An Earthquake is a shaking of
the ground caused by the sudden breaking and shifting of large sections of
Earth's rocky outer shell. Earthquakes are among the most powerful events
on earth, and their results can be terrifying. A severe earthquake may
release energy 10,000 times as great as that of the first atomic bomb.
Rock movements during an earthquake can make rivers change their course.
Earthquakes can trigger landslides that cause great damage and loss of
life. Large earthquakes beneath the ocean can create a series of huge,
destructive waves called tsunamis (tsoo NAH meez)that flood coasts for
Earthquakes almost never kill people directly. Instead, many deaths and
injuries result from falling objects and the collapse of buildings,
bridges, and other structures. Fire resulting from broken gas or power
lines is another major danger during a quake. Spills of hazardous
chemicals are also a concern during an earthquake.
The force of an earthquake
depends on how much rock breaks and how far it shifts. Powerful
earthquakes can shake firm ground violently for great distances. During
minor earthquakes, the vibration may be no greater than the vibration
caused by a passing truck.
On average, a powerful
earthquake occurs less than once every two years. At least 40 moderate
earthquakes cause damage somewhere in the world each year. Scientists
estimate that more than 8,000 minor earthquakes occur each day without
causing any damage. Of those, only about 1,100 are strong enough to be
This article discusses
Earthquake (How an earthquake begins) (How an earthquake spreads) (Damage
by earthquakes) (Where and why earthquakes occur) (Studying earthquakes).
Earthquake of June 16,
1964, Niigata, Japan. The magnitude 7.4 earthquake killed 26 and destroyed
3,018 houses and moderately or severely damaged 9,750 in Niigata
prefecture. Liquefaction-Differential Settlements Aerial view of leaning
apartment houses in Niigata produced by soil liquefaction and the behavior
of poor foundations. Most of the damage was caused by cracking and unequal
settlement of the ground such as is shown here. About 1/3 of the city
subsided by as much as 2 meters as a result of sand compaction. Photo
Credit: National Geophysical Data Center
How an earthquake begins
Most earthquakes occur
along a fault -- a fracture in Earth's rocky outer shell where sections of
rock repeatedly slide past each other. Faults occur in weak areas of
Earth's rock. Most faults lie beneath the surface of Earth, but some, like
the San Andreas Fault in California, are visible on the surface. Stresses
in Earth cause large blocks of rock along a fault to strain, or bend. When
the stress on the rock becomes great enough, the rock breaks and snaps
into a new position, causing the shaking of an earthquake.
Earthquakes usually begin
deep in the ground. The point in Earth where the rocks first break is
called the focus, also known as the hypocenter, of the quake. The focus of
most earthquakes lies less than 45 miles (72 kilometers) beneath the
surface, though the deepest known focuses have been nearly 450 miles (700
kilometers) below the surface. The point on the surface of Earth directly
above the focus is known as the epicenter of the quake. The strongest
shaking is usually felt near the epicenter.
From the focus, the break
travels like a spreading crack along the fault. The speed at which the
fracture spreads depends on the type of rock. It may average about 2 miles
(3.2 kilometers) per second in granite or other strong rock. At that rate,
a fracture may spread more than 350 miles (560 kilometers) in one
direction in less than three minutes. As the fracture extends along the
fault, blocks of rock on one side of the fault may drop down below the
rock on the other side, move up and over the other side, or slide forward
past the other.
At 4:31 am local time
(12:31 GMT) on Monday, January 17, 1994, a magnitude 6.8 earthquake twenty
miles west northwest of downtown Los Angeles awoke nearly everyone in
southern California. Damage was most extensive in the San Fernando Valley,
the Simi Valley, and in the northern part of the Los Angeles Basin. A view
of the parking structure on the campus of California State University. The
bowed columns are of reinforced concrete. The structure has precast
moment- resisting-concrete frames on the exterior and a precast concrete
interior designed for vertical loads. The inside of the structure failed,
and with each aftershock the outside collapsed slowly toward the inside
until finally the west side failed totally. The reinforced concrete
columns were extremely bent. Photo Credit: M. Celebi, U.S. Geological
North San Fernando Valley
One of the most spectacular effects of the earthquake was the collapse of
several freeway overpasses. Pictured here is the collapse at the Antelope
Valley (SR14) and Golden State Freeway (I-5) interchange. Two sections of
highway fell in this earthquake, and there were displacements of a number
of inches between some of the span sections of the structures that
remained standing. I-5 is the primary traffic artery between northern and
southern California. Sections of this interchange also collapsed in the
San Fernando earthquake of 1971, while it was under construction. It was
later rebuilt using the same specifications. A policeman was killed when
his motorcycle tumbled off the edge of the freeway. Photo Credit: J.
