Earthquake Engineering notes- module 1 ppt

Earthquakes
• Causes - tectonics and faults
• Magnitude - energy and intensity
• Earthquake geography
• Seismic hazards - shaking, etc.
• Recurrence - frequency and regularity
• Prediction?
• Mitigation and preparedness

Causes: accumulated strain
leads to fault rupture
- the elastic rebound model

North American tectonic regimes
(much simplified)

The earth’s crust is constantly experiencing pressure from forces
within and around it. This pressure builds up over time, and
eventually causes the crust to break. This becomes a fault.
Causes:
fault movement releases
energy as seismic waves
radiating from rupture
Seismic
waves

3. Shear stress causes rocks
to slide past each other
resulting in strike-slip faults
2. Compression squeezes
rock together resulting in
reverse faults
1.Tension pulls rocks
apart resulting in
normal faults
Fault Types
(Credit: U.S. Geological Survey
Department of the Interior/USGS)
Fault surfaces are surfaces along which rocks move under, over, or
past each other. Rocks may get “stuck” along the fault surface, causing
a build-up of strain energy, and resulting in an earthquake when the
rocks break free of each other. There are 3 types of stress that can
affect rocks, resulting in 3 different types of faults:

Faults are divided into three main groups:
- when two plates are moving apart and one
side of the fracture moves below the other; (caused by
tension forces!)
Normal

- when two plates collide and one side of the
fracture moves on top of another; (caused by compression
forces!!)
- when two plates slide past each other. (caused
by shear forces!)
Reverse
Strike-Slip

 Faults caused by blocks of crust pulling apart under the forces of tension are called normal
faults. Entire mountain ranges can form through these processes and are known as fault block
mountains (examples: Basin and Range Province, Tetons).
 In a normal fault, the hanging-wall block moves down relative to the foot-wall block.
 The footwall is the underlying surface of an inclined fault plane.
 The hanging wall is the overlying surface of an inclined fault plane.
Faults: Normal Faults
Relative movement of two blocks indicating a
normal fault. (Credit: Modified after U.S. Geological
Survey Department of the Interior/USGS)
Diagrammatic sketch of the two types of
blocks used in identifying normal faults.
Hanging
Wall
Foot Wall
Footwall
block
Hanging
wall block

 Faults caused by blocks of crust colliding under the forces of compression are called reverse
faults.
 Reverse faults are a prevalent feature in continent-continent collisions. Usually, there is also
accompanying folding of rocks.
 During reverse faulting, the hanging wall block moves upward (and over) relative to the
footwall block.
Faults: Reverse Faults
Hanging
Wall
Foot Wall
Footwall
block
Hanging
wall block
Relative movement of two blocks indicating a
reverse fault. (Credit: U.S. Geological Survey Department of
the Interior/USGS)
Diagrammatic sketch of the two
types of blocks used in identifying
reverse faults.

In the example above, the fence has been offset
to the right, therefore it is called a right lateral
strike-slip fault. (Credit: U.S. Geological Survey Department of
the Interior/USGS)
 Strike-slip faults occur when two blocks move in
horizontal but opposite directions of each other.
 Depending on the direction of offset, it can be a
“right-lateral offset” or a “left-lateral offset.”
The photograph above displays a light-
colored pegmatite vein offset to the right
in a schistose matrix. Photo courtesy of K.
McCarney-Castle.
Faults: Strike-slip faults
Right-lateral offset

How is direction of fault motion determined?

“First motion” studies can determine the type
of fault that produced an earthquake

Seismicity and Plate Boundaries

Convergent Plate Boundaries
• Deep earthquakes
typically are found only
at convergent plate
boundaries
• Shallow earthquakes
also occur at these
boundaries

Intraplate Earthquakes
• Earthquakes also occur distant from plate boundaries, typically with shallow-
foci
• Some of the most destructive earthquakes (e.g. New Madrid) have been
intraplate, but may be associated with ancient plate boundaries. Their cause is a
mystery.

