Class Notes:
Earthquakes
Early theories on the origin of Earthquakes:
Japan: The demigod daimyojin hold a giant catfish, the
earthquake namazu down with a giant pivot stone. When the catfish
flailed, the ground shook.
Greece: Earthquakes were commonly thought to be the result of the power
and capricious behavior of the Gods, and in particular, temper tantrums of
Poseidon, the God of the Sea. Aristotle, on the other hand, believed strong
winds trapped in caves caused earthquakes as the struggle to escape.
Many ancient cultures thought earthquakes were the result of the earth being balanced on the back of one or more animals, e.g.
Native America: a giant tortoise
Mongolia: a large frog
China: a giant ox
India: seven serpents
India: four elephants standing on a giant turtle that was standing on a
cobra.
East Africa: balanced on the horns of a cow standing on a stone on
the back of a giant fish.
Etc. etc. (more available on request)
The 1906 Earthquake (fig. 11.4) led to the development of the Elastic
Rebound Theory (fig. 11.5)
Vertical displacement due to the 1964 Alaska Earthquake
is the point on the fault plane where the
slip (movement) was initiated. (fig. 11.2)
is the point on the ground directly above
the point where the slip (movement) first occurred. (fig. 11.2)
Ground shaking caused by earthquakes is measured by , instruments which can record both vertical
and horizontal ground motion (fig. 11.7 & 11.8).
The record of the shaking is a .
These can record the arrival time, duration, amplitude and period of both body and
surface waves (fig. 11.10).
___________ can travel through the body of the earth. The
two types:
-waves, are compressional waves (fig.11.9a).
They exert a push-pull motion on the rocks parallel to the direction of wave
propagation. They are also referred to as ÒPrimary-wavesÓ.
-waves, are shear waves (fig. 11.9b).They
exert an up and down motion on the rocks perpendicular to the direction of wave
propagation. They are also referred to as ÒSecondary-wavesÓ.
____________ (fig. 11.9c,d) form when the energy of
body waves intersects the surface of the earth. That energy then is trapped at the
surface, where it then travels as waves along the ground.
Properties of Seismic
Waves:
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Type of Wave |
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Speed |
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Amplitude |
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Period |
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Medium |
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Time Lag Between S and P waves increases with Distance from Epicenter
A (fig. 11.11) plots the predicted arrival time of S- and P-waves as a
function of distance.
Q. How can it be used to determine distance from the epicenter using
only data obtained for a seismogram record (fig. 11.11)? See also GEODe disk.
A.
Q. How can you use data obtained from a Travel Time graph to
locate the epicenter of an earthquake on a map? (fig. 11.12)
A.
Global seismicity delineates the worldÕs plate margins.
Map of US epicenters: 1899-1990
Map of US earthquake hazards.
Q. Do earthquakes ever occur in areas other than at plate boundaries?
Examples?
A.
Modified Mercalli Intensity map of the 1886 Charleston EQ.
I = Not felt
except by a very few under especially favorable circumstances
II = Felt only
by persons at rest
IIIÐIV=
Felt by persons indoors only
VÐVI= Felt
by all; some damage to plaster, chimneys
VII =People
run outdoors, damage to poorly built structures
VIII
=Well-built structures slightly damaged; poorly built structures suffer major damage
IX = Damage
considerable in specially designed structures. Buildings shifted off foundations.
Ground cracked conspicuously.
X = Some well-built
structures destroyed. Most masonry and frame structures destroyed. Ground badly
crackedÉÉ
XI = Few masonry
structures remain standing; bridges destroyed
XII =Damage
total; waves seen on ground; objects thrown into air
Modified Mercalli Intensity Map of the New Madrid
Earthquakes, 1811-1812
Modified Mercalli Intensity Map of the 1994 Northridge
Earthquake
Q. What is odd about the maximum values of intensity as measured in
this (Northridge) earthquake?
A.
measures the damage caused by an earthquake
- which in turn depends on:
1)
2)
3)
4)
measures the energy released during
an Earthquake!
For example:
Mr
=log10 (Amplitude*)
*measured on bedrock 100 km from epicenter
Q. How can you determine this from a seismogram? (fig. 11.15)
A.
This is a Logarithmic
Scale, i.e.
Increase1
unit = 10 times greater shaking
Increase1
unit ~ 32 times greater energy
In theory,
it is an open ended scale, however the largest quake ever recorded had a M =
8.9. Why should there be an upper limit in nature?
(N.B.
