Visualizing
Geology
First Edition
Barbara W. Murck
Brian J. Skinner
Dana Mackenzie
Chapter 5
Earthquakes and Earth’s Interior
Copyright © 2008 by John Wiley & Sons, Inc.
On December 26, 2004 the most powerful earthquake
in 40 years – and the third most powerful of the last
century.
Earthquakes around this region are quite common, the
great difference with this earthquake is that it
provoked the largest and deadliest tsunami in history.
The quake began when part of the Indian Plate, which
is subducting under the Eurasian Plate, suddenly
slipped downward approximately 15 meters. The
motion pushed the seafloor as much as 5 meters on the
Eurasian Plate.
On the surface killer waves generated by the sudden
movement and started to sweep towards Indonesia,
Thailand, Sri Lanka, and India. When the waves
reached shore they were between 20 to 30 meters tall
and swept far inland, obliterating everything in their
path.
The resulting devastation caused hundreds of billions
of dollars in damages and at least 275,000 deaths.
Scientist cannot prevent earthquakes but the science is
getting better in understanding the mechanisms
associated with plate motions and preparation for
future disasters.
Seismology – The scientific study of earthquakes and
seismic waves.
Most earthquakes are caused by the sudden movement of
stressed blocks of Earth’s crust along a fault. The friction
between huge blocks of rock causes them to size up,
bringing the motion along that part of the fault to a
temporary stop. While the fault remains locked by friction
energy continues to build up as a result of the plate
motion, causing rocks adjacent to the jammed section to
bend and buckle. Finally the the stress becomes great
enough to overcome the friction along the fault. The
energy released by the slip of two blocks of rock becomes
a violent earthquake.
EVIDENCE OF LATERAL FAULT MOTION –
San Andreas Fault - California
EVIDENCE OF VERTICAL FAULT MOTION –
Alaska, 1964 – Good Friday Earthquake
Elastic rebound theory – The theory that continuing stress
along a fault results in a buildup of elastic energy in the rocks,
which is abruptly released when an earthquake occurs.
The elastic rebound theory suggests that rocks, like all solids,
are elastic (within limits). This means that they will stretch or
bend when subjected to stress, and snap back when stress is
removed.
Like a guitar string after is plucked, they continue vibrating.
These vibrations are called seismic waves. Like sound waves
these waves travel long distances from their place of origin.
Seismic wave – An elastic shock wave that travels outward in
all directions from an earthquake’s source.
Earthquake hazards and prediction
Each year more than a million earthquakes occur
around the world. Fortunately, only few are large
enough, or close enough to populated areas. Geologist
are working hard to improve their forecasting ability
to the point where effective and accurate early
warnings can be produced.
Earthquake hazards – Earthquakes can cause a total
devastation in a matter of seconds. The most
disastrous quake in history occurred in the Shaanxi
Province, China, in 1556, killing an estimated 830,000
people.
Path of the Tsunami
Experiment to better understand the force of a tsunami
Earthquake hazards –
Primary hazards - Ground motion, with the
resulting collapse of buildings, bridges, and
other structures. To make matters worse,
movement on one part of the of a fault can cause
stress along another part, generating other
earthquakes, called aftershocks.
Secondary hazards – Landslides, fires, ground
liquefaction, and tsunamis are considered
secondary hazards caused by an earthquake.
Secondary hazards, landslide – Huascaran, Peru
Secondary hazards, open fissure – Golcuk,
Turkey
Secondary hazards, fire – San Francisco,
California
Secondary hazards, tsunami – Kalutara, Sri
Lanka
Earthquake prediction –
Charles Ritcher, inventor of the Ritcher
scale for quantifying on the severity of
earthquakes, once said, “Only fools,
charlatans, and liars predict earthquakes”.
Today unfortunately, this is more or less
correct. However scientist understand the
mechanisms a lot better and may prove
Ritcher wrong in the near future.
There are two aspects to the problem of earthquake prediction:
Short term and long-term predictionShort-term prediction – Identifies the exact time, magnitude, and location
of an earthquake in advance to the actual event, providing authorities to
issue and early warning. Short term prediction has not been very
successful to date. We measure magnetic properties of the rock, the level
of well water could drop, an increment in radon gas in the groundwater,
strange animal behavior, glowing auras, unusual radio waves,
development of small cracks or fractures, swarms of tiny earthquakes.