Dewey, U.S. Geological Survey
about the Los Angeles Northridge earthquake of 1994, showing extensive
aftermath damage, including fires, highway damage, and collapse of the
Northridge Meadows apartment
How an earthquake spreads
When an earthquake occurs,
the violent breaking of rock releases energy that travels through Earth in
the form of vibrations called seismic waves. Seismic waves move out from
the focus of an earthquake in all directions. As the waves travel away
from the focus, they grow gradually weaker. For this reason, the ground
generally shakes less farther away from the focus.
There are two chief kinds
of seismic waves: (1) body waves and (2) surface waves. Body waves, the
fastest seismic waves, move through Earth. Slower surface waves travel
along the surface of Earth.
Body waves tend to cause
the most earthquake damage. There are two kinds of body waves: (1)
compressional waves and (2) shear waves. As the waves pass through Earth,
they cause particles of rock to move in different ways. Compressional
waves push and pull the rock. They cause buildings and other structures to
contract and expand. Shear waves make rocks move from side to side, and
buildings shake. Compressional waves can travel through solids, liquids,
or gases, but shear waves can pass only through solids.
Compressional waves are the
fastest seismic waves, and they arrive first at a distant point. For this
reason, compressional waves are also called primary (P) waves. Shear
waves, which travel slower and arrive later, are called secondary (S)
Body waves travel faster
deep within Earth than near the surface. For example, at depths of less
than 16 miles (25 kilometers), compressional waves travel at about 4.2
miles (6.8 kilometers) per second, and shear waves travel at 2.4 miles
(3.8 kilometers) per second. At a depth of 620 miles (1,000 kilometers),
the waves travel more than 11/2 times that speed.
Surface waves are long,
slow waves. They produce what people feel as slow rocking sensations and
cause little or no damage to buildings.
There are two kinds of
surface waves: (1) Love waves and (2) Rayleigh waves. Love waves travel
through Earth's surface horizontally and move the ground from side to
side. Rayleigh waves make the surface of Earth roll like waves on the
ocean. Typical Love waves travel at about 23/4 miles (4.4 kilometers) per
second, and Rayleigh waves, the slowest of the seismic waves, move at
about 21/4 miles (3.7 kilometers) per second. The two types of waves were
named for two British physicists, Augustus E. H. Love and Lord Rayleigh,
who mathematically predicted the existence of the waves in 1911 and 1885,
A destructive earthquake of
magnitude Mw = 6.8 hit the region of Boumerdes and Algiers (Algeria) on
May 21, 2003. This is among the strongest seismic events of the
mediterranean region and the most important event in the capital Algiers
since 1716. It caused a widespread damage in the epicentral region,
claimed 2271 human lives, injured 10000, about 20000 housing units
affected and left about 160000 homeless. The main shock was felt about 250
km far from the epicenter and triggered sea waves of 1-3 m in amplitude in
Balearic islands (Spain). Based on field observations and press report an
intensity IX (MSK scale) is attributed to the epicentral area. The main
shock was followed by many aftershocks among them several are of magnitude
greater than 5.0, which added panic to inhabitants. The main shock
triggered ground deformation, particularly liquefaction whose features are
in different forms and sizes and caused damage and collapse of roads. The
focal mechanism determined by worldwide institutions yield a pure reverse
faulting with a compressional axis striking NE-SW. The epicenter is
located offshore about 7 km from the Boumerdes-Dellys coast. Field
observations show 0.7 m of coseismic uplift of shoreline between Boudouaou
and Dellys. This uplift is about a half of the extracted coseismic slip
from the seismic moment. On the other hand there is no clear surface break
onshore, confirming hence, that the causative active fault is offshore.
However, the rupture may propagate onshore to the SE near the Boudouaou
region where ground cracks showing reverse faulting are observed a long a
corridor of about 1 km wide. These fissures may correspond to a diffuse
coseismic deformation.Pancake collapse in Kahouat-Chergui, Algeria. Photo
Credit: Djillali Benour, University of Bab Ezzour, Algeria.
Damage by earthquakes
How earthquakes cause
Earthquakes can damage
buildings, bridges, dams, and other structures, as well as many natural
features. Near a fault, both the shifting of large blocks of Earth's
crust, called fault slippage, and the shaking of the ground due to seismic
waves cause destruction. Away from the fault, shaking produces most of the
damage. Undersea earthquakes may cause huge tsunamis that swamp coastal
areas. Other hazards during earthquakes include rockfalls, ground
settling, and falling trees or tree branches.
