Three Basic Types of Plate Boundaries
Divergent Plate Boundaries

New crust is generated as the plates pull apart.

Occurs at spreading ocean ridges and at continental rifts.
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Earthquakes are shallow and small.
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Examples:
1.
2.
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4.
5.
Convergent Plate Boundaries
Plates push together.
A) The denser plate subducts (is pushed under the other), or
B) two continental plates crunch together to form high mountains.
Earthquakes along Convergent Zones with
Subducting Oceanic Lithosphere
Shallow earthquakes:
The most destructive of these occur between the plates on the plate boundary.
Shallow earthquakes also occur within the subducting plate and within the overriding
plate near the plate boundary.
Intermediate and Deep earthquakes:
The depth range defined as “intermediate” is 100 – 300 km deep while “deep” earthquakes are in
the 300 – 700 km depth range. Intermediate and deep earthquakes occur only within the
subducting oceanic lithosphere.
Transform Plate Boundaries
Lithosphere is neither produced nor destroyed as the plates slide horizontally past each other. Stress
can build up and when released, cause earthquakes.
Deforming Earth’s Crust
Types of stress: Tensional, Compression, Shear
What is the Elastic Rebound Theory?
Explains how energy is stored in rocks
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Rocks bend until the strength of the rock is exceeded
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Rupture occurs and the rocks quickly rebound to an undeformed shape
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Energy is released in waves that radiate outward from the fault
What are Earthquakes?
•
The shaking or trembling caused by the sudden release of energy along fault lines.
•
Usually associated with faulting or breaking of rocks.
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Faults are found at all three kinds of plate boundaries as well as within the plates!!
Where does the energy come from?
The edges of the fault lines tend to stick together due to friction while the rest of the block is
moving.
The energy that would normally cause the blocks to slide past one another is being stored up.
When the force of the moving blocks finally is greater than the force of friction of the jagged edges
of the fault, it “unsticks” and all that stored energy is released!
Where Do Earthquakes Occur and How Often?

80% of all earthquakes occur in the circum-Pacific belt. Most of these result from convergent
boundary activity

~15% occur in the Mediterranean-Asiatic belt

remaining 5% occur in the interiors of plates and on divergent boundaries

more than 150,000 quakes strong enough to be felt are recorded each year
What are Seismic Waves?
•
Energy that travels in all directions in the form of waves. This energy is released when the rock
fractures or slips due to a buildup of stress.
•
Two types:
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Body waves
•
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Primary (P) and Secondary (S)
Surface waves
•
Rayleigh and Love
The Focus and Epicenter of an Earthquake
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The point within Earth where faulting begins is the focus
•
The point directly above the focus on the surface is the epicenter.
•
The epicenter has the POTENTIAL to have the greatest damage since it receives the most
energy.
Two Types of Body Waves
(1) P or primary waves
•
fastest waves
•
travel through solids, liquids, or gases
•
compressional wave, material movement is in the same direction as wave
movement
(2) S or secondary waves
•
slower than P waves
•
travel through solids only
•
shear waves - move material perpendicular to wave movement
Surface Waves: R and L waves
–
Travel just below or along the ground’s surface
–
Slower than body waves; rolling and side-to-side movement
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Especially damaging to buildings
How do scientists detect earthquakes?
When an earthquake occurs the seismic waves travel through the Earth to the seismic station where
the information is transmitted to distant computers.

A seismograph detects and records EQs.

A seismogram is the EQ record.
How is an Earthquake’s Epicenter Located?
(1) Seismic wave behavior
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P waves arrive first, then S waves, then L and R
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Average speeds for all these waves is known
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After an earthquake, the difference in arrival times at a seismograph station can be used
to calculate the distance from the seismograph to the epicenter.
(2) Time-distance graph showing the average travel times for P- and S-waves.

The farther away a seismograph is from the focus of an earthquake, the longer the
interval between the arrivals of the P- and S- waves
(3) Three seismograph stations are needed to locate the epicenter of an earthquake

A circle where the radius equals the distance to the epicenter is drawn

The intersection of the circles locates the epicenter
How are the Size and Strength of an Earthquake Measured?
Magnitude
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Richter scale measures total amount of energy released by an earthquake.
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Richter Scale rates earthquakes from 0 to 10
Intensity
•
Modified Mercalli Scale is a subjective measure of the kind of damage done and people’s
reactions to it
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Depends on magnitude of quake as well as soil types, time of day of the quake, building styles
and whether the area is densely populated.
•
Ranges from 1 to 12.