The most successful prediction was made in China in 1975, where slow
tilting of the land, fluctuations of the magnetic field, and numerous
foreshocks advice scientists of a large earthquake.
Long time prediction – Involves the prediction of a large earthquake
years or even decades in advance of its occurrence.
Paleoseismology – The study of prehistoric
earthquakes.
Designing for earthquake safety
Japanese schoolchildren practice for an
earthquake
Apartment buildings in Niigata, Japan – after an
earthquake caused liquefaction of the ground
A major quake in Kobe, Japan
How does the elastic rebound theory explain the
violent tremors that occur during earthquakes?
How do scientist predict earthquakes?
What are some of the primary and secondary
hazards associated with earthquakes?
Explain the connection between earthquakes and
plate tectonics.
The science of seismology
Seismograph – An instrument that
detects and measures vibrations of
Earth’s surface.
Seismogram – The record made by a
seismograph.
Ancient Chinese seismograph
The most advanced seismographs measure the
ground’s motion optically and amplify the signal
electronically. Vibrations as tiny as one
hundredth-millionth of a centimeter can be
detected.
Travel path of seismic body waves
Seismic waves
The energy released by an earthquake is
transmitted to other parts of Earth in the form of
seismic waves.
Body wave – A seismic wave that travels through
Earth’s interior.
Surface wave – A seismic wave that travels along
Earth’s surface.
Focus – The location where rupture commences
and an earthquake's energy is first released.
Body waves are divided into two types:
Compressional wave – A seismic body wave
consisting of alternating pulses of compression and
expansion in the direction of wave travel; P wave
or primary wave.
Shear wave – A seismic body wave in which rock
is subjected to side-to-side or up-and-down forces
perpendicular to the wave’s direction of travel; S
wave or secondary wave.
Surface waves – Travel long or near Earth’s
surface, like waves along the surface of the
ocean. They travel more more slowly than
P and S waves, and they pass around the
Earth, rather than through it. Thus, surface
waves are the last to be detected by a
seismograph.
Determining the earthquake epicenter
Stations:
Eureka
Elko
Las Vegas
S-P interval 49 seconds
S-P interval 72 seconds
S-P interval 64 seconds
Eureka 49 seconds - 480 km
Elko 72 seconds – 700 km
Las Vegas 64 seconds – 620 km
Calculating the Ritcher Magnitude
S wave amplitude 235
Calculating the Ritcher Magnitude
S wave amplitude 60
Calculating the Ritcher Magnitude
S wave amplitude 100
Magnitude 7.1 Ritcher scale
Read, study and complete the following
exercise at the following web page:
http://www.sciencecourseware.org/VirtualEarthquake/
Method of triangulation
Measuring earthquakes
Geologist use several different scales to
quantify the strength or magnitude of an
earthquake, by which we measure the
amount of energy released during the
quake.
The most familiar of these is the Ritcher
magnitude scale.
Measuring earthquakes
Ritcher magnitude scale – A scale of
earthquake intensity based on the recorded
heights, or amplitudes, of seismic waves
recorded on a seismograph.
Charles Ritcher developed his famous
magnitude scale in 1935.
Measuring earthquakes
The Ritcher scale is logarithmic, which
mean that each unit increases by a tenfold.
For example a magnitude 6 earthquake has
a wave amplitude ten times larger than a
magnitude 5 earthquake.
The Modified Mercalli Intensity Scale is
another type of measuring an earthquake
based on damage estimates.
Measuring earthquakes
Moment magnitude – A measure of
earthquake strength that is based on the
rupture size, rock properties, and amount of
displacement on the fault surface.
All scales measure the same thing, the
amount of energy released. In either
system, magnitude 9 is catastrophic,
whereas magnitude 3 is imperceptible to
humans.
