1. What is the Definition of an Earthquake?
An earthquake is the sudden, sometimes violent movement of the
surface from the release of energy in the earth's crust.
2. What Causes Most Earthquakes?
The crust of the earth when it is subject to tectonic forces, bends
slightly. But, because the crust is rigid, when the stress or pressure
exceeds the strength of the rocks, the crust breaks and snaps into a
position. Vibrations called seismic waves are generated and
both through the earth and along its surface. These seismic waves
the movement we call earthquakes.
3. Where Are Earthquakes Likely to Occur?
Within areas of the crust are fractures, known as faults, along
which two crustal blocks have slipped or moved against each other.
block may move up while the other moves down, or one may move
horizontally in one direction and the other in the opposite direction.
Geologists and seismologists (scientists who study earthquakes
the processes that create them) have found that earthquakes occur
repeatedly at faults, which are zones of weakness in the
4. How Many Earthquakes Happen Each Year?
There are over a million quakes annually, including those too small
felt. The following table shows the average frequency of different
||Frequency per year
|Minor (damage slight)
||less than 2.0
From Earthquakes and the Urban Environment, Vol. 1, G. Lennis
5. How Many Earthquakes Happen Every Month? Day?
Using the previous table:
And, per second, one earthquake is felt
every 30 seconds.
Of these only a relative few are capable of causing damage.
are common natural events.
6. How Deep Do Earthquakes Occur in the World?
Earthquakes occur in the crust or upper mantle which ranges from
surface to about 800 kilometers deep (about 500 miles).
7. Where Do Most Earthquakes Occur in the World?
The surface of the earth is divided like a jigsaw puzzle into giant
pieces called tectonic or crustal plates. These giant pieces
slowly over partially melted rock known as the mantle. As
move, they slide along each other, move into each other, move away
each other, or one slips under another. On these active plate
about 95% of all the world's earthquakes occur. California, Alaska,
Japan, South America, and the Philippines are all on plate
Only 5% are in areas of the plates far away from the boundaries.
called mid-plate or intra-plate earthquakes and are, as yet,
8. Where do the Most Earthquakes Occur in the United States ?
Alaska has more earthquakes per year than the combined total of
of the United States. As many as 4,000 are recorded there every
Alaska is on a plate boundary where one plate is sliding along
another, a subduction zone.
9. Where Did the Largest Known Earthquake Occur?
A magnitude 9.5 earthquake in Chile in 1960 was the largest known
earthquake and resulted in over 6,000 deaths. It triggered a tsunami or seismic wave (incorrectly known as a tidal wave)
killed people as far away as Hawaii and Japan. Chile is also on a subduction zone.
10. What Was the Largest Earthquake in the United States?
The great Alaska earthquake of March 27, 1964, is the largest
in the United States. It had a magnitude of 9.2. 115 people died,
most of the deaths due to the tsunami it generated. Shaking
felt for an estimated 7 minutes, and raised or lowered the ground
as much as 2 meters (6.5 feet) in some areas and 17 meters (approx.
feet) in others. The length of the ruptured fault was between 500
1,000 kilometers (310.5 and 621 miles). The amount of energy
equal to 12,000 Hiroshima-type blasts, or 240 million tons of TNT.
11. Where Was the Largest Earthquake in the Continental United
A series of four great earthquakes occurred in the central United States on December 16, 1811, and January 23, and February 7, 1812. All had estimated magnitudes greater than 7.5 on the Richter Scale, the largest happening on February 7, 1812. They are collectively known as the New Madrid earthquakes (after a small town in Missouri) and were felt as far away as Washington D.C., and Boston, Massachusetts. These events were felt over a region far greater than any other in the United States, an estimated 2 million square miles. There were fewer than 100 deaths, because of the small number of people living in the area. The earthquakes raised and lowered land levels several feet, created one large lake and several smaller lakes, and formed waterfalls on the Mississippi River. One small town was destroyed and there was extensive damage to structures and changes to land surfaces throughout the region. These earthquakes were far away from a plate boundary, and are the largest known to have happened in a mid-plate area.
12. It Seems that Large Earthquakes in the U.S. Are Responsible
Relatively Few Deaths. Is This True Around the World?
No. In other areas of the world smaller earthquakes are responsible
the deaths of many thousands of people. This is primarily because of
buildings which are poorly designed and constructed for earthquake
regions, and population density. The following table shows some of
major earthquakes around the world in the last twenty years, and
number of deaths associated with them.
13. What Was the Greatest Number of People Killed in One
An earthquake in China in 1556 killed approximately 830,000
14. How Are Earthquakes Measured?
A seismometer is an instrument that senses the earth's motion;
a seismograph combines a seismometer with recording
obtain a permanent record of the motion. From this record scientists
calculate how much energy was released in an earthquake, which is
to decide its magnitude. Calculations are made from several different
seismograms, both close to and far from an earthquake source to
its magnitude. Calculations from various seismic stations and
seismographs should give the same magnitude, with only one
any given earthquake.
Richter Magnitude is the scale most people are familiar with,
scientists use other more accurate scales. Another nonscientific way
measuring earthquakes is by their intensity or degree of shaking.
