Elastic rebound theory states that the waves of energy from an earthquake result from the sudden release of stored up strain energy in rock as it deforms. When the rock ruptures the rock on either side of a fault snaps suddenly to a new position, releasing the stored up strain energy in the process.
Earthquakes and Their Damages: Shaking Ground,
Earthquake Prediction and Tectonic Environment
Earthquakes are vibrations of
the earth caused by the
rupture and sudden movement
of rocks that have been
strained (deformed) beyond
their elastic limit. The forces
that cause deformation and
the build-up of strain energy in
the rock are referred to as
Earthquakes occur along
faults. Faults are fractures
in the lithosphere where
regions of rock move past each
• The focus is the
point on the fault
where rupture occurs
and the location from
which seismic waves
• The epicenter is the
point on the earth’s
above the focus.
• When the fault
ruptures, waves of
energy called seismic
waves spread out in
Types of Faults
Faults can be divided depending on the direction of relative
displacement. Relative displacement is largely a function of the type
of tectonic stress the rock is under.
There are 2 main categories.
• Dip-slip faults - where the
displacement is vertical
• Strike-slip faults - where the
displacement is horizontal. 7
Dip-Slip Faults - Normal Faults
• Normal faults result from
tensional stresses along divergent
• The hanging wall block moves down
relative to the footwall block.
• Low Richter magnitudes due to
the tendency of rocks to break
easily under tensional stress.
• Shallow focus (less than 20 km)
because the lithosphere is
relatively thin along diverging
Examples - all mid-ocean ridges;
Continental Rift Valleys such as the
basin and range province of the
Western U.S. and the East African
Left: Fault scarp near Hebgen Lake,
Montana, after the magnitude 7.1
earthquake of August 18, 1959, shows a
displacement of 5.5 to 6.0 m.
Right: This section of the normal fault
scarp was produced by the earthquake of
October 28, 1983, at Borah Peak, Idaho.
Left: Dixie Valley-Fairview Peaks, Nevada
December 16, 1954
Normal Fault Examples
Normal Fault Example –
The Basin and Range of Nevada, Utah, and Adjacent
The Faults of the Basin and Range
occupy a spreading zone
accompanying the Northwest drag of
the Pacific Plate against the North
American Plate, which moves slightly
south of west.
Lower Right: The
Wasatch Front is a high
fault scarp east of the
Salt Lake basin. There has
not been an earthquake of
any consequence since the
founding of Salt Lake city
in 1847. However, the
Salt Lake City area should
still consider themselves
at high risk for major
Dip-Slip Faults - Reverse Faults
• Reverse faults result from
compressional stresses along
• The hanging wall block has moved
up relative to the footwall block.
• A Thrust Fault is a special case
of a reverse fault where the dip
of the fault is less than 15o
• There are two types of
converging plate boundaries.
1. Subduction boundaries
2. Collision boundaries
• At subduction boundaries there is a continuum of stress along the
subducting plate. Shallow focus earthquakes can be generated near
the trench, but focal depths can reach down to 700 km as
earthquakes are generated along the subducting plate.
• Rocks are strong under compression and can store large amounts of
strain energy before they rupture. Therefore, these earthquakes
can be very powerful.
– 1960 Southern Chili = 9.5
– 1964 Alaska = 9.2
On May 22, 1960 the
largest earthquake on
record struck the coast of
Chile with a Mw of 9.5. The
earthquake ruptured along a
1,000 km length of the
subduction zone. In Chile,
the earthquake and the
tsunami that followed took
more than 2,000 lives. From
Chile the tsunami radiated
outward, killing 61 people in
Hawaii and 122 in Japan.
Left: Stuck to the
subducting plate, the
overriding plate gets
Right: An earthquake along a
subduction zone happens
when the leading edge of the
overriding plate breaks free
and springs seaward, raising
the sea floor and the water
above it. This uplift starts a
• At collision boundaries two plates of continental lithosphere
collide resulting in fold-thrust mountain belts.
