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• Chapter 10 Objectives
1. Discuss the elastic rebound theory.
2. Explain why earthquakes generally
occur at plate boundaries.
3. Compare the 3 types of seismic
waves.
4. Discuss the method scientists use
to pinpoint an Earthquake (EQ).
5. Discuss the method most commonly
used to measure the magnitude of
EQs.
Earthquakes & Plate Tectonics
• Earthquakes (EQs):
vibrations of the
Earth’s crust
– Occur when rocks
under stress suddenly
shift
• Elastic Rebound Theory: the theory
geologists use to explain many EQs
– Rocks on each side of fault are moving
– If fault is locked, stress increases
– At certain point rocks fracture & rebound
• Focus: the area along a fault where
slippage 1st occurs.
– Depths vary
• Epicenter: the point on the Earth’s
surface directly above the focus.
• Aftershocks: series of smaller
tremors.
– Caused by released energy (seismic
waves)
• Increases stress in other rocks along
the fault
• Focus depths vary
– Shallow focus - w/in 70 km of surface
• Tend to cause the most damage
• Focus depths vary
– Intermediate focus - 70 to 300 km
• Focus depths vary
– Deep focus (occur in subduction zone)
• 300 to 650 km
• Focus depths vary
– About 90% of Continental EQs are
shallow focus
3 Major EQ Zones
• Links btwn EQs & Plate Tectonics
– Plate movements = stress buildup
• 1. Pacific Ring of Fire: ring of
tectonic/volcanic activity surrounding
the pacific.
• 2. Mid-ocean ridges: spreading motion
= stress
• 3. Eurasian-Melanesian mountain
belt: Eurasian plate colliding w/African
& Indian plates = stress
• Fault Zones: are
groups of
interconnected
faults.
– Form at plate
boundaries b/c of
intense stress
• e.g. San Andreas fault
• Not all EQs occur at plate
boundaries!
– e.g. New Madrid, Missouri, 1812
• Ancient fault? (600 million yrs ago)
3 major Types of Seismic Waves
• P waves: primary wave.
– Move fastest
– 1st to be recorded
– Travels through solids & liquids
• More rigid matl. = faster
3 major Types of Seismic Waves
• S waves: secondary waves.
– 2nd waves to be recorded
– Only travel through solids
3 major Types of Seismic Waves
• L waves: surface waves, also called long
waves.
– Converted from P & S waves when they
reach the surface
– Slowest-moving waves
• Last to be recorded
– Cause surface to rise & fall
Recording EQs
• Seismograph: instrument used to
detect and record seismic waves.
– 3 separate devices
• 1 records vert. motion
• 2 record horz. motion
– One N to S
– One E to W
• Seismogram: seismograph’s record of
waves; has wavy lines recorded on
paper.
Locating an EQ
• Scientists compare time diff. btwn
P & S waves.
– Plot on graph to get distance to epicenter
Locating an EQ
• Need 3 plots to locate epicenter
– Point where all 3 circles meet is location of
epicenter
Blue primary waves followed by
red secondary waves move
outward in concentric circles
from the epicenter of an
earthquake off British
Columbia and Washington
State.
EQ Measurement
• Richter Scale: used to express
magnitude of an EQ.
– Measure of energy released
EQ Measurement
• Richter Scale: used to express
magnitude of an EQ.
– Measure of energy released
– Major EQ = 7 or above
• Causes widespread damage
EQ Measurement
• Richter Scale: used to express
magnitude of an EQ.
– Measure of energy released
– Moderate EQ = 6 to 7
EQ Measurement
• Richter Scale: used to express
magnitude of an EQ.
– Measure of energy released
– Minor EQ = 2.5 to 6
EQ Measurement
• Richter Scale: used to express
magnitude of an EQ.
– Measure of energy released
– Microquakes = < 2.5
• Not felt by people
EQ Measurement
• Mercalli Scale: expresses the
intensity of an EQ.
• Intensity: the amount of damage the
EQ causes.
– Roman numerals from I to XII
Seismic Monitor
EARTHQUAKE DAMAGE
Movement of the ground itself seldom
causes many deaths or injuries.
Most injuries result from the collapse of
the buildings, flying glass and falling
objects.
EARTHQUAKE DAMAGE
EQs and under water landslides can cause
huge ocean waves called tsunamis.
Tsunamis can cause massive death and
damages.