Ritcher magnitude 6 – Damage on surface: small objects broken,
sleepers awake (Mercalli Intensity VII) – Energy released: about the
same as one atomic bomb – Parkfield, CA 2004
Ritcher magnitude 7 – Damage on surface: some walls fall, general
panic (Mercalli Intensity IX) – Energy released: about the same as 32
atomic bombs – Kobe, Japan 1995
Ritcher magnitude 8 – Damage on surface: wide destruction,
thousands dead (Mercalli Intensity XI) – Energy released: about the
same as 1000 atomic bombs – San Francisco, CA 1906
How does a seismograph detect earthquakes?
What are the major types of seismic waves?
What is the difference between the epicenter and
the focus of an earthquake?
How do P and S reveal both the strength and
location of an earthquake?
Describe the Ritcher and moment magnitude
scales
Studying Earth’s Interior
Studying Earth’s Interior
When scientists cannot study something by
direct sampling, a second method comes to
the forefront: indirect study or remote
sensing.
The seismic waves from an earthquake are
much like X-rays, in the sense that they
enter Earth near the surface, travel all the
way through it and emerge on the other
side.
How geologists look into Earth’s interior –
seismic methods
Before 1906, scientist’s understanding of
seismic waves was limited. But that year,
British geologist Richard Dixon Oldham
first identified the difference between P
waves and S waves. Based on this study
he suggested that the earth has a liquid
core.
Three distinct things can happen to seismic
waves when they meet such a boundary,
called seismic discontinuity:
1. The waves can be refracted, or bent, as
they pass from one material into another.
2. They can be reflected, which means that
all or part of the wave energy bounces
back.
3. They can be absorbed, which means that
all or part of the wave energy is blocked.
Seismic discontinuity – A boundary inside
Earth where the velocities of seismic
waves change abruptly.
Refraction – the bending of a wave as it
passes from one material into another
material, trough which it travels at a
different speed.
Reflection – The bouncing back of a wave
from an interface between two different
materials.
Absorption – All the energy is blocked.
Seismic tomography – CAT scanning allow
scientist to make up three dimensional
layers from two dimensional data.
How geologists look into Earth’s interior –
other methods.
Direct observation: drilling and xenoliths.
Deepest mine - South Africa 3.6 km
Deepest hole ever drilled – Kola
Peninsula, Russia 12 km
Xenoliths – Magma carries fragments from
unmelted surrounding rock.
Diamonds Messengers from the Deep –
They form at depths of 100 to 300 km
Uncut diamond
From South
Africa
How geologists look into Earth’s interior –
other methods.
Indirect observation: methods from
physics, astronomy, and chemistry
Magnetism –
Mass of or planet – Density of Earth is
5.5g/cm3 – Surface rocks 2.8 g/cm3
Meteorites -
Earth’s magnetic
field
Charged particles from the Sun entering
Earth’s atmosphere along magnetic field
lines
Why do seismic waves undergo refraction as they pass through the
Earth?
Explain why seismic data point to the existence of a liquid core
Identify several ways in which scientists can study Earth’s interior
indirectly or remotely
What kind of geological features are revealed as seismic discontinuities?
Why can’t geologist drill a hole down to the mantle?
How do scientists obtain mineral samples that come from the mantle?
What does the Earth’s magnetic field tell us about its interior?
Describe three pieces of evidence that indicate that Earth has a molten
iron-rich core.
A multi-layered planet
Crust, mantle, and core
Crust – The outermost compositional layer
of the solid Earth; part of the lithosphere
Composition of the crust is 95% igneous or
metamorphic rocks.
The boundary that separates the crust from
the mantle is called the Mohorovicic
discontinuity. Mantle rocks being denser
transmit P waves much more quickly.
Mantle – The middle compositional layer
of Earth, between the core and the crust.
The mantle consists mainly of iron and
magnesium silicate minerals.
Asthenosphere – Layer of weak ductile
rock in the mantle that is close to melting
but not actually molten.
Lithosphere – Earth’s rocky, outermost
layer, comprising the crust and the
uppermost part of the mantle.
Mesosphere – The boundary to the core and
mantle.
It is important to remember that the mantle
is mostly solid rock, except for small
pockets of melt in the asthenosphere. We
know that mantle has to be solid because
P and S waves travel through it.
Nevertheless, pressures and temperatures
deep within the Earth are so high that
even solid rock can flow, in very, very
small convection currents as described
before.
