Download Earthquakes

Bellringer# 27
• What is the Richter Scale?
• What is the greatest magnitude on the Richter
scale?
• Where would an earthquake with the greatest
magnitude be felt?
Ch.21 S.2 Earthquakes
Focus
• What is an earthquake?
• What causes and earthquake?
• How are earthquakes rated?
• How are earthquakes measured?
Key Ideas and Key Terms
• Where do most earthquakes occur?
• How do scientists learn about earthquakes and the
Earth’s interior?
•
•
•
•
•
•
Focus
Epicenter
Surface wave
Seismology
Richter scale
Vent
What are Earthquakes?
• An earthquake is the vibration of Earth
produced by the rapid release of energy.
• They are caused by slippage along a fault in
Earth’s crust.
• The energy released is radiated in all directions
from its source or focus.
Earthquakes
• Sensitive instruments located around the world
record the event.
• Over 30,000 earthquakes occur wolrdwide each
year. (only strong enough to be felt)
• Only 75 significant earthquakes occur each year.
• An earthquake is one of the most destructive
natural forces on Earth.
Where do Earthquakes Occur?
7
Where do earthquakes occur?
• By looking at maps showing past seismic
activity, one can see that earthquakes occur
mostly at the boundaries of tectonic plates,
where the plates move with respect to each
other.
•Earthquakes occur at plate boundaries
•Earthquakes are vibrations resulting from
rocks sliding past each other at a fault.
•Seismic waves are waves of energy
released during in earthquake.
focus: the location within Earth along a
fault at which the first motion of an
earthquake occurs
epicenter: the point on Earth’s surface
directly above an earthquake’s starting
point, or focus
• Energy from earthquakes is transferred by
waves.
– The energy released from an earthquake is
measured as shock waves.
– Earthquakes generate three types of waves:
• Longitudinal waves, also known as P waves
• Transverse waves, also known as S waves
• Surface waves
– Longitudinal waves travel
by compressing and
stretching crust. They are
also called primary waves
(P waves).
– Transverse waves travel in
an up and downward
movement. They are also
called secondary waves
(S waves).
• Waves move through Earth and along its
surface.
– Both P waves and S waves spread out from the
focus in all directions through the earth.
– Surface waves move only on Earth’s surface.
• surface wave: a seismic wave that travels
along the surface of a medium and has a
stronger effect near the surface of the medium
than it has in the interior
Foreshocks and Aftershocks
• The adjustments that follow a major earthquake
often generate smaller earthquakes called
aftershocks.
• Aftershocks occur for several days after a quake.
• Sometimes they can destroy already weakened
structures.
• Foreshocks are small earthquakes that happen after
a major earthquake for days or years.
Measuring Earthquakes
〉How do scientists learn about earthquakes and
the Earth’s interior?
〉Because energy from earthquakes is
transferred by waves, scientists can measure
the waves to learn about earthquakes and about
the interior of Earth through which the waves
travel.
Seismology and Seismographs
• Seismology the study of earthquakes including
their origin, propagation, energy, and prediction.
– Seismos= shake, ology= the study of
– The Chinese were the first to study the waves.
• Seismographs are instruments that record seismic
waves
– Seismos= shake, graph= write
• Seismograms are records of seismic activity
– Seismos= shake, gramma= what is written
Horizontal Seismograph
• The mass swings from side to side.
• It detects horizontal ground motion
• 2 horizontal seismographs are
needed one that measures NorthSouth and one for East-West
• The mass (weight) is motionless
• The recording drum is anchored to
the bedrock and vibrates with the
seismic waves
Vertical Seismograph
• The mass is suspended by
a spring.
• The mass stays motionless
while the Earth and support
move due to inertia.
• It detects vertical ground
motion
• Three seismograph stations are necessary to
locate the epicenter of an earthquake.
– There are more than 1000 seismograph stations
across the world.
– Because P waves travel faster, the difference
between the arrival of P waves and the arrival of S
waves allows scientists to calculate how far away
the focus is.
• P-waves move faster than S-waves. So, if the distance
of the earthquake to the recording station increases,
the time between the arrival of P- and S-waves will
increase. From this P-S travel time difference we can
determine the distance of the earthquake to the
seismometer. Note that we only get the distance, not
the direction of the earthquake focus.
• The solution is to determine
the distance for more than
one seismic station, and
draw this on a map. Two
determinations result in two
possible locations (two
overlapping circles have
two points in common).
Three or more
determinations can only
have one location in
common. This location is
called the epicenter.
• Geologists use seismographs to investigate
Earth’s interior.
– The way P and S waves travel through Earth’s
interior help scientists make a model of Earth with
layers of different densities.
– Scientists have used this information to develop a
model of Earth’s interior structure.
• The Richter scale is used to measure
earthquakes.
– Although the Richter scale was used popularly for
much of the 20th century, scientists today more
often use other scales, such as the momentmagnitude scale.
• Richter scale: a scale that expresses the
magnitude of earthquakes
• Magnitude scales alone cannot predict how severe
an earthquake will be.
– Many factors, such as distance from the focus and
local construction techniques, affect how much
damage an earthquake will cause.
• Scientists are trying to predict earthquakes.
– Scientists are trying to measure changes in Earth’s
crust that might signal an earthquake.
– The ability to predict an earthquake could save
thousands of lives.
– http://videos.howstuffworks.com/hsw/11898earthquakes-the-science-of-earthquake-predictionvideo.htm
San Andreas Fault
The most studied fault system in the world.
http://videos.howstuffworks.com/hsw/11897-earthquakesfault-lines-and-earthquakes-video.htm
San Francisco in flames after the 1906 earthquake.
Protecting Structures from Damage
• http://videos.howstuffworks.com/hsw/11899-earthquakes-protectingstructures-from-damage-video.htm
Homework
• Earthquakes East of the Rockies
• Read and answer the questions on the back.
• Don’t forget about the vocab quiz Thursday!!