Introduction to Earthquakes
‫مقدمة علم الزالزل‬
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Topics
What are earthquakes ?
Why do earthquakes occur ?
How do we describe earthquakes ?
Where do earthquakes occur ?
How often do earthquakes occur ?
How do we record earthquakes ?
Can we predict earthquakes ?
What to do in an earthquake ?
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What is an Earthquake?
 An
earthquake is a sudden movement of the
ground that releases elastic energy stored in
the rocks and generates seismic waves.
 After
the initial ground movement along the
fault, seismic waves propagate outward and
vibrate the ground.
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What is an Earthquake?
 Other
names for earthquakes include:
event (or seismic event)
 teleseism - an earthquake from far away
 seismicity - the general pattern of earthquakes in a region
 mainshock, foreshock, aftershock - earthquakes that form
part of a sequence over time
 tremblor - mainly used by media

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Why Do Earthquakes Occur?
 Short
answer: Most earthquakes occur
naturally as a consequence of the cooling of
the Earth. Earth was very hot when it formed
4.5 billion years ago.
 Long answer: involves plate tectonics and
mantle convection, which we will thoroughly
discuss later in the class.
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Why Do Earthquakes Occur?
 Some
earthquakes are actually induced by
human activity.
 These earthquakes are very rare and quite
small.
 The most common type of induced seismicity
has to do with the injection of high pressure
waste fluid into the Earth
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How are Earthquakes
Described?
By Location:
– We use two numbers to describe where an
earthquake has occurred: latitude and longitude
– Latitude varies from 90˚ S to 90˚ N, or -90 to +90
– Longitude varies from 180˚ W to 180˚ E, or
-180˚ to + 180˚
– These two numbers together are known as the
earthquake’s epicenter
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How are Earthquakes
Described?
By Location:
– Distances from earthquake epicenters to seismic
stations are usually given in terms of degrees
– 1 degree is about 111 km. How do you figure this out
? (Hint: Earth’s radius is 6371 km)
– Distance is given along great circle paths (the same
paths that airplanes take)
– Why was Steve Fossett’s trip bogus ?
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How are Earthquakes
Described?
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How are Earthquakes
Described?
By Location:
– It turns out that earthquakes often occur beneath the
earth’s surface at various depths
– When we combine the earthquake’s epicenter with its
depth we call it the hypocenter
– Another word for hypocenter is focus
– The world’s deepest earthquakes occur at a depth of
about 700 km
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How are Earthquakes
Described?
By Time:
– The time when an earthquake starts is known as
the origin time
– Origin times are reported in terms of Greenwich
Mean Time (GMT)
– Sometimes the day and month are combined into
the julian day
– Example:
2002 073 20:23:14.2 , means March 14, 2002 at
3:23:14.2 pm (St. Louis time)
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How are Earthquakes
Described?
By size:
– The oldest method for describing the size of an
earthquake is its maximum intensity (amount of
damage caused)
– These methods were used extensively in the past
before seismometers had been invented
(seismometers have been around for only about
100 years)
– The most well known intensity scale is known as
the modified Mercalli intensity scale. It varies from
I for very small damage to XII for very extensive
damage
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How are Earthquakes
Described?
By size:

Definition of modified Mercalli intensity V:
Felt by nearly everyone, many awakened. Some
dishes, windows, and so on broken; cracked
plaster in a few places; unstable objects
overturned. Disturbances of trees, poles, and
other tall objects sometimes noticed. Pendulum
clocks may stop.
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How are Earthquakes
Described?
By size:
– The more modern method of describing
earthquake size is by magnitude
– Magnitudes are related to the size of the
waves that are recorded by seismometers
– Magnitudes are much more accurate than
maximum intensities in describing
earthquake size
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How are Earthquakes
Described?
By size:
– Magnitudes are defined with a logarithmic
scale
– That means that 1 unit is really a factor of
ten
– For instance, a magnitude 5.0 earthquake
has 10 times larger seismic waves than a
magnitude 4.0 earthquake
– In terms of energy (different than
amplitude) a 5.0 is 30 times bigger than a
4.0
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How are Earthquakes
Described?
By size:
– There are four main magnitude scales
used by seismologists.
– The Richter scale is never used anymore.
It is strictly valid only for earthquakes in
southern California.
– It is impossible to convince journalists of
this fact.
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How are Earthquakes
Described?
By size:
– The four main magnitude scales are:
Ml (local magnitude)
Mb (body wave magnitude)
Ms (surface wave magnitude)
Mw (moment magnitude)
– Each one has pluses and minuses. We will
discuss them in detail later in this class.
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Where Do Earthquakes Occur ?
Earthquakes occur in along geological
structure known as faults.
A fault is a fracture in a rock a that has
been offset.
Faults generally occur in groups known as
fault zones.
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Where do Earthquakes Occur ?
 Faults
may be 100’s to 1000’s of km long, 10’s
of km deep, but are generally relatively
narrow.
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How Often Do Earthquakes Occur?
 Earthquakes
are quite common:
Earthquakes occur in the outer 700 km of
Earth and most are located in well-defined
belts.
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How Often Do Earthquakes Occur?