Potential Earthquake Hazards, USA

Severity of damage is controlled by more than just earthquake
magnitude - Ground material properties also affects Shaking
Amplitude: bedrock is better than loose soil

Folding
 During mountain building processes rocks can undergo folding as well as faulting.
 Sometimes rocks deform ductilely, particularly if they are subjected to heat and pressure. At
elevated temperature and pressure within the crust, folds can form from compressional forces.
 Entire mountain rages, like the Appalachians, have extensive fold systems.
 The conditions of whether a rock faults or folds vary with temperature, pressure, rock
composition, and strain rate. In the same location, some rocks can fold while others fault. Often
folding is just a precursor to faulting.
Z-fold in schist with white felsic dike (hammer for
scale). Near Lake Murray, S.C. Photo courtesy of K.
McCarney-Castle
Large fold in outcrop (geologists for scale). Near
Oakridge, Tennessee, Appalachian Mtns. Photo courtesy
of K. McCarney-Castle.

Measuring earthquakes
• Seismometers:
instruments that detect
seismic waves
• Seismographs
Record intensity, height
and amplitude of seismic
waves

Seismographs
• Seismic waves
are recorded by
seismographs –
now mostly digital
recordings
• Record both
horizontal and
vertical earth
movements

Measuring Earthquakes
• The movement of materials in the outer core (which is a liquid) of the
Earth is inferred to be the cause of Earth’s magnetic field.
• A compass needle will align with the lines of force of Earth’s magnetic
field. Iron and Nickel are metals that easily magnetize, and are
inferred to be the metals in Earth’s core.
• The energy spreads outward in all directions as vibrations called
Seismic Waves. Seismic waves can be measured and recorded by a
seismograph.
• Seismographs are instruments or a device that detects and records
seismic or earthquake waves. It measures the vertical ground motion
and the horizontal ground motions (N-S/E-W). It also traces wave
shapes onto paper and translates waves into an electronic signal.
• The vibration record, called a seismogram, looks like jagged lines on
paper. Seismograms are traces of amplified, electronically recorded
ground motion made by seismographs.
• Measuring the time between the arrival of the P and S waves
determines the distance between the recording seismograph and the
earthquake epicenter.

Earthquake Waves
Measuring Earthquakes
 Seismographs are instruments that
record earthquake waves.
 Seismograms are traces of amplified,
electronically recorded ground motion
made by seismographs.

Seismic waves: properties
• Velocity: function of the physical properties of the
rock the wave is traveling through
– Velocity increases with rock density
– Velocity changes when passing from one material
to another (increases/decreases)
– Liquids: S-waves do not get transmitted through
liquid; P-waves slow down
• Why is this important?
–If we know the velocity of the wave, we can infer
the type of rock it traveled through- that’s how we map
the interior of the Earth!!!

 Seismic waves are generated by the release of energy during an earthquake. They
travel through the earth like waves travel through water.
 The location within the Earth where the rock actually breaks is called the focus of
the earthquake. Most foci are located within 65 km of the Earth’s surface; however,
some have been recorded at depths of 700 km. The location on the Earth’s surface
directly above the focus is called the epicenter.
 The study of seismic waves and earthquakes is called seismology, which is a branch
of geophysics.
Seismic Waves
Two types of seismic waves are generated at the earthquake focus:
1. Body waves spread outward from the focus in all directions.
2. Surface waves spread outward from the epicenter to the Earth’s surface, similar to
ripples on a pond. These waves can move rock particles in a rolling motion that
very few structures can withstand. These waves move slower than body waves.

There are two types of Body Waves:
1. Primary Wave (P wave):
• Compressional wave (travels in the same direction the waves move).
• A type of seismic wave that compresses and expands the ground.
• Very fast (4-7 km/second)
• Can pass through a fluid (gas or liquid)
• Arrives at recording station first
Example: A slinky.
Seismic Waves, continued

2. Secondary Wave (S wave):
• Transverse wave (travels perpendicular to the wave movement).
• A type of seismic wave that moves the ground up and down or side
to side.
• Slower moving (2-5 km/second)
• Caused by a shearing motion
• Cannot pass through a fluid (gas or liquid)
Example: Shaking a rope.

Earthquake Engineering notes- module 1 ppt

Primary or
“P” Wave
Secondary
or “S”
Wave

 Surfaces waves are produced when earthquake energy reaches the Earth’s
surface. Surface waves moves rock particles in a rolling and swaying
motion, so that the earth moves in different directions.
 These are the slowest moving waves, but are the most destructive for
structures on earth.
Diagram representing the damaging back-
and-forth motion of a surface wave.
(Credit: McGraw Hill/Glencoe, 1st ed., pg. 163)

Types of Seismic Waves
• Seismographs detect three main wave types:
– P Waves (primary waves) the fastest of the two body types (~6-
8 km/s); they are the first to arrive
– S Waves (secondary or shear waves) body waves travel
typically half as fast as P-waves
– Surface Waves travel at and near the air-earth interface, are
the slowest and last to arrive, travel at speeds lower than shear
waves.