• Earthquakes occur due to the thrust faulting and range in depth
from shallow to about 200 km.
Reverse Fault Example – Collision Zones
Bam, Iran 2003
Earthquakes in Iran
regions (e.g., Turkey
and Afghanistan) are
closely connected to
their position within
the active Alpine-
Himalayan belt near
the convergence of
the Arabian and
Below: Before and after
pictures of the 2,000 yr.
old citadel in the city of
Bam, the largest mud-brick
structure in the world.
The Mw 6.5
nearly 80% of
Strike-Slip Faults - Transform Faults
• Strike-slip faults result from shear stresses acting on the
lithosphere along transform boundaries.
• Horizontal motion can be right lateral or left lateral.
• Earthquakes along these boundaries tend to be shallow focus with
depths usually less than about 100 km. Richter magnitudes can be
Transform Fault Example
The San Andreas Fault
The San Andreas Fault is the main strand of
a zone of parallel faults resulting from
interaction between the relative northwest
movement of the Pacific Plate compared with
the North American Plate.
These are earthquakes that occur in the stable portions of continents.
Many of them occur as a result of re-activation of ancient faults,
although the causes of some intraplate earthquakes are not well
understood. Refer to pages 42-44.
Charleston, South Carolina, 1886
Mw = 7.3, Mercalli Intensity of IX
Right: The Charleston Earthquake resulted from
movements along a segment of the East Coast Fault
system, a series of faults trending Northeast near the
boundary between continental crust and Atlantic
Oceanic crust. The ancient faults are believed to be
associated with the early stages of opening of the
Atlantic Ocean 200 mya.
• Body waves travel
through the interior
(body) of the earth as
they leave the focus.
They include P-waves
• Surface waves travel
parallel to the earth’s
surface. They are the
slowest and most
damaging. They include
Love and Rayleigh
• “Primary” waves – fastest. Arrive at
distant locations first.
• Push-pull waves - Compress and
extend in the direction of wave
• “Secondary” waves – second fastest.
Arrive at distant locations second.
• Shear waves – Travel at right angles
to the direction of wave travel.
Surface waves travel parallel to the earth’s surface.
They are the slowest, involve the greatest ground
motion, and are therefore most damaging.
• Love Waves - complex, horizontal motion
• Rayleigh Waves - Rolling or elliptical motion. 22
Seismographs are instruments that
detect and record ground shaking
produced by earthquake waves.
Due to their different speeds, the
different waves arrive at the
seismograph at different times:
first P-waves arrive, then S-waves,
then surface waves.
Seismogram - the record
of an earthquake as
recorded by a
seismograph. It is a plot
of vibrations versus time.
Locating the Epicenter Via Seismograms
• P-waves are faster
than S-waves, and the
time gap between their
arrival at a
precisely with distance
from the focus.
• Basically, the lag time
between the arrival of
your first recorded P-
wave and first
recorded S-wave is
Locating the Epicenter Via Seismograms
We can use the lag time
between the P-waves and
S- waves to calculate the
distance to an earthquake!
If we do this for a
minimum of three
different seismic stations,
we can precisely locate the
epicenter. In the figure,
each circle has a radius
equal to the distance to
the earthquake from three
separate seismic stations.
All three circles intersect
at only one point -- the
Richter Magnitude Scale
- ML; based on the highest
amplitude wave measured on
a seismogram, corrected for
distance from the
seismograph to the epicenter
- ranges from 1.0 (smallest) to
infinity, but 9.0 is typically
the highest possible value
for an earthquake.
- logarithmic scale: each whole
unit on the Richter scale
represents a ten-fold
increase in wave amplitude
(ground shaking) and an ~
thirty fold increase in the
Above: The magnitude of the earthquake can be estimated
using an earthquake nomograph, on which a straight line is
plotted between the P-S time (distance) and the maximum
wave amplitude. This line intersects the central line at the
approximate magnitude of the earthquake.
