2. 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!
3. • 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.
4. • 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.
5. 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.
6. 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.
8. • 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.
9. • 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!
10. 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.
11. • 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.
12. • 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.
13. • Earthquakes can also occurwithin plates,
although plate-boundary earthquakes are
much more common. Less than 10 percent of
all earthquakes occurwithin plate interiors.
The New Madrid earthquakes of 1811-1812
and the 1886 Charleston earthquake
occurred within the North American plate.
14. • 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.
15.
16.
17. The point beneath the Earth's surface
where the rocks breakand 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.
18. • 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.
19. • PrimaryWaves
• Seismic waves that travel the fastest are
called primary waves, orPwaves. Pwaves
arrive at a given point before any other
type of seismic wave. Pwaves travel
through solids, liquids and gases.
20. • Pwaves are push-pullwaves. As Pwaves
travel, they push rockparticles into the
particles ahead of them, thus compressing
the particles. The rockparticles then
bounce back. They hit the particles behind
themthat are being pushed forward. The
particles move backand forth in the
direction the waves are moving.
21. • Secondary Waves
• Seismic waves that do not travel through the
Earth as fast as Pwaves do are secondary
waves, orS waves. S waves arrive at a given
point afterPwaves do. S waves travel through
solids but not through liquids and gases.
22. • Surface Waves
• The slowest-moving seismic waves are called surface
waves, orL waves. L waves arrive at a given point
afterprimary and secondary waves do. L waves
originate at the epicenter. Surface waves travel along
the surface of the earth, ratherthan down into the
earth. Although they are the slowest of all the
earthquake waves, L waves usually cause more
damage than PorS waves.
23. • 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.
24. • 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.
