Earthquakes and Seismology
I. Earthquakes and their definition
A. Sudden release of energy by the Earth
1. The vibration of the Earth in several directions as a result of faulting
2. Type of faulting will determine the severity of the earthquake
3. Usually a displacement of rock along preexisting fracture zones- either deep or
shallow
B. Aftershocks result and are adjustments along a fracture zones to the movement at one
spot along the fault line
C. Earthquakes result from the faulting and adjustments of tectonic plates
II. Elastic Rebound Theory
A. Developed by H.F. Reid after the 1906 earthquake in San Francisco
1. The rocks on opposite sides of the fault would be progressively bent or warped
until it exceed the strength of the rocks when it would rupture
2. The rocks would then snap back and the energy stored up would be released
B. The greater the resistance or strain the greater the release of energy
III. Seismology is the study of earthquakes
A. In 1880 the first seismograph was built to detect , record and measure the distinctive
vibrations produced by earthquakes
B. The energy from fault movement is released in the form of seismic waves
1. These waves radiate out from the source like the ripples produced in water when
a rock is thrown in a pond
2. These waves move outward in concentric circles from the source
C. Most earthquakes result when rocks rupture from buildup of pressure along a fracture
zone
D. Once a rupture starts it will move along the fracture at several kilometers per second
1. This continues along the fault as long as stress along the fracture is great enough
to cause movement
2. The length of the rupture can be a few meters to 100’s of kilometers
3. The longer the rupture, the longer it takes to release the stored energy and the
longer the ground shaking and rolling
E. The location of the first rupture is the focus or hypocenter
F. The point on the Earth’s surface above the focus is the epicenter- which is the point that
we hear about in news reports
G. Three categories of earthquakes are recognized
1. Shallow focus earthquakes- focal depth of < 70 km
2. Intermediate focus- foci between 70 and 300 km
3. Deep focus- deeper than 300 km
4. Almost 90% of all earthquakes are shallow focus and these are the most
destructive
5. Deep focus earthquakes make up only 3% of all quakes measured
H. Earthquake types reflect the type of plate margins
1. Shallow earthquakes occur along divergent or transform plates
2. Intermediate and deep earthquakes occur along convergent margins, especially in
the circum-Pacific belt
3. A pattern has emerged when the depth of earthquakes is plotted
4. The earthquakes define a region known as the Benioff Zone in a subducting plate
boundary
5. This defines the angle of the subduction zones or the angle of descent of the plate
IV. Frequency and distribution of earthquakes
A. Most earthquakes occur along plate boundaries
B. The ones that are far from boundaries are rare but can be devastating
C. 80% of all earthquakes occur around the Pacific, most are from convergent plate
boundaries
D. The San Andreas Fault is along a transform plate boundary where plates slide past each
other
E. Mediterranean-Asian belt is the second major belt making up 15% of the earthquakes
1. This occurs from the Himalayas, across Turkey and Iran and west through the
southern part of Europe
2. The 1990 earthquake in Iran killed 40,00 people and the 1993 earthquake killed
30,000 people
F. The remaining 5% take place in the interiors of plates and along spreading ridges most are not strong
G. Although the New Madrid earthquake that occurred in Missouri in 1812 is thought to be
particularly strong and nearly destroyed the town of New Madrid
1. The earth movements were felt from the Rock Mountains to Boston, From
Canada to Mexico
2. The land sank several meters in some areas and land flooded
3. The Mississippi River changed its course and is said to have run backwards
4. The cause of these quakes is not well understood but are thought to be the result
of flexure in the plate and the result of built up interior stresses
5. These quakes may be associated with ancient or presumed inactive faults
6. This may also be the result of a failed rifting zone that was created over 100
million years ago
H. More than 150,000 earthquakes strong enough to be felt occur annually
1. 900,000 more are recorded by seismographs that are too small to be felt
2. The small ones are the result of energy released along fractures that reduce the
chances of a large one occurring
V. Seismic Waves
A. Two types of seismic waves
1. Body Waves- or waves that travel through the body of the Earth
2. Surface Waves- waves that travel along the surface of the Earth
B. Two types of Body Waves generated by earthquake