Seismic wave forms
P wave
S wave
L wave
(Rayleigh wave)
L wave
(Love wave)

P waves (compressional) 6–8 km/s. Parallel to direction of movement
(slinky), also called primary waves. Similar to sound waves.

S waves (shear) 4–5 km/s. Perpendicular to direction of movement
(rope), also called secondary waves. Result from the shear strength of
materials. Do not pass through liquids.

Surface Waves
• Travel at and near the surface-air interface.
• Amplitude of motion decreases exponentially with depth. (scaled
by wavelength)
• Usually largest amplitude, longest period, and most destructive.

Propagation of Earthquake Waves

Locating Earthquakes
If the P- and S- velocities are
known, then differences in
travel times of P and S waves
may be used to estimate
distance from the station to
the epicenter.

 The deepest well ever drilled was 12 kilometers in Russia. The Earth’s radius is 6,370 km, so
drilling can barely “scratch the surface.”
 A branch of geology called seismology lets us obtain information about the interior of the Earth
without directly observing it. Seismologists apply principles and laws of physics to study the
internal structure of the earth.
 Geophysics includes the study of seismic waves and the Earth’s magnetic and gravity fields and
heat flow. Because we cannot directly observe the Earth's interior, geophysical methods allow us to
investigate the interior of the Earth by making measurements at the surface. Without studying
these things, we would know nothing of the Earth’s internal structure.
 Keeping in mind that seismic waves have similar properties to other types of waves (e.g. light
waves), the wave properties of reflection and refraction are useful for learning about the Earth’s
interior. Results of early studies showed that the internal structure of the Earth consists of distinct
layers:
Mapping the earth: Seismic waves
1. Seismic reflection: Seismic waves bounce (reflect) off rock boundaries of different rock type,
and their travel times are recorded on a seismogram. The seismogram records the time it took
for the waves to travel to the boundary, reflect off of it and return to the surface. Seismologists
can measure the time this takes and calculate the depth to the boundary.
Seismograph
Station
Layer B
Reflecting
Boundary
Layer A
Seismic waves reflect off of a rock
boundary in the earth and return to a
seismograph station on the surface.

2. Seismic refraction: Waves change velocity and direction (refract) when they enter a medium of
different density than the one they just passed through.
Low-velocity layer: Seismic wave travels slow.
Example: Granite
High-velocity layer: Seismic wave travels fast.
Example: Gabbro
Research from seismic reflection and refraction has led to many important discoveries such as:
1. There are three main layers of the Earth: The crust, mantle, and core.
2. The continental crust is thicker than oceanic crust and seismic waves travel slower in the
continental crust meaning that they are made up of different kinds of rock (granite/basalt).
3. There is a distinct boundary between the crust and the mantle called the Mohorovicic
discontinuity, or, simply, the Moho. At this boundary, seismic waves are refracted.
4. There is a layer within the mantle up to 70 km thick beneath the ocean and up to 250 km thick
beneath the continents where waves travel slower than in more shallow layers. This layer is
called the low-velocity zone, and scientists have concluded that this zone is at least partially
liquid. In plate-tectonic theory, it is called the asthenosphere, which is the semi-molten region
of the earths’ interior just below the earth’s rigid crust that allows for tectonic plate movement.
5. P-waves can pass through the outer core but S-waves cannot. The outer core is a molten liquid.
Layer A
Layer B

 In the early 1900’s scientists discovered that parts of the Earth’s surface did not receive direct
earthquake waves.
A cross-sectional figure of the layers of the earth with P-
and S-wave propagation. McGraw Hill/Glencoe, 1st ed., pg. 172
 Scientists found that direct P-waves “disappear” from seismograms in a region between 104 and
140 degrees away from an epicenter. The seismic waves are bent, or refracted, upon encountering
the core-mantle boundary, casting a shadow called the P-wave shadow zone.
 Direct S-waves are not recorded in the entire region more than 104 degrees away from an
epicenter, and this is referred to as the S-wave shadow zone. The S-wave shadow zone, together
with the knowledge that liquids do not transmit S-waves, is evidence that the outer core is liquid
(or behaves as liquid).
 S-waves cannot travel through liquids because they are
shear waves, which attempt to change the shape of what
they pass through. Simply put, a liquid “doesn’t care”
what shape it’s in—for example, you can empty a bottle
of water into an empty box, and it will change shape with
the shape of container. Liquids cannot support shear
stresses, so shearing has no effect on them. Therefore, a
liquid will not propagate shears waves.

