Earthquakes, (also known as quakes or a tremors) are violent movements of the rocks in the Earth's crust. Earthquakes are usually quite brief, but may repeat over a period of time. They are the result of a sudden release of energy in the Earth's crust. This creates seismic waves, which are waves of energy that travel through the Earth. The study of earthquakes is called seismology. Seismology studies the frequency, type and size of earthquakes over a period of time. There are large earthquakes and small earthquakes. Large earthquakes can take down buildings and cause death and injury. Earthquakes are measured using observations from seismometers. The magnitude of an earthquake, and the intensity of shaking, is usually reported on the Richter scale. On the scale, 3 or less is scarcely noticeable, and magnitude 7 (or more) causes damage over a wide area. An earthquake under the ocean can cause a tsunami. This can cause just as much death and destruction as the earthquake itself. Landslides can happen, too. This is an important part of the Earth's rock cycle.

4. An earthquake (also known as a quake, tremor or temblor) is
the perceptible shaking of the surface of the Earth, which can
be violent enough to destroy major buildings and kill
thousands of people. Earthquakes are measured using
observations from seismometers. The moment magnitude is
the most common scale on which earthquakes larger than
approximately 5 are reported for the entire globe.
The more numerous
earthquakes smaller than
magnitude 5 reported by
national seismological
Observatories.

5. The most recent large earthquake of magnitude 9.0 or larger
was a 9.0 magnitude earthquake in Japan in 2011 (as of
March 2014), and it was the largest Japanese earthquake
since records began. Intensity of shaking is measured on the
modified Mercalli scale.

6. The Moment Magnitude Scale
Unfortunately, many scales, such as the Richter scale, do not
provide accurate estimates for large magnitude earthquakes.
Today the moment magnitude scale, abbreviated MW, is
preferred because it works over a wider range of earthquake
sizes and is applicable globally.
But only the moment magnitude
scale is capable of measuring M8
(read ‘magnitude 8’) and greater
events accurately. Magnitudes are
based on a logarithmic scale (base
10). What this means is that for
each whole number you go up on
the magnitude scale, the amplitude of the ground motion
recorded by a seismograph goes
up ten times.

7. Earthquakes are the shaking, rolling or sudden
shock of the earth’s surface. Earthquakes happen
along "fault lines" in the earth’s crust.
Earthquakes can be felt over large areas although
they usually last less than one
minute. Earthquakes
cannot be predicted –
although scientists are
working on it!

8. Most of the time, you will notice an earthquake
by the gentle shaking of the ground. You may
notice hanging plants swaying or objects
wobbling on shelves. Sometimes you may hear a
low rumbling noise or feel a sharp jolt. A survivor
of the 1906 earthquake in San Francisco said the
sensation was like riding a bicycle down a long
flight of stairs.

9. The intensity of an earthquake can be measured. One
measurement is called the Richter scale. Earthquakes below 4.0 on
the Richter scale usually do not cause damage, and earthquakes
below 2.0 usually can’t be felt. Earthquakes over 5.0 on the scale
can cause damage. A magnitude 6.0 earthquake is considered
strong and a magnitude 7.0 is a major earthquake. The Northridge
Earthquake, which hit Southern California in 1994, was magnitude
6.7.

10. Earthquakes often occur when tectonic plate collide. What
happens when plates collide? It depends how the plates
are moving when they meet:When two plates collide
head-on, they push each other up and form mountains.
That's how the Himalayas and other great mountain
ranges (including the Rockies, long ago) were created.

11. When one plate dives below another plate, it
creates a subduction zone as the diving plate is
crushed and melted. This process often creates
volcanoes as the magma
(molten rock)
rises up to the
surface.

12. Earthquakes can happen in any of these situations.
Despite the powerful forces driving plate
movement, the plates themselves spend much of
the time locked in place by the friction of the plates
rubbing against each other. Eventually, however,
they build up so much
pressure that the plates
abruptly snap forward.
Then the ground can shift
a few feet—or a few dozen!
Shock waves from that
sudden motion shoot out
in all directions, creating an earthquake.