The Local Magnitude Scale developed by Richter was
strictly valid only for certain frequency and distance
ranges. Therefore new magnitude scales were developed
all calibrated to Richter's original method . These
include body-wave magnitude, MB, and surface-wave
magnitude, MS, each valid for a particular frequency
Because of the limitations of all three magnitude scales, a
new scale, known as moment magnitude, or MW, was
developed. MW is a measure of the seismic moment, or
total energy expended during an earthquake. MW depends
on the rock strength, area of rock broken, and amount of
offset across the fault.
Modified Mercalli scale
based on people’s reported
perceptions of shaking
(subjective), and the type and
extent of damage produced
-ranges from I (not felt by
people) to XII (catastrophic
An example of the Modified
Mercalli scale follows in the
next slide. 30
Right: A map of Modified
Mercalli intensities for the 1994
Left: TriNet ShakeMap showing
the distribution of shaking during
the 1994 Northridge Earthquake.
Modified Mercalli Scale Example:
The 1994 Northridge, California Earthquake
Modified Mercalli Scale vs. The Richter Scale
Not all fault movements result in violent
earthquakes. Some faults move slowly
and fairly continuously, a movement
called fault creep.
Fault creep never killed anyone, but, as
shown in these pictures, it can cause
damage to roads or other structures.
“earthquakes don’t kill people, buildings do”.
Earthquake Damage Susceptibility Depends on
Magnitude of the earthquake - the higher the magnitude, the more intense the shaking,
the longer the duration of shaking, and the greater the displacement. Refer to slides
Distance from the epicenter – Seismic waves attenuate (amplitude diminishes) with
distance. Refer to slide 38.
Surface Geology – Surface Faulting and ground rupture, and soil amplification. Refer to
Integrity of Structures and Type of Construction - Building codes; Building material:
concrete/masonry vs. wood/steel. Structural Integrity. Refer to slides 43-53.
Integrity of Utilities.
Population Density, building density, time of day, etc. - the more people and buildings,
the greater the potential for structural damage and death. 34
Earthquake Magnitude and Ground Acceleration
Ground acceleration is the rate of increase in velocity, or the strength of the
shaking. During an earthquake, the ground accelerates from being stationary
to a maximum velocity before slowing and reversing its movement.
Acceleration is normally designated as some proportion of the acceleration due
to gravity (g). 1.0 g is the acceleration felt by a freely falling body.
Earthquake Magnitude and Shaking Time
Duration of Strong Ground Shaking in
8 – 8.9 30 – 90
7 – 7.9 20 – 50
6 – 6.9 10 – 30
5 – 5.9 2 – 15
4 – 4.9 0 - 5
An increase in magnitude significantly increases the time
of shaking, and the potential damage.
Magnitude and Fault Displacement
Above: Graph of the relationship between
maximum displacement and earthquake
magnitude on all types of faults.
all types of
The amount of displacement during
fault movement and the length of
surface rupture are generally
proportional to the magnitude of
the accompanying earthquake.
Distance from the Epicenter
Seismic waves attenuate with distance.
Ground rupture and surface faulting occur due
to horizontal or vertical displacement of faults
that break the surface. Damage can result
from the ground shifting upward (called uplift)
or downward (called subsidence).
Above: The photo shows a
fault scarp -- a cliff
created by movement along
a fault. This scarp formed
during the 1992 Landers,
Left: The 1999 Taiwan earthquake
caused ground movement over a fault
rupture of 50 miles. The photo shows a
fresh scarp cutting across the running
track at a local high school.
Surface Faulting and Ground
Land Uplift and Subsidence
Shaking is amplified as waves travel from solid bedrock to
unconsolidated sediment to water-saturated sediment.
Above: Buildings in Niigata,
Japan, fell over when the
sediments below them liquefied
during the 1964 earthquake
The figure below shows how liquefaction can
occur. Shaking of water-saturated soils causes
the particles to settle, driving the water out
from between the particles and forcing it
upward, thus liquefying the areas above.