 Both P- and S-waves slow down when they reach the asthenosphere. Because of this, scientists
know that the asthenosphere is partially liquid.
 The asthenosphere (Greek: weak) is the uppermost part of the mantle, which is partially
molten. Lithospheric, or tectonic, plates are able to “slide” in different directions on top of the
asthenosphere. Because seismic waves travel slowly here, it is also called the “low velocity” zone.
 The boundary between the crust and the mantle is distinguished by jump in seismic wave
velocity. This feature is known as the Mohorovicic Discontinuity.
Seismic Waves: Mapping the earth
 Changes in velocity (km/s) of P- and
S-waves allow seismologists to identify
the locations of boundaries within the
earth such as the Mohorovicic boundary
near the earth’s surface and the
boundary between the mantle and outer
core.
Mohorivicic
Discontinuity
Modified after McGraw Hill/Glencoe, 1st ed., pg. 173
Low
Velocity
Zone
MOLTEN

Seismic Waves: Epicenter location
 Although S-waves, P-waves and surface waves all start out at the same time, they travel at
different speeds. The speed of a traveling seismic wave can be used to determine the location of an
 A seismograph records the arrival time and the magnitude of horizontal and vertical
movements caused by an earthquake. The arrival time between different seismic waves is used to
calculate the travel time and the distance from the epicenter.
 The difference in arrival time between primary waves and secondary waves is used to calculate
the distance from the seismograph station to the epicenter.
 It is crucial that seismic waves are recorded by three different seismograph stations in order to
estimate the location of the epicenter (see next slide.)
This example shows seismic waves
arriving at different times at two
seismograph stations. Station B is
farther away from Station A so the
waves take longer to reach Station
B. Primary waves arrive first,
followed by secondary waves, and
then surface waves. Credit: Modified
after Plummer/McGeary, 7th ed., pg.
Station A
Station B
1st
2nd
3rd
1st
2nd
3rd
R1
R2

 Earthquake scientists, or seismologists, can locate the epicenter of an earthquake as long as
the vibrations are felt at three different seismograph stations.
1. Locate at least 3 stations on a map that recorded the seismic waves
2. Calculate the time difference between arrival of P-waves and arrival of S-waves from a
seismogram. The time difference is proportional to the distance from the epicenter.
Because the direction to the epicenter is unknown, the distance defines a circle around the
receiving station. The radius of each circle equals that station’s distance from the
3. The epicenter is where the circles intersect.
Seismic Waves: Epicenter location
Epicenter location using three stations.
McGraw Hill/ Glencoe, 1st ed., pg 170
Earthquake nomenclature based on
proximity to the epicenter:
- Local event: Epicenter of felt
earthquake < 100km away
- Regional event: Epicenter of felt
earthquake is 100 to 1400 km away
- Teleseismic event: Epicenter of felt
earthquake > than 1400 km away

 The Earth’s magnetic field is used along with seismic waves to help understand the interior of
the Earth. The geomagnetic field surrounds the Earth in a south to north direction and has been
studied for many years due to its importance in navigation.
 The current hypothesis for the origin of geomagnetic field is that it is created by electrical
currents generated within the liquid outer core. The heat of the outer core (5000-6000° C) drives
convection of the molten material. The convection of a metallic liquid creates electric currents,
which, in turn, creates a magnetic field. This is a dynamo model, where mechanical energy of
convection generates an electrical energy and a magnetic field.
 Magnetism has two important components: intensity and direction. Most rocks differ in their
magnetism, depending upon their content of iron-bearing minerals, i.e. magnetic susceptibility.
For example, a body of magnetite or gabbro is strongly magnetic, and their magnetic signature
makes them stand out from other rocks. Additionally, when rocks form, they acquire the direction
(polarity) of the magnetic field existing at that time.
 Magnetic anomalies are deviations from the normal magnetic direction (i.e. today’s), or they are
exceptionally large differences (positive or negative) in magnetic intensity relative to average
values of surrounding rocks. Anomalies occur all over the earth and are sometimes indicative of
different rock types under the earth’s surface.
 Magnetic intensity anomalies can be measured by magnetometers, which are often towed
behind ships or flown over land surfaces to aid in mapping deposits in the earth.
Mapping the earth: Magnetic anomalies

