Chapter 8 Notes
I.
What are Earthquakes?
a. The study of earthquakes is called seismology.
b. Most earthquakes occur on the edges of tectonic plates.
i. Tectonic plates moved around on the mantle- the
plastic rock that makes up most of the volume of
Earth.
ii. Tectonic plates move in different directions and at
different speeds.
iii. Plates can push toward each other (convergent
boundary), pull away from each other (divergent
boundary) or slip past each other (transform
boundary)
iv. A fault is a break in Earth’s crust where plates slide
against each other.
c. What causes Earthquakes?
i. As tectonic plates move past one another, stress
increases as they scrape past one another.
ii. As rocks try to move past one another, it causes rocks
to deform.
iii. Deformation is the bending, tilting and breaking of
rocks in Earth’s crust.
iv. Two types of deformation
1. Plastic deformation- deforms like clay- can be
stretched. Does not lead to earthquakes.
2. Elastic deformation- rock is stretched to its
breaking point- like a rubber band- leads to
earthquakes
a. During elastic rebound, rock is stretched
and breaks, releasing energy.
b. Some of this energy travels as seismic
waves.
d. Faults at Plate boundaries
i. Transform Boundary= side to side motion= strike slip
fault
ii. Convergent Boundary= moving together= reverse fault
iii. Divergent Boundary= moving apart= normal fault
e. How to Earthquake waves travel?
i. Waves of energy that travel through the Earth are
called seismic waves
ii. Body waves- waves that travel through the Earth’s
interior
1. Two types
a. P-Waves- also called primary or pressure
waves
i. Fastest wave
ii. Move rock back and forth squeezing
and stretching rock
b. S- Waves- also called secondary or shear
waves
i. Arrive after P-waves
ii. Cannot travel through liquids
iii. Makes rock move side to side
iii. Surface waves- waves that travel along Earth’s
surface
1. Causes up, down and around motion along the
ground
II.
Earthquake Measurement
a. Seismographs- instruments located at or near the surface
of the Earth that record seismic waves
b. Seismogram- tracing of earthquake waves
c. Determining time and location of an Earthquake
i. Seismologists find an earthquakes start time by
comparing seismograms and noting differences in
arrival times of P waves and S waves to find the
epicenter
ii. Epicenter- the point on Earth’s surface directly above
the starting point of the Earthquake.
iii. Focus- the point inside the Earth where an earthquake
begins.
d. Measuring Earthquake Strength and Intensity
i. Richter Scale- measures ground motion from an
earthquake.
1. Each time the magnitude increases by one unit,
the measured ground movement is 10 times
stronger.
2. Ex. A 4.0 earthquake is 10 times stronger than a
3.0
3. Ex. A 5.0 earthquake is 100 times stronger than
a 3.0
ii. Mercalli Scale- a measure of the degree to which an
earthquake is felt by people and the amount of
damage.
1. Represented by roman numerals