Earthquake

• An earthquake (also known as a quake,

tremor or temblor) is the vibration of Earth
produced by the rapid release of energy.

• Energy radiates in all directions from its

source, the focus
• Energy moves like waves
• Seismographs record the event
Anatomy of Earthquakes

Earthquakes are associated with faults

Fault scarp: Any rise or cliff formed because of faulting
Fault line: Fracture seen on Earth’s surface by faulting
Epicenter: Place on Earth’s surface where the earthquake is first flet
Focus: Where earthquake originates inside the earth
Fault plane: Surface inside the earth where there is movement along a fault
Seismic wave: Energy released during an earthquake
 Earthquakes are caused by
sudden release of
accumulated strain energy
along Faults

•Build up Stress Rocks on sides

•Release stress of fault are
•Elastic rebound deformed by
tectonic forces

Rocks bend
and store
)))
elastic
) ) ) ) energy
(((( (((
Frictional
resistance
holding the
rocks together
is overcome by
tectonic forces
 Earthquake mechanism

– Slip starts at the weakest point (the focus)

– Earthquakes occur as the deformed rock “springs
back” to its original shape (elastic rebound)
– The motion moves neighboring rocks
– And so on.

– Elastic rebound ??

The elastic rebound theory is an explanation for how energy is spread during
earthquakes. As rocks on opposite sides of a fault are subjected to force and shift,
they accumulate energy and slowly deform until their internal strength is
exceeded. At that time, a sudden movement occurs along the fault, releasing the
accumulated energy, and the rocks snap back to their original undeformed
shape.
Relationship
Between
Stress and
Strain

Demo: Rubber Band

Strain can be a change in shape (a deformation) due to an applied stress

Relationship
Between Stress
and Strain at low
Temps and
Pressure or
Sudden Stress

Demo: Pencil
Relationship
Between Stress
and Strain under
High
Temperatures or
Pressure

Demo: gum
 Strike and Dip

Strike is long line, dip is short line

Note the angle of dip given 45o

Strike intersection horizontal, dip perpendicular, angle from horizontal down toward surface
Vertical Movement
along Dip-Slip Faults

Divergent

Convergent
Horizontal Movement Along
Strike-Slip Fault
Reverse Fault Quake - Japan Normal Fault Quake - Nevada
DEMO – Types of faults

Strike Slip Fault Quake - California

 Top ten Earthquake in the History of Earth
Magnitude 9.5 (22 May 1960 – Chile)
The world's most powerful earthquake left 4,485 people dead and injured and 2
million homeless after it struck southern Chile in 1960.

Magnitude 9.2 (28 March 1964 – Prince William Sound, Alaska)

The Gulf of Alaska was devastated by the Prince William Sound earthquake that
caused landslides in Anchorage and raised parts of outlying islands by as much as 11
metres. The resulting tsunami reached heights of 67 metres as it swept into the
shallow Valdez inlet and was responsible for most of the 128 deaths.

Magnitude 9.1 (26 December 2004 – Off the west coast of northern
Sumatra)
The deadliest tsunami in history was felt in 14 countries across Asia and east Africa,
triggered by a "megathrust" as the Indian tectonic plate was forced beneath the
Burmese plate. Indonesia was the worst affected with an estimated 170,000 of the
nearly 230,000 dead.

Magnitude 9 (4 November 1952 – Kamchatka)