The Big Ones: Earthquakes from around the world
 Charleston Earthquake: The Charleston Earthquake struck this eastern South Carolina city on August 31,
1886. East Coast earthquakes are felt over a much larger area than earthquakes occurring on the West Coast,
because the eastern half of the country is mainly composed of older rock that has not been fractured and
cracked by frequent earthquake activity in the recent geologic past. Rock that is highly fractured and
crushed absorbs more seismic energy than rock that is less fractured. The Charleston earthquake, with an
estimated magnitude of about 7.0, was felt as far away as Chicago, more than 1,300 km to the northwest,
whereas the 7.1-magnitude Loma Prieta earthquakes was felt no farther than Los Angeles, about 500 km
south. Approximately 60 people were killed as a result of this historic earthquake.
 New Madrid Earthquake: Three quakes over 8.0 occurred from Dec 16, 1811 to February 7, 1812. This was an
unusual place for an earthquake, and seemed to represent a weak point in the North American tectonic plate.
Survivors reported that they saw the ground rolling in waves. Scientists estimate that the possibility of
another earthquake at the New Madrid fault zone in the next 50 years is higher than 90 percent. The most
widely felt earthquakes ever to strike the United States were centered near the town of New Madrid,
Missouri, in 1811 and 1812. Three earthquakes, felt as far away as Washington D.C., were each estimated to
be above 8.0 in magnitude.
Damage to Stanford University. Source USGS:
http://earthquake.usgs.gov/regional/nca/1906/18april/casualties.php
 San Francisco Earthquake (April 18, 1906, 5:12am): Occurred
along the San Andreas Fault. The total population at the time
was 400,000, the death toll was 3,000 and 225,000 were left
homeless. 28,000 buildings were destroyed and damages were
estimated at $400 million from the earthquake plus fire, but only
$80 million from the earthquake alone. Horizontal displacement
was 15 feet with a visible scar 280 miles long. Fires caused by
broken gas lines raged for 3 days until buildings were destroyed
to create a firebreak. The vast percentage of damage was done
by fires.

 The Loma Prieta Earthquake (5:04pm
October 17, 1989) occurred along the San
Andreas Fault Zone with the epicenter located
in the Santa Cruz Mountains, 70 miles south
of the San Franciso earthquake. It registered
6.9 on the Richter Scale with a final death toll
of 63 people and damages of $6 billion.
Resulting fires were hard to manage due to
broken water lines. The total tremor time
lasted approximately 15 seconds.
 The Good Friday Earthquake (March 27, 1964 at 5:36pm): At 9.2 on the Richter Scale, this was
the largest earthquake to occur in the U.S. in recorded history. The death toll was only 115 due to
the scarcity of people in this area. Damages were approximately $300 million. A 30 mile x 125 mile
block of land was raised 40 ft, and a similar block dropped 3-6 feet. The tremor, which lasted 3
minutes, created a tsunami that drowned 100 people in Alaska, Oregon and California. Landslides
destroyed parts of Anchorage, 90 miles away.
Lisbon, Portugal: November 1, 1755. 9:40 am on All Souls Day. The quake triggered a tsunami
with a wave 50-ft high, which crashed through the city. Buildings collapsed of killing many people,
and waves swept thousands more away. Fires ran unchecked for 3 days, completing the
destruction of the capitol. Over 60,000 people died in the city alone and thousands more in
surrounding areas.
Photo left: Cypress Freeway collapsed- 42 people killed at this
section. Right: Bay bridge section collapse.
Source:http://www.olympus.net/personal/gofamily/quake/famous/pri
eta.html

Earthquake Magnitude:
Scales based on Seismograms
• ML= local (e.g. Richter scale) - based on amplitude of waves with
1s period within 600 km of epicenter.
• Mb= body-wave (similar to above)
• Ms= surface wave (wave periods of 20s measured anywhere on
globe
• Mo= seismic moment
• Mw= moment magnitude

The Richter scale
Steps:
1. Measure the interval (in seconds) between the arrival of the first P
and S waves.
2. Measure the amplitude of the largest S waves.
3. Use nomogram to estimate distance from earthquake (S-P interval)
and magnitude (join points on S-P interval scale and S amplitude
scale).
4. Use seismograms from at least three geographic locations to locate
epicenter by triangulation.

