Earthquakes
IV. Measuring the size of
Earthquakes
Intensity vs. Magnitude
A. Intensity: A measure of
the degree of earthquake
shaking at a given locale
based on the damage to
human structures, extent
of ground rupture, and
human/animal reactions
2) Earthquake Intensity and magnitude
 Mercalli intensity scale
Intensity of shaking & damage at a specific location
Depends on distance to earthquake
& strength of earthquake
 Magnitude
A measure of the energy released in an earthquake
Depends on size of fault that breaks
Earthquakes
IV. Measuring the size of
Earthquakes
Intensity vs. Magnitude
A. Intensity
1. Modified Mercalli Intensity
Scale: after volcanologist
Guiseppe Mercalli who
developed the original
form in 1902
12 levels of intensity,
I, lowest to XII. highest
Compares historical EQ’s, preinstrumental time
Modified Mercalli Intensity Scale
I.
Not felt except by a very few under especially favorable conditions.
II. Felt only by a few persons at rest, especially on upper floors of buildings.
III. Felt quite noticeably by persons indoors, especially on upper floors of
buildings. Many people do not recognize it as an earthquake. Standing
motor cars may rock slightly. Vibrations similar to the passing of a truck.
Duration estimated.
IV. Felt indoors by many, outdoors by few during the day. At night, some
awakened. Dishes, windows, doors disturbed; walls make cracking sound.
Sensation like heavy truck striking building. Standing motor cars rocked
noticeably.
V. Felt by nearly everyone; many awakened. Some dishes, windows broken.
Unstable objects overturned. Pendulum clocks may stop.
VI. Felt by all, many frightened. Some heavy furniture moved; a few instances of
fallen plaster. Damage slight.
VII. Damage negligible in buildings of good design and construction; slight to moderate
in well-built ordinary structures; considerable damage in poorly built or badly designed
structures; some chimneys broken.
VIII. Damage slight in specially designed structures; considerable damage in ordinary
substantial buildings with partial collapse. Damage great in poorly built structures. Fall
of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.
IX. Damage considerable in specially designed structures; well-designed frame
structures thrown out of plumb. Damage great in substantial buildings, with partial
collapse. Buildings shifted off foundations.
X. Some well-built wooden structures destroyed; most masonry and frame structures
destroyed with foundations. Rails bent.
XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent
greatly.
XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air.
Earthquakes
IV. Measuring the size of
Earthquakes
Intensity vs. Magnitude
A.
Intensity
1. Modified Mercalli Intensity
Scale:
Problems: cannot find epicenter
exactly
EQ either far away & strong or close
& weak
Intensity of shaking depends on
rock type
Damage: depends on quality of
building construction
Can’t be used in unpopulated areas
or oceans
Earthquakes
IV. Measuring the size of
Earthquakes
Intensity vs. Magnitude
B. Magnitude: more quantitative,
based on ground
motion/amount of energy
released
1. Richter Scale
1935 Charles Richter, Caltech
Based on the amplitude of the
largest seismic wave recorded
(P, S, or surface)
Magnitude is the most common
measure of an earthquake's size.
It is a measure of the size of the
earthquake source and is the
same number no matter where
you are or what the shaking feels
like.
The Richter scale measures the
largest wiggle on the
seismogram; other magnitude
scales measure different parts of
the earthquake.
Intensity is a measure of the
shaking and damage caused by
the earthquake, and this value
changes from location to location.
Earthquakes
IV. Measuring the size of
Earthquakes
1. Richter Scale
a. Vary so much in strength:
nonlinear scales
Based on logrithmic scale—
each unit corresponds to 10 fold
increase in amplitude
At a distance of 100 kilometers:
M1 = amplitude of 0.001 mm
M2 = amplitude of 0.01 mm
M3 = amplitude of 0.1 mm
M4 = amplitude of 1 mm
M5 = amplitude of 10 mm
M6 = amplitude of 10 cm
M7 = amplitude of 1 meter!
Earthquakes
IV. Measuring the size of
Earthquakes
1. Richter Scale
Magnitude 6 is 10 times larger than
a M5 or
Magnitude 7 is 100 times larger
than M5
b. Each magnitude unit
corresponds to 32 fold increase
in energy
Magnitude 6 released 32 times
more energy than a M5 or
Magnitude 7 releases >1000 times
more energy than a M5!
Richter Magnitude
How many kilograms of TNT would have this much energy?
0
0.6
1.0
20
2.0
600 Smallest EQ people can normally feel
3.0
20 000 Most people near epicenter feel the quake, Nearly
100, 000 occur every year of size 2.5 - 3.0
4.0
60 000 A small fission atomic bomb; EQ’s above 4.5 can cause
local damage
5.0
20 000 00
A standard fission bomb, similar to the first bomb tested
in New Mexico, U.S.