The volcanic Russian peninsula was near the epicentre of the quake. No deaths were
recorded.
Magnitude 9 (13 August 1868 – Arica, Peru (now part of Chile)
The estimated damage was huge and 25,000 peoples were killed.
Magnitude 9 (26 January 1700 – North Pacific coast of America)
Across the pacific, the quake was accurately recorded by Japanese observers of the
large tsunami that struck Japan on 27 January 1700.
Magnitude 8.8 (27 February 2010 – off Bio-Bio, Chile)
The region around Concepción has been recorded as a centre for seismic shocks
since the 16th century, but few have been as devastating as the early morning quake
that generated a Pacific-wide tsunami and cost the lives of 521 people.
Magnitude 8.8 (13 January 1906 – coast of Ecuador)
Emanating from the ocean off Ecuador and Colombia, the quake generated a tsunami
that killed between 500 and 1,500 people along a coastline from Central America to
San Francisco.
Magnitude 8.7 (1 November 1755 – Lisbon)
The near-total destruction of Lisbon and the deaths of a quarter of the city's
population were caused by an earthquake, followed by a tsunami and fire, that was
felt in north Africa, France and northern Italy.
Magnitude 8.6 (15 August 1950 – Assam-Tibet)
Seventy villages simply disappeared in the string of disasters generated by an
earthquake with an epicentre in Tibetan Rima but which wrought most destruction in
India's Assam state. Across the region, landslides claimed the lives of 1,526 people.
 San Andreas is the most studied transform fault system in the world
 discrete segments 100 to 200 kilometers long
 slip every 100-200 years producing large earthquakes
 Some portions exhibit slow, gradual displacement known as fault creep
 Fires caused by 1906 San Francisco Earthquake

Gas mains break, fires shaken out of furnaces. Water mains break, cannot
fight fires. Debris in streets, Fire department cannot reach fires.
Tectonic setting of the 2004 and 2005 Sumatra earthquakes. (a) The larger aftershocks of the Mw
9.3, 26 December, 2004 (red) and the Mw 8.7 March 28, 2005 earthquakes (orange),
superimposed on the the GEBCO (2003) bathymetry data, are shown. Thick black lines show the
location of the extinct spreading centre that according to Deplus et al. (1998) generated oceanic
crust in the Wharton basin. (b) Satellite-derived gravity anomaly based on Sandwell and Smith
(1997)-Version 15.2. Grey dashed lines and red lines show fracture zones and magnetic lineations
respectively (Larson et al. 1978).
Depth of Earthquake Focus
On the basis of the depth of focus, an earthquake may be termed as shallow focus (0-
70 km), intermediate focus (70-300 km), and deep focus (> 300 km).

Shallow-focus earthquakes are most common; they account for 85% of total quake
energy released. Intermediate-(12%) and deep-(3%) focus quakes are rarer because
most deep rocks flow in a ductile manner when stressed or deformed; they are
unable to store and suddenly release energy as brittle surface rocks do.
Vertical cross-sections
of hypocentre distributions
beneath modem arc-
trench
systems. Each diagram
shows earthquakes for 7-
10 year periods between
1954 and 1969. T =
trench axis; V = recently
active volcanic chain.
Distance is measured
horizontally from each
trench axis in kilometres.
Landscape Shifting, Wallace Creek

San Andreas Fault, a Transform Margin

Liquefaction
Liquefaction is a phenomenon in which the strength and stiffness of a
rock/soil is reduced by earthquake shaking or other rapid loading.
Liquefaction and related phenomena have been responsible for tremendous
amounts of damage in historical earthquakes around the world.

Soil liquefaction occurs when a saturated or partially saturated soil

substantially loses strength and stiffness in response to an applied stress
such as shaking during an earthquake or other sudden change in stress
condition, in which material that is ordinarily a solid behaves like a liquid.

Some effects of liquefaction after

the 1964 Niigata earthquake
 Liquefaction

Demo: Liquifaction
Locations of earthquakes with magnitude M≥ 6. Shaded regions represent
continents and the dots for locations of earthquakes.
 Seismology
Seismology is the science of earthquakes and other ground vibration.
At present, seismology is the study of seismic sources, the waves
they produce, and the properties of the media through which these
waves travel.
Elevated highway knocked over by a
strong horizontal jolt during the 1995
Kobe, Japan, earthquake.
Seismic Waves
Energy released from an earthquake.

P-wave
Primary waves
Fastest
Compressional
Weakest
Travel trough solids and fluids

P-waves are a type of body wave, called seismic waves in seismology, that travel through a continuum and
are the first waves from an earthquake to arrive at a seismograph. The continuum is made up of gases (as
sound waves), liquids, or solids, including the Earth. P-waves can be produced by earthquakes and recorded
by seismographs. The name P-wave can stand for either pressure wave as it is formed from alternating
compressions and rarefactions or primary wave, as it has the highest velocity and is therefore the first wave
to be recorded.