Determining the location of an earthquake
First, distance to earthquake is determined.
1. Seismographs record seismic waves
2. From seismograph record called the seismogram, measure time delay between P
& S wave arrival
3. Use travel time curve to determine distance to earthquake as function of P-S
time delay
Now we know distance waves traveled, but we don't know the direction from
which they came.
We must repeat the activity for each of at least three (3) stations to triangulate a
point (epicenter of quake).
Plot a circle around seismograph location; radius of circle is the distance to the
quake.
Quake occurred somewhere along that circle.
Do the same thing for at least 3 seismograph stations; circles intersect at epicenter.
Thus, point is triangulated and epicenter is located.

How is an Earthquake’s Epicenter Located?
Seismic wave behavior
– P waves arrive first, then S waves, then L and R
– Average speeds for all these waves is known
– 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.

How is an Earthquake’s Epicenter Located?
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.

How is an Earthquake’s
Epicenter Located?
• 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

• With multiple stations,
the location of the
epicenter can be
estimated.
• Focus (depth) can be
inferred from travel
times and ‘depth’ phases
due to free-surface
reflections.
Determining Epicenter Location and
Focal Depth by Triangulation

Earthquake magnitude:
scales based on rupture dimensions (equivalent to
energy released )
• Mo= seismic moment.
= m * A * d, where m is the shear modulus
of rock; A is the rupture area, and d is
displacement
• Mw= moment magnitude.
= 2/3 * log Mo - 10.7
N.B. moment scales do not saturate

Saturation of
non-moment
scales

How are the Size and Strength of an Earthquake Measured?
• Modified Mercalli Intensity Map
– 1994 Northridge, CA earthquake,
magnitude 6.7
• Intensity
– subjective measure
of the kind of
damage done and
people’s reactions
to it
– isoseismal lines
identify areas of
equal intensity

How are the Size and Strength of an Earthquake Measured?
• Magnitude
– Richter scale
measures total amount
of energy released by
an earthquake;
independent of
intensity
– Amplitude of the
largest wave produced
by an event is
corrected for distance
and assigned a value
on an open-ended
logarithmic scale

 Earthquakes can be very destructive at the Earth’s surface. The magnitude of an earthquake is a
measure of how destructive it is. Basically the magnitude corresponds to how much energy is
released.
 The Richter Scale is used to express earthquake magnitude on the basis of the height (amplitude)
of the largest line (seismic wave, P or S) on a seismogram. The Richter scale was originally
developed for earthquakes in Southern California. The utility of this scale was its ability to account
for decreased wave amplitude with increased distance from the epicenter. Richter’s scale is also a
logarithmic scale.
 Today, a standard magnitude scale is used, Seismic Moment, which more accurately represents
the energy released in an earthquake, especially large magnitude events.
 The majority of earthquakes are minor and have magnitudes of 3-4.9 on the Richter scale. These
can be felt, but cause little or no damage, and there are about 55,000 of these earthquakes each year.
 Thousands of earthquakes are recorded every day with magnitudes < 3.0 but are almost never
felt.
 The Mercalli scale is different from the Richter scale because it measures the intensity of how
people and structures are affected by the seismic event. In essence, it measures damage. It is much
more subjective and uses numbers ranging from 1 (no damage) to 12 (total destruction).
Earthquake classification scales

Earthquake Magnitude
• Earthquake strengths range from imperceptible to
catastrophic. Several scales are used:
– Richter Magnitude – a logarithmic measure of how much the ground
moved at the seismograph as seismic waves pass by
– Moment magnitude – a logarithmic measure proportional to total
area of fault rupture and seismic energy released
– Modified Mercalli scale – Not a magnitude, but a “measure” of the
perception of the earthquake – what people felt, and how much
damage there was, useful for historical events preceding the invention
of the seismograph. This scale is useful for studying historic
earthquakes that occurred prior to modern seismographs.

Earthquake size: two ways to
measure
1) Magnitude: Richter Scale
• Measures the energy released by fault
movement
• related to the maximum amplitude of the S
wave measured from the seismogram
• Logarithmic-scale; quantitative measure
• For each whole number there is a 31.5 times
increase in energy
• eg. an increase from 5 to 7 on the Richter scale =
an increase in energy of 992 times!!