Liquefaction is a quicksand like condition that occurs in water-
saturated soil and rock. The shaking of earthquake waves causes the
soil or rock to turn into a weak, fluid-like mass. Structures built on
areas that liquefy may fall over or sink.
Vibration of water saturated sediment can force water into pore
spaces between sediment grains reducing friction and permitting the
mass to slide down slope.
Integrity of Structures and Type of
The Uniform Building Code
provides a seismic zonation map
that indicates the level of
construction standards for
•Concrete and masonry
structures are brittle and thus
more susceptible to damage.
•Wood and steel structures are
more flexible and thus less
susceptible to damage.
Most damage and collapse of structures occurs due to sideways movement of
the ground from earthquake waves. This process is called horizontal ground
acceleration, or base shear.
•Base shear causes the building to deform from a rectangle into a parallelogram.
• Base shear causes buildings constructed on so-called “cripples” -- short
walls that raise the building up from its foundation -- to fall sideways.
• The most deadly type of failure from base shear is “story-shift”, in which the
sideways acceleration causes floors to shift resulting in the collapse of either
individual floors or the whole building -- a situation called pancaking. Few or no
occupants survive such collapses.
Mitigation of Base Shear
• Shear walls: diagonal braces or plywood sheeting built into the
walls, to keep the building from deforming during base shear.
• Bolting to the foundation, so the building does not slide off.
Basement of Veterans
Administration Medical Center, Long
Mitigation of Base Shear
• Base isolation: putting the building on large rubber pads, rolling
wheels, or slippery Teflon plates. This allows the ground to move
under the building, thus isolating the building somewhat from
The photograph shows a seismic
joint (the dark vertical stripe of
rubbery material) between two
halves of a building.
Mitigation of Base Shear
• Seismic joints: areas of flexible material, like rubber, form
connections between different parts of a building. These allow
the separate areas of the building to shake independently.
• The building practices
described on the last
few slides make a huge
difference in quake
survival! The bar graph
shows us the connection
between year of
construction and amount
of damage to buildings
during the 1995 Kobe,
Japan, earthquake. More
recent buildings were
built to stricter codes,
and thus fared much
better during the quake.
Left and Right:
• In addition to buildings, highway overpasses, bridges, and multi-decked
freeways also suffer major damage from base shear, most commonly due to
the failure of the concrete supporting columns.
Mitigation of Base Shear
New construction practices to reduce failure of highway overpasses
• 1. Horizontal rebar wrapping added when casting concrete columns
• 2. Retro fitting existing columns with steel jackets to make them
stronger and more flexible (right below).
Building Vibration and
heights will sway
collide during an
Buildings have natural vibration
frequencies in the same range as
earthquake waves. Shaking of the
building will be amplified when the
frequency of the building is close to
the frequency of the waves produced
by the earthquake.
• Aftershocks are earthquakes
that follow the largest shock of
an earthquake sequence. The
main earthquake changes the
stress pattern in areas around
the epicenter, and the crust
must adjust to these changes.
• Aftershocks can continue over a
period of weeks, months, or
years. In general, the larger the
main shock, the larger and more
numerous the aftershocks, and
the longer they will continue.
Photos show an uncontrolled fire in San
Francisco after the 1989 Loma Prieta
• Fires commonly break out during quakes due to ruptured gas lines or downed
electrical lines. Impassible roads and ruptured water mains compound the
problem. In some urban quakes, fires have caused more damage than the ground
Some of the Most Catastrophic Earthquakes in Terms
The largest earthquakes do not necessarily kill the most people. Most of the
high death counts come from countries notable for poor building construction
or unsuitable building sites.
• Based on knowledge of earthquakes and the tectonic
environments that control them scientist can make reasonable
forecasts about where and how large an earthquake will occur at
some future time.
