ENGINEERING GEOLOGY
SPRING 2017
LECTURE SEVEN AND EIGHT:
EARTHQUAKES
PREPARED BY
DR. KHAYYUN A. RAHI
DEPARTMENT OF
ENVIRONMENTAL ENGINEERING
APRIL 5TH AND 6TH, 2017
OUTLINE
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Mechanics of EQ
Seismic waves
Seismology and Seismograph
Locating the sources of EQ
Measuring the size of EQ
EQ Destruction
EQ Prediction
EARTHQUAKES: NORTHRIDGE, CA,
USA, 1994
EARTHQUAKES IN THE PAST
WHAT IS AN EARTHQUAKE
 An earthquake is the
vibration of Earth produced
by the rapid release of
energy
 Energy released radiates
in all directions from its
source, the focus
 Focus - the place within
Earth where earthquake
waves originate
 Epicenter – location on
the surface directly above
the focus
Earthquake focus and epicenter
MECHANISM OF EARTHQUAKE GENERATION
 Elastic rebound
– Mechanism for
earthquakes was first
explained by H.F. Reid
» Rocks on both sides of
an existing fault are
deformed by tectonic
forces
» Rocks bend and store
elastic energy
MECHANISM OF EARTHQUAKE GENERATION
 Elastic rebound

– Slippage at the weakest
point (the focus) occurs
– Vibrations (earthquakes)
occur as the deformed
rock releases the stored
energy (elastic rebound)
Earthquakes most often
occur along existing faults
whenever the frictional forces
on the fault surfaces are
overcome
TYPES OF SEISMIC WAVES
 Body waves
 Primary (P) waves
– Push-pull (compress and expand) motion,
changing the volume of the intervening material
– Travel through solids, liquids, and gases
TYPES OF SEISMIC WAVES
 Body waves
 Secondary (S) waves
– Slower velocity than P waves
– Slightly greater amplitude than P waves
– Second to appear at recording station
TYPES OF SEISMIC WAVES
 Surface waves
– Travel along outer part of
Earth
– Complex motion
– Cause greatest destruction
– Waves exhibit greatest
amplitude and slowest
velocity
– Waves have the greatest
periods (time interval
between crests)
– Often referred to as long
waves, or L waves
SEISMOLOGY
 The study of earthquake waves,

seismology, dates back almost
2000 years to the Chinese
Seismographs, instruments that
record seismic waves
– Records the movement of
Earth in relation to a stationary
mass on a rotating drum or
magnetic tape
ancient seismograph
modern seismograph
PRINCIPLE OF OPERATION OF THE SEISMOGRAPH
Seismograph: Device used to detect and
record seismic waves based on the
inertia of a suspended mass
Consists of:
1) seismometer - instrument for detecting
seismic waves
2) seismogram - the record of seismic
waves
 horizontal mode- records
horizontal vibrations (E-W and NS directions)
 vertical mode- records vertical (up
and down motions)
LOCATING THE SOURCE OF EARTHQUAKES
 Terms

– Focus - the place
within Earth where
earthquake waves
originate
– Epicenter – location
on the surface
directly above the
focus
Epicenter is located
using the difference in
velocities of P and S
waves
P-wave arrived 50 s before S wave
LOCATING THE SOURCE OF EARTHQUAKES
 Three or more
station recordings
are needed to locate
an epicenter
– 1) First determine the
difference in arrival
times between the P
and S waves for each
station from your
seismic record called
a seismogram
LOCATING THE SOURCE OF EARTHQUAKES
– 2) Next a travel-time
graph is used to
determine each
station’s distance to
the epicenter
LOCATING THE SOURCE OF EARTHQUAKES
– 3) A circle with a
radius equal to the
distance to the
epicenter is drawn
around each station
– The point where all
three or more circles
intersect is the
earthquake epicenter
MEASURING THE SIZE OF EARTHQUAKES
 Intensity – a measure of
the degree of
earthquake shaking at a
given locale based on
the amount of damage
MEASURING THE SIZE OF EARTHQUAKES
 Intensity scales
– Modified
Mercalli
Intensity Scale
was developed
using California
buildings as its
standard
 The drawback of intensity scales is that destruction may
not be a true measure of the earthquakes actual severity
– Very subjective – based on eye witness accounts
– Bedrock geology can influence the amount of damage
caused
MEASURING THE SIZE OF EARTHQUAKES
Zones of destruction associated
with the Loma Prieta EQ (1989)
using the Modified Mercalli scale
Zones of destruction
determined using the
Modified Mercalli scale
MEASURING THE SIZE OF EARTHQUAKES
 Richter scale
– a measure of the amount
of ground motion during
an earthquake
– Magnitudes less than 2.0
are not felt by humans
– Each unit of Richter
magnitude increase
corresponds to a tenfold
increase in wave
amplitude and a 32-fold
energy increase
Graphical determining the
Richter magnitude of an EQ
RICHTER SCALE
The Richter scale is based on a standard
measurement: an earthquake that can be
felt 100km away with amplitude of 1mm is
given a magnitude measurement of 3.0.
This is the base measurement and all
other measurements of magnitude are
made to this reference. As a result, an
earthquake that is 100km away, but has
an amplitude measurement of 10mm
would measure 4.0. The following graph
shows this relationship and outlines the
base reference standard at magnitude 3.0.
A straight line is drawn from the distance
measurement to the amplitude measurement
of the three seismographs. The three lines
should all meet at a single point on the
magnitude scale in the middle, giving a
magnitude reading for the earthquake.
EARTHQUAKE BELTS
 About 95 percent of the energy released by earthquakes

