built to withstand
that shaking. In other cases, smaller earthquakes have caused great
shaking and/or buildings collapsed that were never designed or built
to survive shaking. Much depends on two variables: geology and engineering.
From place to place, there are great differences in the geology
at and below the ground surface. Different kinds of geology will
do different things in earthquakes. For example, shaking at a site
with soft sediments can last 3 times as long as shaking at a stable
bedrock site such as one composed of granite. Local soil conditions
also play a role, as certain soils greatly amplify the shaking in
an earthquake. Seismic waves travel at different speeds in different
types of rocks. Passing from rock to soil, the waves slow down but
get bigger. A soft, loose soil will shake more intensely than hard
rock at the same distance from the same earthquake. The looser and
thicker the soil is, the greater the energy movement will be. Fires
are another major risk during earthquakes as gas lines may be damaged
and particularly hazardous.
During an earthquake you should head for the doorway.
FICTION: That’s outdated advice. In past earthquakes
in unreinforced masonry structures and adobe homes, the door frame
may have been the only thing left standing in the aftermath of an
earthquake. Hence, it was thought that safety could be found by
standing in doorways. In modern homes doorways are no stronger than
any other parts of the house and usually have doors that will swing
and can injure you. YOU ARE SAFER PRACTICING THE “DROP, COVER,
AND HOLD” maneuver under a sturdy piece of furniture like
a strong desk or table. If indoors, stay there. Drop to the floor,
make yourself small and get under a desk or table or stand in a
corner. If outdoors, get into an open area away from trees, buildings,
walls and power lines. If in a high-rise building, stay away from
windows and outside walls, stay out of elevators, and get under
a table. If driving, pull over to the side of the road and stop.
Avoid overpasses and power lines. Stay inside your car until the
shaking is over. If in a crowded public place, do not rush for the
doors. Crouch and cover your head and neck with your hands and arms.
You should practice the “DROP, COVER AND HOLD” method
at work and at home at least twice a year.
 Everyone
will panic during the Big One.
FICTION: A common belief is that people always panic and
run around madly during and after earthquakes, creating more danger
for themselves and others. Actually, research shows that people
usually take protective actions and help others both during and
after the shaking. Most people don't get too shaken up about being
shaken up!
You can’t plan ahead for an earthquake.
FICTION: There are plenty of things you can do right now
to prepare if you live in an earthquake-prone area.
1. Make sure each member of your family knows what to do no matter
where they are when earthquakes occur:
o Establish a meeting place where you can all reunite afterward.
o Find out about earthquake plans developed by children's school
or day care.
o Remember transportation may be disrupted, so keep some emergency
supplies--food, liquids, and comfortable shoes, for example--at
work.
2. KNOW where your gas, electric and water main shutoffs are and
how to turn them off if there is a leak or electrical short. Make
sure older members of the family can shut off utilities.
3. LOCATE your nearest fire and police stations and emergency medical
facility.
4. TALK to your neighbors--how could they help you, or you them
after an earthquake?
5. TAKE Red Cross First Aid and CPR Training Course.
6. MAKE your disaster supply kit. Beyond the usual flashlights,
batteries and radios, include a first-aid kit; work gloves; sturdy
shoes or boots; a week’s supply of any presciption medications
you or your family might need; credit card and cash; personal identifcation;
extra set of keys; matches in a waterproof container; map of your
area; phone numbers of family and other important people (doctors,
veterinarians, etc.); copies of insurance policies and other important
documents; special needs equipment (diapers, baby formula, hearing
aid batteries; spare eyeglasses, etc.); three gallons of water per
person; three-day supply of food per person; hand tools; a portable
ABC fire extinguisher; sanitation supplies for you and your family;
entertainment (toys, books, coloring books and crayons, playing
cards)
7: BOLT bookcases, china cabinets, tall furniture, file cabinets,
etc. to wall studs. Brace or anchor heavy electronics and other
heavy items. Secure items that might fall. Move heavy or fragile
items to lower shelves. Fasten drawers and cabinet doors with latches
or locks. Brace overhead light fixtures. Strap your water heater
to wall studs and bolt down any gas appliances. Look for other non-structural
steps you can take in your home and workplace to reduce your chances
for injury and loss.
8. ASK AN ENGINEER about the seismic safety of your home and/or
business. It’s well known that unreinforced masonry structures
can fail quickly during earthquakes. An inspection by a structural
engineer now can help you decide if retrofitting will help your
property withstand shaking.
USGS is a worldwide leader in earthquake hazard research,
information and mitigation.
FACT: The U.S. Geological Survey performs the following
functions related to earthquake hazard mitigation:
Receives, analyzes, maintains, and distributes data on earthquake
activity worldwide. Seismograph stations located around the country
and the world are linked in real-time to USGS offices and to analysis
centers run by our partner institutions that continuously monitor,
detect and locate earthquake activity.
Provides rapid notification of earthquake events to civil defense
and government officials in the affected area, and to the public
through the Internet and news media.
Produces regional assessments of earthquake hazards in conjunction
with State and local governments. This information is used by: local
planners and building officials in setting appropriate building
and retrofitting standards, government and civil defense officials
in planning for disaster recovery; professionals conducting detailed
site assessments; and researchers engaged in engineering and seismologic
research.
Engages in basic research to learn more about the nature of earthquake
activity.
Provides education on earthquake hazards and safety to the public
by publishing and distributing literature, and through a variety
of other outreach efforts.
 But
more can be done.
ABSOLUTELY: Earthquakes pose a significant risk to 75 million
Americans in 39 states. The USGS is the only Federal agency with
responsibility for recording and reporting earthquake activity nationwide.
Citizens, emergency responders and engineers rely on USGS for accurate
and timely information on where an earthquake occurred, how much
the ground shook in different locations and what is the likelihood
of future significant ground shaking. USGS is working to improve
its earthquake monitoring and reporting capabilities through the
Advanced National Seismic System (ANSS). In the past three years,
USGS has installed about 300 new earthquake- monitoring instruments
in the San Francisco, Seattle, Salt Lake City, Anchorage, Reno,
Las Vegas and Memphis areas. Full implementation of the ANSS will
result in nearly 7,000 new high-tech instruments in the ground or
in buildings. And once in place, the ANSS will provide emergency
personnel with near real-time information on the intensity and distribution
of ground shaking that can be used to guide emergency response efforts.
Similarly, information on building shaking will equip engineers
with the data they need to improve building designs in the future.
There’s great information available online about
preparing for an earthquake
FACT! There are literally hundreds of websites that focus
on earthquake safety. For more factual information on earthquakes,
earthquake myths and earthquake preparedness, see the Earthquakes,
“Mega-quakes,” and the Movies sheet that accompanies
this sheet and visit:
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