The rock on either side of
a fault may shift only slightly during an earthquake or may move several
feet or meters. In some cases, only the rock deep in the ground shifts,
and no movement occurs at Earth's surface. In an extremely large
earthquake, the ground may suddenly heave 20 feet (6 meters) or more. Any
structure that spans a fault may be wrenched apart. The shifting blocks of
earth may also loosen the soil and rocks along a slope and trigger a
landslide. In addition, fault slippage may break down the banks of rivers,
lakes, and other bodies of water, causing flooding.
Ground shaking causes
structures to sway from side to side, bounce up and down, and move in
other violent ways. Buildings may slide off their foundations, collapse,
or be shaken apart.
In areas with soft, wet
soils, a process called liquefaction may intensify earthquake damage.
Liquefaction occurs when strong ground shaking causes wet soils to behave
temporarily like liquids rather than solids. Anything on top of liquefied
soil may sink into the soft ground. The liquefied soil may also flow
toward lower ground, burying anything in its path.
An earthquake on the ocean
floor can give a tremendous push to surrounding seawater and create one or
more large, destructive waves called tsunamis, also known as seismic sea
waves. Some people call tsunamis tidal waves, but scientists think the
term is misleading because the waves are not caused by the tide. Tsunamis
may build to heights of more than 100 feet (30 meters) when they reach
shallow water near shore. In the open ocean, tsunamis typically move at
speeds of 500 to 600 miles (800 to 970 kilometers) per hour. They can
travel great distances while diminishing little in size and can flood
coastal areas thousands of miles or kilometers from their source.
Structures collapse during
a quake when they are too weak or rigid to resist strong, rocking forces.
In addition, tall buildings may vibrate wildly during an earthquake and
knock into each other. Picture San Francisco earthquake of 1906 A major
cause of death and property damage in earthquakes is fire. Fires may start
if a quake ruptures gas or power lines. The 1906 San Francisco earthquake
ranks as one of the worst disasters in United States history because of a
fire that raged for three days after the quake.
Other hazards during an
earthquake include spills of toxic chemicals and falling objects, such as
tree limbs, bricks, and glass. Sewage lines may break, and sewage may seep
into water supplies. Drinking of such impure water may cause cholera,
typhoid, dysentery, and other serious diseases.
Loss of power,
communication, and transportation after an earthquake may hamper rescue
teams and ambulances, increasing deaths and injuries. In addition,
businesses and government offices may lose records and supplies, slowing
recovery from the disaster.
How do scientists measure earthquakes?
There are two ways in which scientists quantify the size of earthquakes:
magnitude and intensity.
Magnitude is a measure of the amount of energy released during an
earthquake, and you've probably heard news reports about earthquake
magnitudes measured using the Richter scale. Something like, "A
magnitude 7.3 earthquake struck Japan today. Details at ten." Did you
ever wonder why, if it's that important, they just don't tell you right
The Richter scale was invented, logically enough, in the 1930s by Dr.
Charles Richter, a seismologist at the California Institute of Technology.
It is a measure of the largest seismic wave recorded on a particular kind
of seismograph located 100 kilometers (about 62 miles) from the epicenter
of the earthquake.
Think of a seismograph as a kind of sensitive pendulum that records the
shaking of the Earth. The output of a seismograph is known as a
seismogram. In the early days, seismograms were produced using ink pens on
paper or beams of light on photographic paper, but now it's most often
done digitally using computers. The seismograph that Dr. Richter used
amplified movements by a factor of 3000, so the waves on the seismograms
were much bigger than those that actually occurred in the Earth. The
epicenter of an earthquake is the point on the Earth's surface directly
above the source, or focus, of the movement that causes the quake.
Dr. Richter studied records from many earthquakes in southern
California, and realized that some earthquakes made very small waves
whereas others produced large waves. So, to make it easier to compare the
sizes of the waves he recorded, Richter used the logarithms of the wave
heights on seismograms measured in microns (1/1,000,000th of a meter, or
1/1000th of a millimeter). Remember, you have to be using a particular
kind of seismograph located 100 km from the epicenter when you make the
measurement; otherwise, all sorts of complicated calculations have to be
made. That's why seismologists spend so many years in college!
A wave one millimeter (1000 microns) high on a seismogram would have a
magnitude of 3 because 1000 is ten raised to the third power. In contrast,
a wave ten millimeters high would have a magnitude of 4. For reasons that
we won't go into, a factor of 10 change in the wave height corresponds to
a factor of 32 change in the amount of energy released during the
earthquake. In other words, a magnitude 7 earthquake would produce
seismogram waves 10 x 10 = 100 times as high and release energy 32 x 32 =
1024 times as great as a magnitude 5 earthquake.