Richter
Magnitudes
Description Earthquake Effects
Frequency of
Occurrence
Less than 2.0 Micro Micro-earthquakes, not felt. About 8,000 per day
2.0-2.9 Minor Generally not felt, but recorded. About 1,000 per day
3.0-3.9 Minor Often felt, but rarely causes damage. 49,000 per year (est.)
4.0-4.9 Light
Noticeable shaking of indoor items, rattling noises.
Significant damage unlikely.
6,200 per year (est.)
5.0-5.9 Moderate
Can cause major damage to poorly constructed
buildings over small regions. At most slight damage
to well-designed buildings.
800 per year
6.0-6.9 Strong
Can be destructive in areas up to about 100 miles
across in populated areas.
120 per year
7.0-7.9 Major Can cause serious damage over larger areas. 18 per year
8.0-8.9 Great
Can cause serious damage in areas several hundred
miles across.
1 per year
9.0-9.9 Great Devastating in areas several thousand miles across. 1 per 20 years
10.0+ Great
Never recorded; see below for equivalent seismic
energy yield.
Extremely rare
(Unknown)
The Richter Scale
Complications of the Richter scale include:
1. The Richter scale originally only applied to shallow-focus earthquakes in southern
California so now must be modified.
2. Magnitudes calculated from seismograms above 7 tend to be inaccurate.

2) Intensity: Mercalli Scale:
– Assigns an intensity or rating to measure an earthquake at a
particular location (qualitative)
– I (not felt) to XII (buildings nearly destroyed)
– Measures the destructive effect
• Intensity is a function of:
• Energy released by fault
• Geology of the location
• Surface substrate: can magnify shock waves e.g.
Mexico City (1985) and San Francisco (1989)

I. Not felt except by a very few under especially favorable conditions.
II. Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended
objects may swing.
III. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do
not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the
passing of a truck. Duration estimated.
IV. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes,
windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking
building. Standing motor cars rocked noticeably.
V. Felt by nearly everyone; many awakened. some dishes, windows broken. Unstable objects
overturned. Pendulum clocks may stop.
VI. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster.
Damage slight.
VII. Damage negligible in buildings of good design and construction; slight to moderate in well-built
ordinary structures; considerable damage in poorly built or badly designed structures; some
chimneys broken.
VIII. Damage slight in specially designed structures; considerable damage in ordinary substantial
buildings with partial collapse. Damage great in poorly built structures. Fall of chmineys, factory
stacks, columns, monuments, walls. Heavy furniture overturned.
IX. Damage considerable in specially designed structures; well-designed frame structures thrown out
of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off
foundations.
X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with
foundations. Rail bent.
XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.
XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air.
The Mercalli scale of earthquake intensity Credit: USGS

Frequency of Occurrence of Earthquakes
Descriptor Magnitude Average Annually
Great 8 and higher 1 ¹
Major 7 - 7.9 17 ²
Strong 6 - 6.9 134 ²
Moderate 5 - 5.9 1319 ²
Light 4 - 4.9
13,000
(estimated)
Minor 3 - 3.9
130,000
(estimated)
Very Minor 2 - 2.9
1,300,000
(estimated)
¹ Based on observations since 1900.
² Based on observations since 1990.

Large earthquakes occur much less frequently
than smaller ones - longer recurrence interval

Seismic hazards
1. Locating faults
2. Estimating recurrence: history and
geology
3. Measuring relative motions and crustal
deformation
4. Learning from analogies
5. Assessing probabilities

1. Locating faults:
Seattle Fault (LIDAR image)

The Hayward
fault runs
through UC
Berkeley
campus
(US $1 billion
seismic upgrade
program)
Lawrence
Livermore
UC Berkeley

San Francisco
City Hall, 1906
2. Recurrence - historical records

3. Prediction:
current crustal
deformation

3. Prediction: crustal velocity (mm/yr)
from repeated GPS measurements at permanent stations
Why are all stations
moving to NW?

4. Learning from analogues
(Turkey - California)

The Bay Area:
earthquake
probabilities
(AD2000-2030)
N.B. A probability of
70% over 30 years is
equivalent to a daily
probability of
1 : 15 000
5. Assessing probabilities

Probabilities, yes!
but prediction, no!
• 1996 - Earthquake prediction group of Japanese
Seismological Survey voluntarily disbands (after Kobe)
• 2000 - British researcher argues that prediction of main
shock impossible at present; immediate goal should be
prediction of aftershock location and magnitude

Individual seismic hazards
• Shaking = accelerated ground motion
• Liquefaction = failure of waterlogged sandy
substrates
• Landslides, dam failures, etc.
• Tsunamis = seismic sea waves
• Fire, etc.