Earthquakes< M = 2 are not felt by people)
|
Magnitude |
Amplitude |
Energy |
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1 |
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2 |
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3 |
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4 |
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Richter Magnitude versus Energy (table 11.3)
There is much truth to the statement:
ÒEarthquakes donÕt
kill people, buildings do!Ó
Summary of Earthquake Hazards:
1) Building failure caused by
a)
Fault Rupture
b)
Ground shaking
c)
Liquefaction of sediment
d)
Landslides
3) Tsunamis
4) Dam Failure
5) Fires
Some Earthquakes of Special Interest
1906 San Francisco Earthquake and Fire
M ~ 8.1-8.2
Lives Lost: 700
Damage: ~$400 million
(most due to fire). Greatest damage on Òmade-landÓ.
Isoseismal Map of the 1906 San Francisco Earthquake (Max = XI)
Some effects:
Earthquakes Associated with Convergent Plate Margins (fig. 2.22,
11.27)
Q. What are Seismic Gaps? (fig. 11.25)
A.
1964 Alaskan Earthquake (The Good Friday Earthquake) fig. 11.16
M = 8.3-8.4
Lives Lost: 131
Damage: ~$310 million
Notable for large tsunami and pattern of re-building
Some effects (fig. 11.17,
Landslide at Turnagin Heights (fig. 11.22)
Prince William Sound, 1964
Q. What is a Tsunami and how is it formed (fig. 11.20)?
A.
Hilo, Hawaii, 1946 (fig. 11.21, 11D)
Q. What was different about Hilo in 1960?
A.
Q. What can you do to reduce the potential damage from tsunamis?
A.
M = 6.5
Lives Lost: 65
Damage: ~ $
550Million
Lesson learned: existing building codes for highways not adequate;
dam almost failed
Transform Plate Boundary
Some effects
M =
7.8 (8.2?)
Lives
Lost: 242,419 (500,000?)
Damage
~$363 million
Most
lives lost due to an earthquake in the 20th century.
Strike-slip
fault associated with continental collision
M = 8.1
Lives Lost: 9500
Damage: ~$4 billion
Notable for damage occurring >350 km from epicenter due to differential
ground amplification and building characteristics (fig. 11.18)
Convergent Plate Boundary
Some effects (fig. 11.18, 11B)
M = 6.9
Lives Lost: ~25,000 (100,000?)
Damage: $14.2 billion
Lessons: poor construction techniques resulted in high losses for moderate
earthquake.
Transform (?) Plate Boundary
1989 Loma Prieta Earthquake (the World Series Earthquake)
M = 7.1
Lives Lost: 62
Damage: >$
6Billion
Lessons: Bridges and overpasses still not safe. Role of local geology.
Transform Plate Boundary
Some effects (fig. 11.14)
- Collapsed Oakland Freeway
- Role of Geology in Freeway Collapse (fig. 11C)
- California Speed Bump
- FAULT!
M = 6.7
Lives Lost: 61
Damage:
$15-25?Billion
Notable for the amount of damage in a moderate EQ and the implication
of a Òblind thrustÓ
Transform Plate Boundary
Mercalli Intensity Map of the 1994 Northridge Earthquake
Some effects
Highway Failure in 1994 Earthquake (fig. 11.23)
Comparison of the 1971 (San Fernando) and 1994 (Northridge) Earthquake
Foci
Q. What is a ÒBlindÓ Thrust?
A.
M = 6.9
Lives Lost: 5472
Damage: >$120 Billion
Notable for amount of damage for size of earthquake (a sign of the
future?)
Convergent Plate Boundary
Effects of the 1995 Kobe, Japan, Earthquake (fig. 18.18)
M = 7.4
Lives Lost: ~15,000
Damage: ~6.5 Billion
Notable for:
Transform Plate Boundary
Some effects
What have we learned these lessons?
Housing built along the 1906 trace of the San Andreas Fault
Q. Where is the Hayward Fault located?
A.
Q. WhatÕs odd about the Berkeley Football Stadium?
A.
Key public facilities located along the Hayward Fault (e.g. Schools,
Hospitals, Fire and Police Stations
Shaking Intensity map for the San Jose, CA region, assuming a
M=7earthquake on Hayward Fault.
Q. What was the calculated probability of a major earthquake
occurring in the San Francisco area during the 30 years after 1988?
A.
Q. How had that probability changed by the year 2000 and why?
A.