originates in a few relatively narrow zones that wind
around the globe
Major earthquake zones include the Circum-Pacific belt,
Mediterranean Sea region to the Himalayan complex, and
the oceanic ridge system
Distribution of
magnitude 5 or
greater
earthquakes,
1980 - 1990
EARTHQUAKE DEPTHS
 Earthquakes originate

at depths ranging from
5 to nearly 700
kilometers
Earthquake foci
arbitrarily classified as
shallow (surface to 70
kilometers),
intermediate (between
70 and 300 kilometers),
and deep (over 300
kilometers)
EARTHQUAKE DESTRUCTION
 Amount of structural
damage attributable to
earthquake depends on
– Ground shaking
– Movement/displacement
along faults
– Proximity to
focus/epicenter
EARTHQUAKE DESTRUCTION
 Amount of structural
damage attributable to
earthquake vibrations
depends on
– Intensity and duration of
the vibrations (ground
motions)
– Nature of the material
upon which the structure
rests
– Design of the structure
EARTHQUAKE DESTRUCTION
 Nature of the material
upon which the
structure rests
Greater vibration on
unconsolidated sediments
Mexico City is built on dried up
lake bed
EARTHQUAKE DESTRUCTION
 Secondary: Liquefaction of
the ground
– Unconsolidated materials
saturated with water turn
into a mobile fluid
Tilted building rests on
unconsolidated sediments
EARTHQUAKE DESTRUCTION
 Secondary: Landslides
and ground subsidence
Turnagain Heights slide caused by
the 1964 Alaskan EQ
200 m of the Turnagain Heights bluff
area slid into the ocean
EARTHQUAKE DESTRUCTION
 Secondary: Fires
Arnold Genthe's famous
photograph of San Francisco
following the earthquake, looking
towards the fire on Sacramento
Street
http://en.wikipedia.org/wiki/1906_
San_Francisco_earthquake
EARTHQUAKE DESTRUCTION
 Secondary: Tsunamis, or seismic sea
waves
– Destructive waves that are often
inappropriately called “tidal waves”
– Result from vertical displacement
along a fault located on the ocean
floor or a large undersea landslide
triggered by an earthquake
– In the open ocean height is usually less
than 1 meter
– In shallower coastal waters the water
piles up to heights that occasionally
exceed 30 meters
CAN EARTHQUAKES BE PREDICTED?
 Short-range predictions

» Currently, no reliable method exists for making
short-range earthquake predictions
Long-range forecasts
» Give the probability of a certain magnitude
earthquake occurring on a time scale of 30 to
100 years, or more
» Based on the premise that earthquakes are
repetitive or cyclical
» Using historical records or paleoseismology
CAN EARTHQUAKES BE PREDICTED?
 Long-range forecasts
– Based on the premise
that earthquakes are
repetitive or cyclical
– Using historical records
or PALEOSEISMOLOGY
– SEISMIC GAPS have a
higher probability of
next EQ
NORTH
ANATOLIAN
SLIP HISTORY
Can earthquakes be predicted?
 Short-range predictions
» Currently, no reliable method exists for making
short-range earthquake predictions
» Seeks to determine geographic area, expected
magnitude, and probable time of occurrence
1) monitoring of foreshocks
2) establishing a network of seismographs
3) landform studies ( fault movement, uplift, or
subsidence)
4) release of radon gas
5) animal behavior
Can earthquakes be predicted?
How successful are the predictions?
 Location? Yes for the most part
 Magnitude? Yes for the most part
 Time? No! (in Years or decades maybe)
END OF LECTURES 7
AND 8