Predictions of shaking intensity on
San Andreas fault (long segment) in the Bay Area

Ground motion, structural damage and basin
morphology: Mexico City, 1985
periodic periodic
random
bodysurface surface/body
Damage
heavy light heavy
ridge
basin basin

Cascadia: megaearthquakes at the
plate boundary
Mw = 9.2?

Parkfield, CA
“Earthquake Capital of the World”
Earthquake Hazard Potential Map

What causes
earthquakes?
 Elastic Rebound
 Sources of stress
 Plate Tectonics
 Other stresses

Seismic (Earthquake) Waves
• Travel outward from focus
• Focus: site of initial rupture
• Epicenter: point on surface above the focus

Elastic Rebound and Fault Rupture
• Forces applied, often due to
plate tectonics
• Rocks accumulate strain
(deformation)
• Elastic rebound when fault
ruptures
• Seismic waves radiated
• Slip along fault apparent if
near the surface
• Frequency of earthquakes
depends both on earthquake
size and rate of strain
accumulation

What Happens During an Earthquake?

What Happens During an Earthquake

Damage: Causes
 Ground motion
 Duration of Shaking
 Surface Rupture
 Poor building design

Effects
 Rupture
 Death
 Bldg collapse

Effects
 Fires
 Liquifaction
 Landslides

Damage:
Key Factors
 Amount & duration of shaking
 Water content of soil
 Population concentration
 Building construction
 Distance from Epicenter
 Depth of focus
 Direction of rupture
 Material amplification

Tsunamis
• Tsunamis are waves generated by fault motion or slumps on the seafloor .
• An underwater earthquake with a magnitude of 8 or higher on the Richter
scale can affect the earth’s oceans by causing a tsunami. Less commonly,
tsunamis are also caused by submarine landslides or volcanic explosions.
• Tsunamis are sometimes called seismic sea waves. They can destroy
everything in the coastal zone, but they occur as small unnoticeable waves
out at sea.
• Tsunamis can travel at speeds up to 800 km/hr and form waves over 20m
high as they break.

The earthquake generates a rolling wave out in the open water;
however, as the waves approach shore, they start to “feel” the bottom
of the sea floor. The waves slow down near the bottom, causing a
huge wave to build up on top as the top is still moving at its original
speed. This is the same as the way regular waves form in the surf
zone. The tsunami, however, can be a one-hundred foot high wall of
water moving at 450 miles per hour.
Sketch showing the generation of a Tsunami
from an underwater earthquake.
Credit: McGraw Hill/Glencoe, 1st ed., pg. 178
Earthquakes: Tsunamis
Dislocation of seafloor

Can Earthquakes be Predicted?
Earthquake Precursors
– changes in elevation or tilting of land surface,
fluctuations in groundwater levels, magnetic field,
electrical resistance of the ground
– seismic gaps

Can Earthquakes be Predicted?
Earthquake Prediction Programs
– include laboratory and field studies of rocks before, during,
and after earthquakes
– monitor activity along major faults
– produce risk assessments

Earthquake Prediction -Still Science Fiction
• Large earthquakes do tend to follow a cycle of rupture, followed by declining
aftershocks and a period of quiescence
• During the quiet period strain is building toward another rupture
• Recurrence intervals vary from 10’s of years to 100’s
• Great interest in seismicity prior to written history
• -Helps us predict where, but when is another matter

Earthquake risk and prediction
• Long-term methods
1) seismic hazard maps
2) probability analysis based
on:
- historical EQ records
- geologic EQ records
- slip-rate on active faults
- frequency and magnitude
of recent EQ's
Real-time 24 Hour
Forecast

Short-term predictions
Precursor phenomena (<1 year to days)
1. Foreshocks: usually increase in magnitude
2. Ground deformation
3. Fluctuations in water well levels
4. Changes in local radio wave characteristics
5. Anomalous animal behavior???

Impacts of Earthquake Prediction

…And that was
just a 7.2 on
the Richter
scale!

Earthquake Engineering notes- module 1 ppt

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