S-wave
Secondary waves
Slower
Shearing
Stronger
Only travel through solids
S-waves, secondary waves, or shear waves (sometimes called an elastic S-wave) are a type of elastic wave,
and are one of the two main types of elastic body waves, so named because they move through the body of
an object, unlike surface waves.

The S-wave moves as a shear or transverse wave, so motion is perpendicular to the direction of wave
propagation. The wave moves through elastic media, and the main restoring force comes from shear effects.

L-wave
Surface waves
Slowest
Compressional and shearing
Strongest
Solids only

Love waves (also known as Q waves (Quer: German for lateral)) are surface seismic waves that cause
horizontal shifting of the Earth during an earthquake. Love waves travel with a lower velocity than P- or S-
waves, but faster than Rayleigh waves. These waves are observed only when there is a low velocity layer
overlying a high velocity layer/ sub–layers.

Rayleigh waves behave like rolling ocean waves. Rayleigh waves are a type of surface acoustic wave that
travel on solids. They can be produced in materials in many ways, such as by a localized impact or by piezo-
electric transduction, and are frequently used in non-destructive testing for detecting defects. They are part of
the seismic waves that are produced on the Earth by earthquakes. When guided in layers they are referred to
as Lamb waves, Rayleigh–Lamb waves, or generalized Rayleigh waves)
Conceptualized P- and S-wave propagation
 P and S waves

Demo: P and S waves

Smaller amplitude than surface (L) waves, but faster, P arrives first, then S, then L
Earthquake focus and epicenter
Note how much bigger the surface waves are
LOCATING AND MEASURING EARTHQUAKES

The invention of instruments that could accurately record seismic waves was
an important scientific advance. These instruments measure the amount of
ground motion and can be used to find the location, depth, and size of an
earthquake. The instrument used to measure seismic waves is a
seismometer.

Seismometers are the instruments that record seismic waves. Records the
movement of Earth in relation to a stationary mass on a rotating drum or
magnetic tape. It includes:

Seismograph: Machine that records earthquake activity

Seismogram: Record of earthquake activity.

 A seismograph designed to
record vertical ground motion

The heavy mass doesn’t move much

The drum moves
The principle of the seismometer is to keep a heavy suspended mass as
motionless as possible—suspending it by springs or hanging it as a
pendulum from the frame of the instrument. When the ground moves, the
frame of the instrument moves with it; however, the inertia of the heavy
mass suspended inside keeps the mass motionless to act as a point of
reference in determining the amount of ground motion. Seismometers are
usually placed in clusters of three to record the motion along the x, y, and z
axes of three-dimensional space.

Because the different types of seismic waves travel at different speeds, they
arrive at seismograph stations in a definite order:
first the P waves, then the S waves, and finally the surface waves.
These three different waves can be distinguished on the seismograms.
By analyzing these seismograms, geologists can learn a great deal about an
earthquake, including its location and size.
 Lateral Movement Detector

In reality, copper wire coils move around magnets, generating current which is recorded.
Because of the difference in travel times, intervals between P waves, S
waves, and surface waves increase with distance from the focus.
Determining the Location of an Earthquake
P and S waves start out from the focus of an earthquake at essentially the
same time. As they travel away from the quake, the two kinds of body
waves gradually separate because they are traveling at different speeds. On
a seismogram from a station close to the earthquake, the first arrival of the
P wave is separated from the first arrival of the S wave by a short distance
on the paper record. At a recording station far from the earthquake,
however, the first arrivals of these waves will be recorded much farther apart
on the seismogram. The farther the seismic waves travel, the longer the
time intervals between the arrivals of P and S waves and the more they are
separated on the seismograms.
Graph to find distance to epicenter
Travel-time curve