 Earthquakes
seldom occur alone, but are
generally part of a sequence of events
consisting of a mainshock and aftershocks
(occasionally foreshocks also occur).
 Generally
aftershocks are usually much
smaller than the mainshock - but can still be
damaging in large sequences.
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How Often Do Earthquakes Occur?
 Larger
mainshocks tend to have larger
aftershocks and longer aftershock sequences
 The
decay rate of aftershocks is given by
Omori’s Law:
earthquake frequency ~ 1/time
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How Often Do Earthquakes Occur?
 Some
earthquakes repeat on the same fault at
periodic intervals.
 These earthquakes are called characteristic
earthquakes and the time between such events
is called the recurrence interval.
 Typical recurrence intervals are hundreds to
thousands of years.
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How Do We Record
Earthquakes?
Devising systems that can accurately
measure and record ground motions is a
difficult task.
Large amounts of time and effort are spent
on developing and improving seismic
recording systems because of the
enormous value of seismic data.
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How Do We Record
Earthquakes?
seismoscope – an instrument that documents the
occurrence of ground motion (but does not record it
over time)
seismometer – an instrument that senses ground
motion and converts the motion into some form of
signal
accelerometer – a seismometer that records
acceleration, also known as strong ground motion
geophone – another name for a seismometer,
commonly used in active source seismology
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How Do We Record
Earthquakes?
seismograph – a system of instruments that
detects and records ground motion as a
function of time
seismogram – the actual record of ground
motion produce by a seismograph
seismometry – the design and development of
seismic recording systems
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How Do We Record
Earthquakes?
•First seismoscope!
• Constructed by Chang Heng
(a Chinese philosopher)
around 130 AD.
• Ball would roll out of
dragon’s mouth into frog’s
mouth in the direction of
ground motion.
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How Do We Record
Earthquakes?
132 – first seismoscope (Heng, China)
1751 – seismoscope which etched in sand (Bina, Italy)
1784 – first attempt to record ground motion as a
function of time using a series of seismoscopes
(Cavalli, Italy)
1875 – first true seismograph (Cecchi, Italy)
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How Do We Record
Earthquakes?
1889 – first known seismogram from a distant earthquake
is generated (Rebeur-Paschwitz, Germany)
1914 – first seismometer to use electromagnetic transducer
to sense ground motion (Galitzin, Russia)
1969 – first digital seismograph (data recorded in discrete
samples on a magnetic tape) (U.S. researchers)
1990s – broadcast of real time seismic data via internet
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How Do We Record
Earthquakes?
The first SLU seismometer was installed in 1909 (in
basement of DuBourg Hall)
It was part of the first North American seismological
network – the Jesuit Seismological Network
The first geophysics department in the Western
hemisphere was established at SLU in 1925, by J.B.
Macelwane, S.J.
At SLU in 1948 Florence Robinson became the first
woman geophysicist to obtain a PhD in North America
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Can We Predict Earthquakes ?
No!! There is no proven, reliable method
for predicting earthquakes.
A method must provide statistically
significant results.
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Can We Predict Earthquakes ?
Example:
– Every day I could predict a magnitude 5.5
earthquake in Southern California and
eventually I would be right and I will have
“predicted” the earthquake.
– But this would not be statistically significant.
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Can We Predict Earthquakes ?
Because earthquakes cannot be predicted
most of the work is directed toward
mitigating the hazard from earthquakes.
However, there are legitimate scientists
who work on methods of predicting
earthquakes.
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Can We Predict Earthquakes ?
Some of the things people study for
possible use in predicting earthquakes:
– Seismicity patterns
– Electromagnetic precursors
– Variations in depth of the water table
– Variations in radon concentration in well water
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Why Study Earthquakes?
Large earthquakes can be catastrophic to
cultures and societies:
– Can this damage be mitigated in the future?
(Yes)
– Can earthquakes be predicted? (Probably
not)
– Can earthquakes be prevented? (VERY
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Why Study Earthquakes?
Earthquakes are the dominant source of
seismic waves
Analysis of seismic waves gives us the
best view into the interior of the Earth:
Seismologists use seismic waves to image the
Earth in the same way that doctors use ultrasound
to image unborn babies
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Why Study Earthquakes?
Techniques used in studying earthquakes
can be applied to studying nuclear
weapons tests.
This is called verification seismology and
aims to answer questions like:
– Did nation X test a nuclear device?
– How big was the weapon nation Y tested?
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Who Studies Earthquakes ?
• Earthquake science is broad and
interdisciplinary and is studied by those in
fields such as:
• Seismology, geophysics, geochemistry,
geology, and engineering
• Economics, history, and sociology
• Students in EAS-A193 at SLU
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What to do in an earthquake …
If you are in bed:
– Remain in bed until the shaking stops
– Hold on
– Protect your head with pillow
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What to do in an earthquake …
If you are driving:
– Slow down
– Drive to a clear area and stop
– Remain in the car until the shaking stops
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What to do in an earthquake …
If you are outdoors:
– Find a clear spot away from tree, power lines,
and buildings
– Drop to the ground
If you are near a tall building:
– duck into a doorway to avoid debris from
above
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