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Physical Geology 101
21. Earthquakes I
(p. 296-303; 306)
How many people have been killed by earthquakes in the last 4,000 years? ____________
How many people have been killed by earthquakes in the past century? ____________
What two recent earthquakes resulted in a combined ~360,000 fatalities?
LOCATION
YEAR
1. _______________________
_________ (M 7.6; 86,000 fatalities)
2. _______________________
_________ (M 9.0; 283,000 fatalities)
Faults and Earthquakes
Earthquakes occur along faults because tectonic stresses are constantly trying to push the two sides of a
fault past each other. Which types of faults are associated with the following types of tectonic stresses?
Tensional stresses:
___________________
Compressional stresses:
___________________
Shear stresses:
___________________
Tectonic stresses are always present in the crust, but any one fault doesn’t produce earthquakes every
day. This is because tectonic stresses need to overcome the strength of a fault.
What is it that gives faults their strength? ___________________________
When the frictional strength of the fault is exceeded, the fault jumps forward (slips or ruptures) suddenly,
producing an earthquake.
How much slip generally occurs on a fault during a single earthquake? ____________
What is the largest amount of motion ever recorded at the Earth’s surface during an earthquake, and
where did it occur?
Amount of slip: ____________ Location: _________________________
Is an earthquake produced every single time a fault slips?
YES
or
NO?
Some faults slip continuously in response to tectonic stresses, although very, very slowly. This process is
called ________________.
Why do these creeping faults slip continuously instead of producing earthquakes?
__________________________________________
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Physical Geology 101
Theory of Earthquakes
The reason that faults fail abruptly when the friction is overcome is because the rocks build up a lot of
energy by getting deformed before an earthquake. The rocks get pulled in opposite directions on either
side of the fault, but they are locked together by the friction along the fault.
What type of energy builds up in the rocks? __________________________
In the crust, tectonic stresses are always present. So the energy stored in the rocks is violently released
as fault slips and causes an earthquake. We feel this release of energy in the form of shaking of the
ground.
This theory for how earthquakes form is called the ____________________________.
After the stored energy is released, the fault stops slipping and things quiet down as the stresses build up
towards the next earthquake.
Often, the shaking created by a large earthquake along a fault upsets the balance on nearby faults which
then undergo smaller fault rupture events. This produces a number of smaller earthquakes called
__________________, which can continue for months after the main earthquake.
Earthquake Locations and Frequency
If we look at a map of the world distribution of earthquakes, we can see that they appear to cluster greatly
into zones of high earthquake activity. These zones indicate the ________________________________.
The tectonic plates are only able to slide past each other because of the faults in between them.
What percentage of all earthquakes occurs along the plate boundaries? ___________
There are three main types of faults (normal, reverse, and strike-slip), and all three can cause
earthquakes. So some earthquakes occur where the plates slide past each other, others where plates
collide, and others still where plates move apart.
About ____% of all earthquakes occur around the edges of the Pacific Ocean, in a belt called the
___________________________.
About ____% of all earthquakes occur far from plate boundaries in the continental interiors, where some
of the largest earthquakes ever recorded happened.
Examples of large continental interior earthquakes in the United States:
LOCATION
YEAR
MAGNITUDE
1. _______________________
___________
_____________
2. _______________________
___________
_____________
What is the most prominent earthquake-producing plate boundary fault in the lower 48 states of the US?
_________________________________ (in California).
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Physical Geology 101
What two historical large magnitude earthquakes occurred along this fault?
EARTHQUAKE NAME
YEAR
MAGNITUDE
1. _______________________
___________
_____________
2. _______________________
___________
_____________
What other plate boundary area in the lower 48 states produced earthquakes?
________________________________
What was the largest earthquake ever to be associated with the plate boundary along the Pacific
Northwest? YEAR: __________
Magnitude: _________
The eight largest earthquakes ever to hit the United States all occurred in the state of _______________.
The largest earthquake ever recorded in the United States (and the second largest ever in the world) was
the ______________________________earthquake in the year _________.
What was its magnitude? _________.
The largest earthquake ever recorded was in ____________ in the year _________.
What was its magnitude? _________.
Recording Earthquakes
Every year, there are more than ______________ earthquakes around the world that are strong enough
to be felt.
How many earthquakes are recorded by instruments every year? _________________
The study of earthquakes is a field of geology called __________________.
The instrument used to record the occurrence of an earthquake is called either a
_____________________ or a ________________________.
Old seismographs consisted of a pen attached to a free-swinging pendulum. If an earthquake occurred, the
ground would move back and forth, and the pen would record a series of jagged strokes on a rotating drum of
paper.
The paper record of the earthquake shaking is called a __________________.
Seismographs today are a lot more sophisticated, with electronic sensors recording the ground shaking,
and reporting it directly to a computer screen.
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Physical Geology 101
Earthquake Depth
During an earthquake, the ground shakes because of the release of elastic energy when the fault slips.
How is this energy transmitted through rocks? ________________________.
This effect is similar to energy passing across the surface of the ocean as water waves. It is only because
rocks can behave in an elastic manner that they are able to transmit elastic waves without fracturing into
pieces.
The elastic waves get emitted from the exact location on the fault where the slip first starts. This is called
the ____________________ or ______________________ of the earthquake, which is usually many
kilometers below the surface of the Earth.
The point on the Earth's surface directly above the focus of the earthquake is called the
________________ of the earthquake, which can be plotted on a map.
The depth of the focus of an earthquake is called the ________________________ and may be
anywhere from ________________ deep to ____________ deep.
Seismic Waves
There are a number of different types of seismic waves, which are divided into two categories:
1. ____________________ : these are the waves that radiate away from the focus.
2. ____________________ : these are produced when body waves intersect the Earth's surface
and cause a vibration along the surface.
What are the two types of body waves?:
1. _______________ (also called _______________)
2. _______________ (also called ________________ or _______________)
P-waves move through rock as a series of pulses of compression and expansion.
What direction do these pulses move? ____________________________________
So a single particle in the rock just moves back and forth, back and forth, as the energy wave passes
through.
What other type of wave propagates in this way? _____________________
As P-waves pass through the rock, there is no actual transport of the rock. It just gets a little disturbed
while the energy waves passes through it. We usually feel the P-wave arriving as a sudden vertical jolt. It
travels through rock at about 4 mi/sec.
Are P-waves are the fastest or slowest traveling waves? FASTEST or SLOWEST ?
So in which order does it arrive at any location after an earthquake? ___________
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Physical Geology 101
Which types of media can P-waves travel through?
Solids
Liquids
Gases
YES or NO ?
YES or NO ?
YES or NO ?
S-waves propagate like a snake as a back and forth motion of particles.
What direction is this back and forth motion? _______________________________
They are the ____________ fastest waves (~2 mi/sec) so we feel them only after the P-wave has passed by.
Which types of media can S-waves travel through?
Solids
Liquids
Gases
YES or NO ?
YES or NO ?
YES or NO ?
Surface waves travel along or near the surface of the Earth, like water waves. Body waves can produce
sharp jolts and extreme shaking, but surface waves produce a gentle rolling motion, like being on a boat
on the ocean.
Surface waves are the slowest moving waves, and there are two types
1. _________________________
2. _________________________
Love waves involve a back and forth horizontal motion, like an S-wave, and can be very damaging to
buildings.
What is the order in which a Love wave is recorded by a seismograph? _____________
Rayleigh waves cause particles in the rock to trace out an elliptical rolling motion, like the waves on the
ocean.
FINAL QUESTION:
Rayleigh waves are the last seismic waves to be felt because they are the:
____________________________
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