Because the time interval between the first arrivals of P and S waves
increases with distance from the focus of an earthquake, this interval can be
used to determine the distance from the seismograph station to a quake.
The increase in the P-S interval is regular with increasing distance for
several thousand kilometers and so can be graphed in a travel-time curve ,
which plots seismic-wave arrival time against distance.
Locating Earthquake Epicenter
A single station can determine only the distance to a quake, not the
direction. A circle is drawn on a globe with the center of the circle being the
station and its radius the distance to the quake. The scientists at the station
know that the quake occurred somewhere on that circle, but from the
information recorded, they are not able to tell where. With information from
other stations, however, they can pinpoint the location of the quake. If three
or more stations have determined the distance to a single quake, a circle is
drawn for each station. If this is done on a map, the intersection of the
circles locates the epicenter.
Epicenter located using three seismographs
Focal Depth Measurement
Earthquake focal depth is a critical parameter for seismological research,
seismotectonic study, seismic hazard assessment, and event discrimination.
For most earthquakes with MW ≥4.5, the focal depth can be estimated from
the arrival times of the teleseismic depth phase sP (or pP) and its reference
phase P.
For smaller earthquakes, focal
depths can be estimated jointly
while being located with the arrival Surface of the Earth

times of the Pg and Sg phases

recorded at close stations. Because
stations in a regional network are
generally not dense enough to Earthquake
control focal depth, operators often Focal Depth= h

use default focal depths for Depth

regional events.

pP
sP

P
Figure shows the
sketch paths of these
regional depth phases.
If regional depth phases can be identified, an alternative solution for
moderate and small earthquakes is to use regional depth phases to estimate
focal depth. The P portion of regional waveform records contains three major
parts:
(1) the P-wave travels directly to the station
(2) the P- or S-wave travels upward to the surface in the source region, is
reflected or converted at the surface and then travels downward to the
Moho (or interfaces), is reflected or refracted there, and then travels
upward to the station; and
(3) the P-wave travels downward to the Moho (or interfaces), is reflected
there and then travels upward to the station. One feature of P- and S-
waves is that the amplitude of the S-wave radiated from the source is
generally stronger than that of the P-wave by about five times and the
period of the S-wave is longer than that of the P-wave on the same
record.
From this analysis we know that there are regional depth phases in the P
portion of the record and the usable regional depth phases are
(1) sPg (the S-wave travels upward to the surface, is converted to a P-wave
at the critical angle, then the P-wave travels along or close the surface to
the station)
(2) sPmP (the S-wave travels upward to the surface, is converted to a P-
wave, then the P-wave travels downward to the Moho, is reflected there
and travels upward to the station; Langston et al., 2003), and
(3) sPn (the S-wave travels upward to the surface, is converted to a P-wave,
then the P-wave travels along the Pn path to the station.

Regional depth phases (sPg, sPmP, and sPn) can be used to estimate focal
depth if they and their reference phases (Pg, PmP, and Pn) can be
correctly identified.
where k = Vp1 /Vp2 ; Vp1 and Vp2 are P-wave velocities in and beneath the
crust, respectively; h is the focal depth; and t is the differential time. The
differential time is independent of station distance and crustal thickness.

The time difference sPn–Pn on trace ACTO/HHZ is 4.06 sec.

When we take Vp1 = 6.25 km/sec and Vp2 = 8.0 km/sec, the focal depth
from equation (1) is 11.7 km; this depth is close to the focal depth (about
11.5 km) determined by regional depth-phase modeling (RDPM).
where VP is the P-wave velocity in a one-layered crustal model of Poisson medium, α
is the P-wave take-off angle, t is the differential time, and h is the focal depth. If we
take VP = 6.25 km/sec and α = 29.68° the focal depth from equation (2) is 12 km.
 95% of energy released by earthquakes originates
in narrow zones that wind around the Earth

These zones mark of edges of tectonic plates

Broad are subduction zone earthquakes, narrow are MOR. Lead to recognition of plates
Types of Earthquakes
Earthquakes are classified by their mode of generation.

The most common types of natural events are

(i) tectonic earthquakes,
(ii) volcanic earthquakes and
(iii)collapse earthquakes.

