Download Chapter 8 - Earthquakes

Chapter 8, Section 1 – What are
Earthquakes?
Seismology – The study of earthquakes.
Most earthquakes occur near the edges of tectonic plates where giant pieces
of the Earth’s crust are moving over plastic rock. Earthquakes occur at or near
fault lines where the sliding action of these tectonic plates causes vibrations felt
as earthquakes.
Deformation – stress caused by the bending, tilting or folding of rock layers
causing them to change shape.
Plastic Deformation – rocks bend but do not break; these do NOT cause
earthquakes.
Elastic Deformation – rocks stretch and then break creating the vibrations
of seismic waves that cause earthquakes and then return to their pre-deformed
shape.
Strike/Slip Faults – are the cause of most earthquakes. These faults occur at
transform plate boundaries.
Earthquake Zones – Areas where large numbers of faults are located parallel
to tectonic plate boundaries.
• Seismic Waves – Energy waves that travel in all directions
through the Earth.
• Types of Seismic waves:
1. Body Waves – seismic waves that travel through the
interior of the Earth.
a. Primary or P Waves – These are the fastest moving
seismic waves thus, the first to be detected. These waves travel
with a “back & forth” motion.
b. Secondary or S (shear) Waves – These are the
second fastest, hence the second waves to be detected. S
waves travel with a side to side motion.
2. Surface or L (long) waves – These are the slowest of the
waves and travel along the Earth’s surface.
Chapter 8, Section 2 – Earthquake
Measurement
• Seismograph – The instrument that records the time
and intensity of the seismic waves and helps locate the
epicenter of seismic events.
• Seismogram – The tracing or paper record of the
earthquake motion created by the seismograph.
• The most direct method of locating the epicenter of an
earthquake is to measure the time difference between
the arrival of the S waves and the P waves (S – P Time
Method) as recorded on the seismogram.
• Focus – The origin or starting point deep within the
Earth where the earthquake begins along the fault.
• Epicenter – The point on the Earth’s surface located
directly above the focus.
Chapter 8, Section 2 – Earthquake
Measurement
• Richter Magnitude Scale – Measures the strength of
the earthquake by measuring the amount of ground
motion.
• Magnitude – A measure of the strength of the
Earthquake. Each single number increase on the Richter
Scale increases the strength of the earthquake by a
factor of 10. For example, a magnitude 6 earthquake is
10 times greater than a 5 and 100 times greater than a
4. The highest number on this scale is a 9.
• Intensity – How much ground shaking is felt by people
or how much damage a quake causes. This is measured
on the Mercalli Intensity Scale. A single earthquake can
have a variety of intensities levels with the highest near
the epicenter decreasing outwards. The highest number
on this scale is XI.
Chapter 8, Section 3 –
Earthquakes & Society
• Earthquake Hazard – How likely an area is to have an
earthquake based on past and present seismic activity.
The west coast of the U.S. has the highest probability in
our country.
• Earthquakes prediction is very difficult but scientists do
observe patterns and are able to make generalized
predictions.
• Gap Hypothesis – A hypothesis that states sites along
active faults with few earthquakes will have strong
earthquakes in the future.
• Seismic Gaps – Areas along a fault where a relatively
few number of earthquakes have occurred.
Chapter 8, Section 3 –
Earthquakes & Society
• Retrofitting – The process of making older buildings earthquake
resistant by reinforcing their foundations with steel.
• Earthquake Resistant – The addition of weights to dampen seismic
vibrations, cross-bracing and shock absorbers to reduce the “pushpull” effect of earthquakes that causes building to sway.
• Before the Quake:
1. Place heavier objects on lower shelves.
2. Develop a plan to get away from possible falling objects as well as a
meeting place.
3. Stockpile food and water.
During the quake:
1. If indoors, get under a heavy table to protect yourself from falling
objects.
2. If outside, stay away from anything that could fall on you, finding a
clear area is best.
3. If in a car, stop and stay inside the car.