Intensity is descriptive, and is determined by inspection of damage
other effects, with the greatest intensity being close to the epicenter,
and smaller intensities further away. The Modified Mercalli
Scale uses Roman Numerals from I to XII to describe different
earthquake effects is commonly used.
15. What Does the Richter Scale Look Like?
The Richter Scale is not an actual instrument. It is a measure
the amplitude of seismic waves and is related to the amount of
released. This can be estimated from the recordings of an earthquake
seismograph. The scale is logarithmic, which means that each
number on the scale increases by 10. A magnitude 6.0 earthquake is
times greater than a 5.0, a 7.0 is 100 times greater, and a magnitude
is 1,000 times greater.
16. When was the First Instrument for Detecting Earthquakes
The earliest known earthquake detection instrument was invented in
A.D. by Zhang Heng, a Chinese philosopher. The instrument was a
meters or 6.5 feet in diameter) bronze jar, with a central pendulum
inside. Decorating the jar on the outside were a series of dragon
connected to a pendulum, each with a ball in a hinged mouth.
beneath each dragon head, on the surface of the stand, was a bronze
head up, mouth open to receive a ball from the dragon's mouth.
During an earthquake, the ground motion would move the pendulum
one or more balls to fall from a dragon's mouth into a toad's mouth.
direction of the earthquake was indicated by which of the dragon
had dropped a ball.
This instrument was sensitive enough to perceive shaking too small
felt, as it detected an earthquake over 600 kilometers (372 miles)
news of which arrived several weeks later.
Earthquake detectors are mentioned later in oriental manuscripts,
the west earthquake detection instruments did not emerge until
17. What is the Difference Between an Earthquake Prediction
An earthquake prediction involves assigning a specific date, location,
and magnitude for an earthquake. A forecast assigns a series of
probabilities and a range of years and magnitudes to a region. There
no way to accurately predict earthquakes, but forecasts have been
calculated for different areas of the United States. The earthquake in
northern California on October 17, 1989 was not predicted, but did
within the magnitude range, time span, and region forecast by U.S.
Geological Survey staff.
18. Does Animal Behavior Change Before Earthquakes?
Changes in animal behavior before earthquakes have been observed
documented in different parts of the world, most recently in the
California earthquake of October 17, 1989. It has been recorded that
fish in a high school biology lab in California would flip on its side
before some earthquakes.
Dogs, cats, snakes, and horses has also been known to behave
before earthquakes. Since behavior is not earthquake specific, change
animal behavior can therefore result from other events, and it is
impossible to determine beforehand what factor has caused the
Also, the behavior is not consistent. Sometimes earthquakes occur
previous behavior change.
19. Does the Ground Really Open Up and Swallow People?
This is an earthquake myth. Cracks and fissures appearing in the
are a common effect of earthquakes. Most of these are narrow and
In very large earthquakes changes in the level of the land can result
larger cracks that can cause a lot of damage to buildings, but people
buildings do not get swallowed by the ground.
20. Do Earthquakes Cause Volcanoes?
No, there are different earth processes responsible for volcanoes.
Earthquakes may occur in an area before, during, and after a volcanic
eruption, but they are the result of the active forces connected with
eruption, and not the cause of volcanic activity.
21. Are Earthquakes Weather Related?
In the 4th Century B.C., Aristotle proposed that earthquakes were
by winds trapped in subterranean caves. Small tremors were thought
have been caused by air pushing on the cavern roofs, and large ones
the air breaking the surface. This theory lead to a belief in
weather, that because a large amount of air was trapped
weather would be hot and calm before an earthquake.
A later theory stated that earthquakes occurred in calm, cloudy
conditions, and were usually preceded by strong winds, fireballs, and
meteors. There is no connection between weather and earthquakes.
the result of geologic processes within the earth and can happen in
weather and at any time during the year.
22. What Are Earthquake Scientists Called?
Seismologists: seismos-from the Greek meaning earthquakes, and
ologist-meaning a person who studies (something). A seismologist is
person who studies earthquakes and the mechanics of the earth.
23. How Much Energy is Released in an Earthquake?
Earthquakes release a tremendous amount of energy, which is why
be so destructive. The table below shows magnitudes with the
amount of TNT needed to release the same amount of energy.
From Earthquakes and the Urban Environment, Vol. 1, G.
24. Do All Large Magnitude Earthquakes Result in Great Amounts
Death and Destruction?
No.The destructive forces of an earthquake depends on many factors.
earthquakes commonly occur in remote areas of the world, with no
buildings or people, and are not destructive. In addition to
some of the factors that determine damage and deaths are:
densities, the density and types of building construction, local
conditions, distance from the epicenter, earthquake depth, how long
shaking continues, and the degree of earthquake preparedness in the
25. Can Earthquakes Be Prevented?
There is no known way to prevent earthquakes, but it is possible to
lessen the impact. The amount of devastation from an earthquake
greatly diminished by building structures using earthquake resistant
design, making the interiors of buildings safe from falling objects,
educating people about earthquake safety.