13. Two great plates, the Pacific and the North American,
meet in California. The Pacific Plate is moving north,
creating a transform fault (the San Andreas and
related faults) Over the last 20 million years the
Pacific Plate has slid about
200 miles north. If it keeps
moving as predicted, San
Francisco will become
neighbors with Seattle in
20 million years!

14. Because the San Andreas
fault curves around Los
Angeles, and then again
into the Pacific in
northern California, the
two plates cannot slide
smoothly against each
other. Instead, the
complex stresses of plate
movement have fractured
the land and created
dozens of smaller fault
lines.

15. Seismologists have been
studying California's faults for
decades. They now say that
the San Francisco Bay Area
has a 70% chance of a major
earthquake before 2030. This
forecast is based on years of
study of the many faults in
the area. The map shows the
probability of a quake from
each of these faults.

16. The rate of large earthquakes in the San Francisco Bay region
abruptly dropped after the Great 1906 Earthquake. The San
Andreas Fault slipped so much over such a great length in that
quake that the strain was reduced on most faults throughout the
region. Strain has been slowly building up again.

17. Earthquakes can also occur within plates, although plate-
boundary earthquakes are much more common. Less
than 10 percent of all earthquakes occur within plate
interiors. The New Madrid earthquakes of 1811-1812 and
the 1886 Charleston earthquake occurred within the
North American plate.

18. • Earthquakes: Facts and Fiction
• Fiction: Earthquakes usually happen in the morning.
Fact: Earthquakes happen in both the day and the night. There is no
pattern.
• Fiction: There is such a thing as "earthquake weather."
Fact: There is no connection between earthquakes and weather.
Remember, earthquakes happen deep in the earth, far away from
the weather!
• Fiction: Earthquakes are on the increase.
Fact: It may seem like we’re having more earthquakes because
there are more reporting stations, but the truth is we’re not.
• Fiction: We can prevent earthquakes from happening.
Fact: No. You can protect yourself by doing things to secure
buildings, like your home, but earthquakes can’t be prevented -- or
predicted.

19. The point beneath the Earth's surface where the
rocks break and move is called the focus of the
earthquake. The focus is the underground point
of origin of an earthquake.
Directly above the
focus, on the Earth's
surface, is the
epicenter.

20. Earthquake waves are known as seismic waves. There
are three main types of seismic waves.
Each type of wave
has a characteristic
speed and manner
of travel.

21. Primary Waves
Seismic waves that travel the fastest are
called primary waves, or P waves. P
waves arrive at a given point before any
other type of seismic wave. P waves travel
through solids, liquids and gases.

24. water waves are push-pull waves. As P
waves travel, they push rock particles
into the particles ahead of them, thus
compressing the particles. The rock
particles then bounce back. They hit the
particles behind them that are being
pushed forward. The particles move back
and forth in the direction the waves are
moving.

25. Secondary Waves
Seismic waves that do not travel through
the Earth as fast as P waves do are
secondary waves, or S waves. S waves
arrive at a given point after P waves do. S
waves travel through solids but not through
liquids and gases.

26. Surface Waves
The slowest-moving seismic waves are called
surface waves, or L waves. L waves arrive at a
given point after primary and secondary waves do.
L waves originate at the epicenter. Surface waves
travel along the surface of the earth, rather than
down into the earth. Although they are the slowest
of all the earthquake waves, L waves usually
cause more damage than P or S waves.

27. This is an image of a seismograph, an
instrument used to record the energy
released by an earthquake. When the needle
is moved by the motion of the earth, it
leaves a wavy line.

28. Blue primary waves followed by red
secondary waves move outward in concentric
circles from the epicenter of an earthquake
off British Columbia and Washington State.

40. June 5
After: June 7