1. P-waves or primary waves or pressure waves
a) These are fastest waves
b) Travel through solids, liquids, and gases
c) These are push-pull waves like sound waves
d) The material is expanded and compressed as the waves moves through it
e) These even move through air and can be heard
2. S-waves or secondary waves or shear waves
a) Slower and only travel through solids
b) Move perpendicular to the direction of travel
c) They shear the rocks as they move through
d) Do not transmit through liquids or gases because there is no shear
strength
3. Velocities of these waves are determined by the density and elasticity of the
material they travel through
a) Greater the density, the slower the seismic wave moves
b) Greater elasticity, the faster they move
C. Surface Waves travel along the surface and produce a rolling effect
1. Two types of surface waves- Rayleigh and Love waves
2. Named after Lord Rayleigh and A.E. Love
3. Rayleigh waves are slower and act as waves on the ocean
4. Love waves move back and forth in a horizontal plane perpendicular to the
direction of travel
5. These are destructive to buildings
D. The epicenter of an earthquake can be determined by using the time-distance graph and
plotting the distance on a map
VI. The strength of a earthquake is measured by two methods- intensity felt and magnitude of the
energy released by the earthquake
A. Intensity is a subjective measurement obtained by the kind of damage and how people
felt by using the Modified Mercalli Intensity Scale page 186
1. This has been used to understand quakes in the distant past
2. Gives a rough estimate on the effects of the local geology on building
construction
3. Effected by distance from epicenter, depth of quake, local geology, duration of
shaking and population density
4. Damage done is greatest in poorly consolidated fill because it amplifies the
shaking
5. Least damage in solid rock
B. Magnitude is the total amount of energy released at the source
1. It is measured on an instrument called a seismograph invented by Charles F.
Richter and uses the Richter Scale, it is an open ended scale beginning at 1 with
each numerical representing a 10 fold increase in wave amplitude
2. The largest earthquake energy recorded is 8.6
3. Magnitudes of 9’s are possible, but generally rocks are not able to store the
energy to release this magnitude
4. Magnitude is determined by measuring the amplitude of the largest seismic wave
(Page 188)
5. The magnitude measures a ten fold increase but the energy released
corresponds to almost a 30 fold increase (because this is a maximum wave
amplitude reading, and all the various wave energies are not counted)
VII. Destructive Effects
A. Ground shaking, landslides, seismic sea waves
B. Buildings and roads destroyed, water and gas lines broken, fires, dam failure and floods,
C. General panic, loss of life and injury result depending on the time of day and density of
population
1. Ground Shaking is important to the structures
a) Fill material can amplify the seismic waves
b) Liquefaction can occur in water soaked sediments or fill where the
shaking causes the ground to move or flow
c) Brick and unreinforced concrete can crumble
d) California codes require stronger built buildings
2. Fire is a major problem
a) Gas lines break along with water lines to fight the fire
b) In 1906, San Francisco burned for three days because the fire could not
be put out
c) Japan has poorly constructed wood buildings that experienced devastating
fires in 1923, 143,000 people died
VIII.
3. Tsunamis result from earthquake movement on the ocean floor and come to
shore developing heights of 60 meters or more!
a) They travel a several hundreds of kilometers per hour but only reach a
wave crest of 10 feet at sea, so a ship would not even notice it pass by!
b) Hilo, Hawaii experienced one is1946
c) Reported that the children went out to the beach to see that the ocean had
gone way down
d) Fish and shells were everywhere and the people were picking them up by
the hundreds
e) Then the tsunami came in and the people ran but it was too late
f) A whole school was wiped out in the low lying areas
g) Now have a early warning system
4. Landslides and ground failure can be very destructive
a) Rock slides
b) Avalanches
c) Unstable cliffs and overhangs
d) Unstable coastlines
Earthquake prediction has been a difficult science- precursors sought
A. Seismic risk maps have been made
B. Fault maps have been made to determine if an area has had an earthquake recently or if it
is locked in place building up energy
C. Tiltometers can help tell if an area is bulging or tilting
D. Fluctuations in water well levels
E. Local changes in the earth’s magnetic field
F. Water pumped into fractures can release tension and cause small earthquakes to form