Download What is an earthquake?

Chapter 16
Earthquakes
What is an earthquake?
• Shaking of the ground caused by
the sudden release of energy
stored in the rocks.
• When rocks are stressed, potential
energy is stored in them.
• Rocks can behave plastically upto
a certain limit (elastic limit), after
which they fail and release the
energy.
• Energy waves produced by an
earthquake are called Seismic
Waves.
• The generation of an earthquake is
explained by Elastic Rebound Theory.
• According to this theory, the sudden
release of progressively stored strain in
rocks causes movement along a fault
and an earthquake is generated.
Causes of earthquakes
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Movement along a fault
Volcanic eruptions
Plate movement
Subsidence
Earthquake terminology
• Focus: the point inside the earth
where seismic waves first
generate.
• Epicenter: the point on ground
surface directly above the focus. It
may be any geographic locality.
Seismic waves
• 2 types
–Body waves: traveling inside the
earth
–Surface waves: travel on the
earth’s surface.
• Body waves:
–P-waves (primary)
–S-waves (secondary)
• P-wave: is the fastest (4-7 km/sec)
and is the first to arrive at a
recording station.
• P-wave: compressional or
longitudinal wave in which rocks
vibrate back and forth parallel to
the direction of wave propagation.
• S-wave: slower (2-5 km/sec) and
arrives after the P-wave at a
recording station.
• It’s a transverse wave in which the
rock vibrates perpendicular to the
direction of wave propagation.
• Surface waves:
–slowest waves generated by an
earthquake.
–They take longer to pass and
cause more property damage.
EQ measurement
• The instrument used to measure
an EQ is called a seismometer.
• Seismograph: paper or electronic record of
an EQ.
World seismograph stations
Locating EQ Epicenter
• Travel time of the seismic waves
from the focus to the seismograph
station is used to determine the EQ
epicenter.
• P and S waves gradually separate
because they travel at different
velocities.
• The P-S interval is compared with a
standard travel-time curve.
• Data from one station can be used
to determine only distance to the
station and not the direction.
• At least three stations are required
to determine the location of the
epicenter.
Measuring the size of an EQ
• Two ways of determining the EQ
size;
–Intensity
–Magnitude
Measuring the size of an EQ
• EQ Intensity: it is a measure of
how an area has been damaged by
the EQ.
• Modified Mercalli Scale is used to
measure intensity.
• EQ magnitude: it’s a measure of
the amount of energy released by
an EQ.
• EQ magnitude is reported on the
Richter scale.
• It is done by measuring the height
(amplitude) of a specific wave.
• ML = log10A(mm) + (Distance
correction factor)
Magnitude (log Scale)
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Possible Effects
Normally only detected by instruments
Only faintly felt
Faint tremor causing little damage
Structural damage
Distinct shaking, less well-constructed
buildings collapse
Most buildings destroyed
Major structures destroyed
Ground seems to shake-catastrophic
Effects of EQ
• Primary effects: occur immediately
from ground shaking
• Secondary effects: damage caused
by;
Effects of EQ
–Floods
–Fire
–Landslides
–Liquefaction: wet, saturated soil
changes from solid to liquid as a
result of shaking.
–Tsunamis
Global distribution of EQ
• 3 belts;
–Circum-Pacific belt
–Mediterranean-Himalayan belt
–Continental interior
Mitigating EQ effects
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Avoiding EQ prone zones
Foundation design
Structural modifications
Building codes
Predicting EQ
• Rock monitoring: rock properties
begin to change before its failure.
• Water levels: increase or decrease.
• Radon emission: increases before
an EQ.
Predicting EQ
• Seismic gap: regular pattern of EQ
occurrence.
• Animal behavior: snakes, dogs.
Assignment 2
Rivers, groundwater, lakes
Ohio EPA
Great lakes, lake erie
Relative proportions
Maximum Contaminant Level
Primary contaminants
Secondary