The Richter scale is open-ended, meaning there is no limit to how small
or large an earthquake might be. Due to the nature of logarithms, it is
even possible to have earthquakes with negative magnitudes, although they
are so small that humans would never feel them. At the other end of the
spectrum, there should never be an earthquake much above magnitude 9 on
the Earth simply because it would require a fault larger than any on the
planet. The largest earthquake ever recorded on Earth was a magnitude 9.5
that occurred in Chile in 1960, followed in size by the 1964 Good Friday
earthquake in Alaska (magnitude 9.2), a magnitude 9.1 earthquake in Alaska
during 1957, and a magnitude 9.0 earthquake in Russia during 1952. Two
large earthquakes, one a magnitude 9.0 and one a magnitude 8.2, occurred
on Dec. 26, 2004 and March 28, 2005, respectively, along the same fault
zone off the coast of Sumatra, Indonesia.
The list of really large earthquakes in the previous paragraph brings
up another interesting point. Five earthquakes of magnitude 9 or above
have been recorded during the past 45 years, which averages out to one
every decade. It turns out that earthquake occurrences seem to follow what
is called a power-law distribution, meaning that if there is on average on
magnitude 9 earthquake every ten years somewhere in the world, then on
average there should be one magnitude 8 earthquake every year, 10
magnitude 7 earthquakes every year, and 100 magnitude 6 earthquakes every
year. So, if someone "predicts" that a magnitude 6 earthquake
will occur somewhere in the world during the next week, don't be too
impressed if it happens because random probability tells us that there
should be a magnitude 6 earthquake somewhere in the world every 365/100 =
3.65 days! In reality, things are a little more complicated. But, you get
What did people do before the Richter scale was invented? To some
degree, one of the same things that we do today. They observed the
intensity or effects of an earthquake at different locations. Whereas the
magnitude of an earthquake is a single number regardless of where it's
felt, intensity will vary from place to place. In general, the intensity
will be much greater near the epicenter than at large distances from the
epicenter. This decrease in intensity with distance is known as
attenuation. Imagine it this way: If I drop a rock into a pool of water,
the difference between magnitude and intensity is similar to the
difference between the height of the splash exactly where I drop the rock
and the height of the waves all over the pool. Earthquake intensity is
most often measured using the modified Mercalli scale, which was invented
by the Italian geologist Giuseppi Mercalli in 1902 and uses Roman numerals
from I to XII. In the United States, we use the modified Mercalli scale,
which was adjusted to account for differences in buildings between Italy
and southern California. An earthquake intensity of I is generally not
felt, and an intensity of XII represents total destruction of buildings.
Some kinds of geologic deposits, most notably water saturated muds,
amplify seismic waves and may produce intensities much greater than those
for nearby areas underlain by bedrock. Thus, after an earthquake
seismologists can interview people and make maps showing the intensity of
an earthquake in different areas to better understand the influence of
rock or soil type on seismic waves.
the Richter and Modified Mercalli scales
Not felt except by a very few under especially favourable
conditions detected mostly by Seismography.
Felt only by a few persons at rest, especially on upper floors of
Felt quite noticeably by persons indoors, especially on upper
floors of buildings. Many people do not recognize it as an
earthquake. Standing motor cars may rock slightly. Vibration
similar to the passing of a truck.
Felt indoors by many, outdoors by few during the day. At night,
some awakening. Dishes, windows, doors disturbed; walls make
cracking sound. Sensation like a heavy truck striking building.
Standing motor cars rock noticeably.
Strong. Felt by nearly everyone; many awakened. Some dishes,
windows broken. Un-stable objects overturned. Pendulum clocks may
Felt by all, many frightened. Some heavy furniture moved; a few
instances of fallen plaster. Damage slight.
Strong. Damage negligible in buildings of good design
and construction; slight to moderate in well-built ordinary
structures; considerable damage in ordinary structures;
considerable damage in poorly built or badly designed structures.
Damage slight in specially designed structures; considerable
damage in ordinary substantial buildings with partial collapse.
Damage great in poorly built structures. Fall of factory stacks,
columns, monuments, walls. Heavy furniture overturned.
Damage considerable in specially designed structures; well
designed frame structures thrown out of plumb. Damage great in
substantial buildings, with partial collapse. Buildings shifted
Some well-built wooden structures destroyed; most masonry and
frame structures destroyed with foundations. Rails bend greatly.
Disastrous. Few, if any (masonry) structures remain
standing. Bridges destroyed. Rails bend greatly.
Damage total. Lines of sight and level are distorted. Objects
thrown into the air.
Credit: NASA, USGS, BBC,
compiled from The British Antarctic Study, NASA, Environment Canada,
UNEP, EPA and other sources as stated and credited Researched by Charles
Welch-Updated daily This Website is a project of the The Ozone Hole Inc.
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