Anthropogenic events are

(iv) Explosion earthquakes
(v) Reservoir induced earthquakes
(vi) Mine bursts and collapse, and
(vii) Microseisms
Earthquake Depth and Plate Tectonic Setting

Subduction Zones discovered by Benioff

Earthquake in subduction zones
Earthquakes at Divergent Boundaries - Iceland

Crust pulling apart – normal faults

 Seismic Waves 1: Surface waves
– Complex motion, great destruction
– High amplitude and low velocity
– Longest periods (interval between crests)
– Termed long, or L waves
• Types of seismic waves (continued)
• Body waves
– Travel through Earth’s interior
– Two types based on mode of travel
– Primary (P) waves
» Push-pull motion
» Travel thru solids, liquids & gases
– Secondary (S) waves
» Moves at right angles to their direction of
travel
» Travels only through solids
 Measuring the size of earthquakes

• Two measurements describe the size of an earthquake

• Intensity – a measure of earthquake shaking at a given

location based on amount of damage

• Magnitude – estimates the amount of energy released by

the earthquake
 Intensity scales

• Modified Mercalli Intensity Scale was

developed using California buildings as its
standard

• Drawback is that destruction may not be

true measure of earthquakes actual
severity
Magnitude scales

• Richter magnitude - concept introduced by

Charles Richter in 1935
• Richter scale
– Based on amplitude of largest seismic wave
recorded
– LOG10 SCALE
Each unit of Richter magnitude corresponds
to 10X increase in wave amplitude and 32X
increase in Energy
 Magnitude scales

• Moment magnitude was developed because Richter

magnitude does not closely estimate the size of very
large earthquakes

– Derived from the amount of displacement that

occurs along a fault and the area of the fault that
slips
How Are Earthquake Magnitudes Measured?

The Richter Scale

There are a number of ways to measure the magnitude of an earthquake. The first
widely-used method, the Richter scale, was developed by Charles F. Richter in 1934. It
used a formula based on amplitude of the largest wave recorded on a specific type of
seismometer and the distance between the earthquake and the seismometer. That
scale was specific to California earthquakes; other scales, based on wave amplitudes
and total earthquake duration, were developed for use in other situations and they
were designed to be consistent with Richter’s scale.

The Moment Magnitude Scale

Unfortunately, many scales, such as the Richter scale, do not provide accurate
estimates for large magnitude earthquakes. Today the moment magnitude scale,
abbreviated MW, is preferred because it works over a wider range of earthquake sizes
and is applicable globally. The moment magnitude scale is based on the total moment
release of the earthquake. Moment is a product of the distance a fault moved and the
force required to move it. It is derived from modeling recordings of the earthquake at
multiple stations. Moment magnitude estimates are about the same as Richter
magnitudes for small to large earthquakes. But only the moment magnitude scale is
capable of measuring M8 (read ‘magnitude 8’) and greater events accurately.
Magnitudes are based on a logarithmic scale (base 10). What this means is that for
each whole number you go up on the magnitude scale, the amplitude of the ground
motion recorded by a seismograph goes up ten times. Using this scale, a magnitude 5
earthquake would result in ten times the level of ground shaking as a magnitude 4
earthquake (and 32 times as much energy would be released). To give you an idea
how these numbers can add up, think of it in terms of the energy released by
explosives: a magnitude 1 seismic wave releases as much energy as blowing up 6
ounces of TNT. A magnitude 8 earthquake releases as much energy as detonating 6
million tons of TNT. Fortunately, most of the earthquakes that occur each year are
magnitude 2.5 or less, too small to be felt by most people.

Magnitude scales can be used to describe earthquakes so small that they are
expressed in negative numbers. The scale also has no upper limit, so it can describe
earthquakes of unimaginable and (so far) unexperienced intensity, such as magnitude
10.0 and beyond.
Calculation of Moment Magnitude
The symbol for the moment magnitude scale is Mw, with the subscript w meaning
mechanical work accomplished. The moment magnitude Mw is a dimensionless
number defined by Hiroo Kanamori as

where M0 is the seismic moment in dyne⋅cm (10−7 N⋅m). The constant values in
the equation are chosen to achieve consistency with the magnitude values
produced by earlier scales, such as the Local Magnitude and the Surface Wave
magnitude.

The seismic moment is a measure of the size of an earthquake based on the

area of fault rupture, the average amount of slip, and the force that was required to
overcome the friction sticking the rocks together that were offset by faulting.