• However they have been less successful in finding ways to make
short term predictions. Earthquakes appear to be inherently
• Nevertheless, great research effort has gone toward finding a
reliable system for short term earthquake prediction. One main
avenue of research has been the study of earthquake
Earthquake Prediction / Forecasting
• Ground deformation: Measurements taken in the vicinity of active
faults sometimes show that prior to an earthquake the ground is
uplifted or tilts due to the strain building on the fault.
• Foreshocks: Small earthquakes that precede a large quake by a few
seconds to a few weeks. The pattern and intensity of foreshocks
usually increase in magnitude and may cluster or migrate down a fault to
the place where the main shock will eventually occur.
• Water Level in Wells: As rocks become strained in the vicinity of a
fault, new fractures may form causing a change in the path of
groundwater subsequently causing changes in the water levels in wells.
• Emission of Radon Gas - Radon is an inert gas that is produced in the
radioactive decay of uranium. Radon is inert and remains in rock until
some event forces it out. Deformation resulting from strain may form
fracture in the rock which could serve as pathways for the Radon to
escape into groundwater resulting in background radioactivity in wells.
• Abnormal Animal Behavior.
The only successful prediction of a major earthquake was the
1975 magnitude 7.3 Haicheng Earthquake in northeastern China.
The prediction was based on
•bulging of the ground surface near the fault,
•changes in groundwater levels,
•and strange animal behavior.
Based on these observations, over 1 million people were ordered
to evacuate and remain outside in the winter cold. The
earthquake collapsed more than 90% of the houses and, were it
not for the prediction, would have killed hundreds of thousands
Earthquake Prediction / Forecasting
No reliable short-term precursors have been found. Therefore
research today focuses on longer-term warnings or forecasts.
In this approach, geologists attempt to identify regions where
large earthquakes are likely to occur within the next several
years or decades. While this does not provide short-term
warnings, it is useful for long-range planning for building codes
and emergency response services.
Since 1939, there have been
seven earthquakes measuring
over magnitude 7.0 along the
North Anatolian Fault in
Turkey, each occurring at a
point progressively further
west. By analyzing the
stresses caused along the
fault by each earthquake,
they were able to forecast
the 1999 Izmut earthquake.
It is thought that an
earthquake will soon strike
further west along the fault,
possibly in the heavily
populated city of Istanbul.
A few faults show a sequential migration of earthquakes along the
fault with time believed to be the result of progressive earthquake
failure . For faults that follow this pattern it is believed that one
earthquake triggers the next.
A seismic gaps is a
section of an active fault
that has not had a recent
earthquake. The theory
here is that if a portion
of a fault has been
“locked” for some time
(i.e. has not had an
earthquake in a long
time), then strain may
have built up to especially
high levels there, and a
large quake may occur in
the near future.
For nearly 150 years, the San
Andreas Fault moved at regular
intervals near Parkfield
generating earthquakes of ~
magnitude 6 at intervals averaging
about 22 years. This series
prompted seismologist to predict
that there was approximately a
95% chance that the next M6
earthquake would occur between
1985 and 1993. So the USGS
installed a wide variety of
expensive instruments around the
Parkfield area. There, however
was no M6 earthquake in this
region until September 28, 2004.
Recurrence interval is a statistical estimate of the expected time
interval between an event of a given magnitude. It is a statistical
The theory is that faults should behave in the future like they have
behaved in the past, producing a characteristic number of quakes of
particular sizes over a given time interval.
This map, based on
the history of
particular faults in the
San Francisco region,
shows the predicted
probabilities of one or
more magnitude 6.7 or
occurring on these
faults by 2032.
Older sedimentary layers are offset more because they have experienced a
series of fault movements. Radiocarbon dating of organic layers broken along
the fault can reveal the maximum dates of fault movements.
Paleoseismology is the study of the long-term geologic history of
faults to determine their earthquake history and possible future
activity. The methodology assumes that the earthquakes produced
by the fault have a characteristic recurrence interval.