Core – Earth’s innermost compositional
layer, where the magnetic field is
generated and much geothermal energy
resides. The outer core must be liquid (S
waves stop) and because of the great
pressure and in spite of high temperatures
the inner core, must be solid.
What is the nature between the crust and
the mantle?
Why and how the inner core differs from
the outer core?
What are the major layers of the Earth?
Define the composition of the crust, mantle,
and core
Loch Ness
Loch Ness
Loch Ness
What reason can
you suggest for
this stream’s
strange
behavior?
Along which type of plate boundary do you
think that you’ll have the largest
earthquakes?
What is the elastic rebound theory?
List primary earthquake hazards
List secondary earthquake hazards
How many types of seismic waves?
Name the types of wave motion
On the seismogram below label the
following: arrival of P and S waves, S-P
interval, arrival of surface waves,
background noise
What is the difference between the Ritcher
Scale, Moment magnitude scale, and the
Mercalli scale.
What happens to the waves as they reach a
discontinuity?
What is the composition of the mantle and
the core?
What is the asthenosphere?
If we seismographs at three different
stations A, B, and C. A and B had an S-P
interval of 3 seconds and C had an
interval of 11 seconds. To which
station(s) is the earthquake near to?
Which station(s) received the first P waves?
A magnitude 8 earthquake is how many
times greater than a magnitude 7
earthquake?
Label the following
: mantle,
lithosphere,
oceanic crust,
asthenosphere,
outer core,
moho, inner
core, continental
crust,
mesosphere
The scientific study of earthquakes and seismic waves
is known as _______.
A) seismograph
B) seismogram
C) seismology
D) vulcanology
E) Tectonics
The _______ scale is a logarithmic scale that measures
earthquake intensity.
A) Barton
B) Wegener
C) Mercalli
D) Richter
E) none of the above
Fragments of unmelted rocks that are sometimes
incorporated in magma are known as
_______.
Which type of waves would cause more
damage?
_______ are the first waves to leave the focus
after an earthquake.
The _______ is the point on the surface directly
above the point of an earthquake.
Diamonds are incorporated in rocks called
_______ that come deep from within the
Earth.
The _______ is the part of the Earth's
interior where rocks start to melt.
An instrument that measures and detects
vibrations in the Earth is known as a
_______.
In the figure below, what is the approximate time of the
arrival of the P-waves?
A) 1 minute
B) 2 minutes, 15 seconds
C) 3 minutes
D) 4 minutes
E) 5 minutes, 30 seconds
In the figure below, what is the approximate time of the
arrival of the S-waves?
A) 1 minute
B) 2 minutes, 15 seconds
C) 3 minutes
D) 4 minutes
E) 5 minutes, 30 seconds
In the figure below, what is the approximate time of the
arrival of the surface waves?
A) 1 minute
B) 2 minutes, 15 seconds
C) 3 minutes
D) 4 minutes
E) 5 minutes, 30 seconds
According to the figure below, what is the approximate SP travel time?
A) 1 minute, 45 seconds
B) 2 minutes, 15 seconds
C) 3 minutes, 15 seconds
D) 0 minutes, 45 seconds
E) cannot be determined
The _______ is the Earth's rocky, outermost
layer.
The Earth's density as a whole is approximately
2.8 g/cm3.
Secondary hazards sometimes cause more
damage than the earthquake itself. (T/F)
A large, destructive wave sometimes caused by
am earthquake is called a _______.
According to the figure below, _______ is
the location of the earthquake.
A wave could be:
1)
2)
3)
The theory that stress is continually built up
along a fault and released when
earthquake occurs is known as _______.
The method of using data from three seismic
stations to locate an earthquake is known as
_______.
_______ are fragments of unmelted rock that are
sometimes incorporated in magma.
The average density of the Earth's crust is
_______.
A _______ wave is a wave that travels along the
earth's surface.
The area inside the Earth where rocks start
to turn plastic is known as the _______.
The Richter magnitude scale is a _______
scale.
Swarms of tiny earthquakes that may occur
before large earthquakes are known as
_______.
Describe the method of triangulation.
Which type of wave cannot travel through a
liquid.