The seismic moment can be determined from the determination of the moment
tensor or by spectral analysis Mw=2/3log10(M0) – 10.7 [dyn-cm=10-7 Nm],
Mw=2/3(log10(M0) - 9.1) [Nm], Mw=2/3log10(M0) – 6.0 [MKS(SI) unit, (meter-kg-
second], Mw is today seen as the ’most correct’ magnitude measure
Comparative energy released by two earthquakes
As with the Richter scale, an increase of one step on this logarithmic scale
corresponds to a 101.5 ≈ 32 times increase in the amount of energy released, and
an increase of two steps corresponds to a 103 = 1000 times increase in energy.
Thus, an earthquake of MW of 7.0 contains 1000 times as much energy as one of
5.0 and about 32 times that of 6.0.

The following formula, obtained by solving the previous equation for M0, allows one
to assess the proportional difference fΔE in energy release between earthquakes of
two different moment magnitudes, say m1 and m2:

For example, an earthquake with a moment magnitude of 7.0 is approximately 5.62

times greater than a quake with moment magnitude 6.5
 Earthquake Magnitude Scale
Magnitude Earthquake Effects Estimated Number
Each Year
2.5 or less Usually not felt, but can be recorded by seismograph. 900,000

2.5 to 5.4 Often felt, but only causes minor damage. 30,000
5.5 to 6.0 Slight damage to buildings and other structures. 500
6.1 to 6.9 May cause a lot of damage in very populated areas. 100
7.0 to 7.9 Major earthquake. Serious damage. 20
8.0 or Great earthquake. Can totally destroy communities
One every 5 to 10 years
greater near the epicenter.

Earthquake Magnitude Classes

Earthquakes are also classified in categories ranging from minor to great, depending on their
magnitude.
Class Magnitude
Great 8 or more
Major 7 - 7.9
Strong 6 - 6.9
Moderate 5 - 5.9
Light 4 - 4.9
Minor 3 -3.9
Diagram shows the relationship between the moment magnitude of an
earthquake, the number of earthquakes per year throughout the world, and
the energy released during an earthquake.
The Mercalli Scale

Another way to measure the strength of an earthquake is to use the Mercalli scale.
Invented by Giuseppe Mercalli in 1902, this scale uses the observations of the people
who experienced the earthquake to estimate its intensity.

The Mercalli scale isn't considered as scientific as the Richter scale, though. Some
witnesses of the earthquake might exaggerate just how bad things were during the
earthquake and you may not find two witnesses who agree on what happened;
everybody will say something different. The amount of damage caused by the
earthquake may not accurately record how strong it was either.

Some things that affect the amount of damage that occurs are:

•the building designs,

•the distance from the epicenter,
•and the type of surface material (rock or dirt) the buildings rest on.

Different building designs hold up differently in an earthquake and the further you are
from the earthquake, the less damage you'll usually see. Whether a building is built on
solid rock or sand makes a big difference in how much damage it takes. Solid rock
usually shakes less than sand, so a building built on top of solid rock shouldn't be as
damaged as it might if it was sitting on a sandy lot
Nuclear explosions

The energy released by nuclear weapons is traditionally expressed in terms of the

energy stored in a kiloton or megaton of the conventional explosive trinitrotoluene
(TNT).

A rule of thumb equivalence from seismology used in the study of nuclear proliferation
asserts that a one kiloton nuclear explosion creates a seismic signal with a magnitude
of approximately 4.0. This in turn leads to the equation

where mTNT is the mass of the explosive TNT that is quoted for comparison (relative
to megatons Mt).

Such comparison figures are not very meaningful. As with earthquakes, during an
underground explosion of a nuclear weapon, only a small fraction of the total amount
of energy released ends up being radiated as seismic waves. Therefore, a seismic
efficiency needs to be chosen for the bomb that is being quoted in this comparison.
Using the conventional specific energy of TNT (4.184 MJ/kg), the above formula
implies that about 0.5% of the bomb's energy is converted into radiated seismic
energy Es. For real underground nuclear tests, the actual seismic efficiency achieved
varies significantly and depends on the site and design parameters of the test.
Tsunami
Tsunami waves are generated by
a submarine earthquake that
displaces the sea floor and water
column above. Long, low waves
are formed above the displaced
sea floor to compensate for the
momentary rise in sea level and
spread very rapidly (at the speed
of a jetliner) in the deep ocean.
In shallower water, the tsunami
slows to highway speeds and
builds in height until it breaks
and crashes onto the shore with
incredible force, causing
destructive flooding along low-
lying coastal areas.