C E R T E: Arthquakes
C E R T E: Arthquakes
C E R T E: Arthquakes
EARTHQUAKES
EARTHQUAKES
INTRODUCTION
Twenty-six urban areas in all parts of the United States are identified as carrying
significant risk of earthquake:
The Western United States, particularly along the San Andreas Fault in California,
the Cascadia Subduction Zone in western Oregon and Washington, and up the
Alaskan coast
The New Madrid Fault Zone in Missouri
A few pockets on the east coast, including coastal South Carolina and New England
EARTHQUAKE STATISTICS
More than 75 million Americans in 39 states face significant risk from earthquakes.
California’s 17 million people face the highest risk, followed by the residents of
western Washington State.
Four million people are within the destructive reaches of the New Madrid Fault.
Hundreds of tremors are felt each year, particularly in California. Major earthquakes are
rare, however. Five major earthquakes have occurred in the last century in the United
States. They occurred in:
San Francisco, 1906 (700 - 800 lives lost)
Alaska, 1964 (131 lives lost)
San Fernando, California, 1971 (65 lives lost)
Loma Prieta (Northern California), 1989 (66 lives lost)
Northridge (Southern California), 1994 (61 lives lost)
The Richter Scale measures earth movement caused by an earthquake. The Richter
Scale has a logarithmic base, so each increment on the scale is multiplied by a factor of
10.
For example, an earthquake of magnitude 8.6 would not be twice as violent as one of
4.3, but rather would be 10,000 times worse. The 10 fold is in regard to amplitude. The
actual energy released by an earthquake increases 31 times for each whole number
increment.
EARTHQUAKE PREPAREDNESS
Develop a home earthquake plan so that you know what to do during and after an
earthquake.
Conduct earthquake drills with your family or coworkers. Locate safe spots (e.g.,
under a sturdy table), and identify danger zones (e.g., near windows).
Develop a plan for reuniting all family members after an earthquake occurs.
Develop a family communication plan. This includes identifying an out of state
contact, informing that person of the duties and expectations that duty entails.
Keep supplies on hand, including food and water for 3 days, a flashlight with extra
batteries, a portable radio, a fire extinguisher, and tools (see Assembling and Storing
a Disaster Supply Kit in Unit 1).
DURING AN EARTHQUAKE
During earthquakes, most injuries result from people being hit by falling objects and
shattered glass, rather than being hurt in collapsing buildings. Many injuries can be
avoided if people take appropriate steps to prepare.
If outdoors, find a spot away from buildings, trees, streetlights and power lines, and
overpasses. Drop to the ground and stay there until the shaking stops. Injuries can
occur from falling trees, street lights and power lines, or building debris.
If in a vehicle, pull over at a clear location free of hazards and stop. Stay in the
vehicle with seatbelt fastened until the shaking stops. Turn on the radio to get
information regarding the quake and any damage to roadways that may have
occurred.
AFTER AN EARTHQUAKE
Myth: Earthquakes only occur on the West Coast in the United States.
Fact: 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.
Myth: The 1906 San Francisco earthquake was the deadliest ever.
Fact: Though well known, the magnitude 7.8 San Francisco earthquake and
ensuing fire killed 700 - 800 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 to be recorded
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.
Fact: Alaska registers the most earthquakes in a given year, with California
placing second. California, however, has the highest risk and most
damaging earthquakes because of its greater population and extensive
infrastructure. Florida and North Dakota have the fewest earthquakes
each year.
Fact: 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 (2 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!
Fact: 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.
Fact: 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.
Myth: It’s been raining a lot, or very hot--it must be earthquake weather!
Fact: 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. It is also a myth that big
earthquakes always happen at a particular time of day.
Fact: 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. 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.
Fact: In an earthquake, the severity of the shaking can cause manmade and
natural structures and the contents within these to 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 2 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. A soft,
loose soil will shake more intensely than hard rock at the same distance
from the same earthquake. Fires are another major risk during
earthquakes as gas lines may be damaged and particularly hazardous.
Fact: 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!