6.0
60 000 000
A hydrogen bomb; can cause great damage locally, About
100 shallow EQs of size 6.0 every year
7.0
20 billion Major earthquake; about 14 every year. Enough energy
to heat New York City for 1 year. Large enough to be
detected all over globe
8.0
60 billion Largest known: 8.9 in Japan and in Chile/Ecuador; San
Francisco destroyed by 7.8 in 1906
9.0
20 trillion
Roughly the world’s energy usage in a year, Chile, 9.5 in
1960 and Alaska, 9.2 in 1964
Earthquakes
IV. Measuring the size of
Earthquakes
Comparison: Southern California
has ≥ M 8 every ~160 years
Last large EQ in 1857 = 146 years
ago
So does the occasional M6 release
energy to alleviate this
problem?
Energy released in M8 is 1000 times
greater than mag 6
Thus, LA needs 1000 M6’s to equal
an M8 (LA has a M6 every ~5
years)
So LA can expect a seismic event
100 times as violent as any in
recent memory!
Earthquake Magnitude
Earthquakes release a tremendous amount of
energy, which is why they can be so destructive.
The table below shows magnitudes with the approximate
amount of TNT needed to release the same amount of
energy.
Magnitude
4.0
5.0
6.0
7.0
8.0
9.0
Approximate Equivalent
TNT Energy
1,010 tons
3,1800 tons
1,010,000 tons
31,800,000 tons
1,010,000,000 tons
31,800,000,000 tons
Earthquakes
IV. Measuring the size of
Earthquakes
1.
Richter Scale
c. Problems:
Limited to M7 or smaller
M8 wave amplitude is 10 meters =
too big to record on
seismometer
Developed for California rocks—not
necessarily good everywhere
Amount of shaking is partially due to
rock type, so while two EQs
may shake the same, they may
actually release different
amounts of energy
Earthquakes
IV. Measuring the size of
Earthquakes
2. Moment-Magnitude Scale—the
newest scale—used today by
seismologists—measures the EQ
total energy (measures the “cause”
not the “effect”)
The shaded regions on the fault
surface are the areas that
rupture during different size
events
Seismic Moment = (fault rupture area
or L x W)
X (total amount of slip along the
fault)
X (strength of rock) (the force that
was required to overcome the
friction sticking)
M0 = A x D x μ
Seismic moment (M0) = fault area x displacement x strength of rock
Earthquakes
IV. Measuring the size of
Earthquakes
2. Moment-Magnitude Scale
Calibrated so small and moderate
quakes equal Richter scales, but
moment-magnitude much better
for large quakes
A more direct measure of EQ
Data are taken directly from the
physical fault that produced the
quake
Reflects the energy released in a
EQ
Strongest recorded EQ, 1960
Chilean quake with Moment
Magnitude of 9.5
Most Destructive Known Earthquakes on Record in the World
(> 50,000 deaths) (Listed in order of greatest number of deaths)
Date
Location
January 23, 1556
October 11, 1737
July 27, 1976
August 9, 1138
May 22, 1927
December 22, 856+
December 16, 1920
China, Shansi
India, Calcutta**
China, Tangshan
Syria, Aleppo
China, near Xining
Iran, Damghan
China, Gansu
March 23, 893+
September 1, 1923
December 28, 1908
Iran, Ardabil
Japan, Kwanto
Italy, Messina
September, 1290
November, 1667
November 18, 1727
November 1, 1755
December 25, 1932
May 31, 1970
1268
January 11, 1693
May 30, 1935
China, Chihli
Caucasia, Shemakha
Iran, Tabriz
Portugal, Lisbon
China, Gansu
Peru
Asia Minor, Silicia
Italy, Sicily
Pakistan, Quetta
February 4, 1783
June 20, 1990
Italy, Calabria
Iran
Deaths
M
Comments
830,000
300,000 2004, Sumatra >300,000 people, M = 9.3
255,000*
8.0
230,000
200,000
8.3
Large fractures.
200,000
200,000
8.6
Major fractures,
landslides.
150,000
143,000
8.3
Great Tokyo fire
70,000
7.5
Deaths from to 100,000
earthquake & tsunami.
100,000
80,000
77,000
70,000
8.7
Great tsunami
70,000
7.6
66,000
7.8
Great rock slide and flood
60,000
60,000
30,000
7.5
Quetta almost completely
destroyed (~60,000)
50,000
50,000
7.7
Landslides.
* Official casualty figure--estimated death toll as high as 655,000.
+ Note that these dates are prior to 1000 AD. No digit is missing.
** Later research has shown that this was a typhoon, not an earthquake. (1737 Calcutta Earthquake Bilham, 1994)