ISNS 4359
EARTHQUAKES AND VOLCANOES
Spring 2005
Steve Bergman, Instructor
Lecture 6. Earthquake Intensity, Magnitude, and Energy
Seismic Intensity - a measure of violence of ground shaking (what you feel) based on damage done to human-made
structures, surface changes, and felt reports; intensity varies with distance from the hypocenter and ground conditions.
Isoseismals-lines of equal seismic intensity [intensity generally decreases away from the EQ focus, but not always in a
simple pattern -it is highly dependent on rocks and sediments-muds and unconsolidated sediments can amplify shaking].
Modified Mercalli Intensity Scale (1902, modified by H.O. Wood and F. Neumann) [MMI-Unitless]
[typical peak velocity (cm/sec) and acceleration near epicenter as a fraction of g (980 cm/sec2)]
I. People do not feel any Earth movement unless circumstances are exceptionally favorable.
II. A few sensitive people might notice movement if they are at rest and/or on the upper floors of tall buildings.
III. Many people indoors feel movement. Hanging objects swing back and forth. People outdoors might not realize that an
earthquake is occurring; very little to no damage.
IV. Most people indoors feel movement. Hanging objects swing. Dishes, windows, and doors rattle. The earthquake feels like
a heavy truck hitting the walls. Parked cars rock. [v=1-2, a~0.015-0.02g]
V. Almost everyone feels movement. Sleeping people are awakened. Doors swing open or close. Dishes are broken. Pictures
on the wall move. Liquids might spill out of open containers; no structural damage [v=2-5, a~0.03-0.05g]
VI. Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture
moves. Plaster in walls might crack. Damage is slight in poorly built buildings. No structural damage. [v=5-8, a\~0.06-0.09g]
VII. People have difficulty standing. Drivers feel their cars shaking. Some furniture breaks. Loose bricks fall from buildings.
Damage is slight in well-built buildings; major in poorly built buildings. [v=8-12, a~0.1-0.19g]
VIII. Houses not bolted down may shift on their foundations. Tall structures such as towers and chimneys might twist and
fall. Well-built buildings suffer slight damage. Poorly built structures suffer severe damage. [v=20-30, a~0.2-0.3g]
IX. Well-built buildings suffer considerable damage. Houses not bolted down move off their foundations. Some
underground pipes are broken. Reservoirs suffer serious damage. [v~50, a~0.3-0.5g]
X. Most buildings and their foundations are destroyed. Some bridges are destroyed. Dams are seriously damaged. Large
landslides occur. Water is thrown on the banks of rivers & lakes. Railroad tracks are bent slightly. [v~60-80, a~0.6-0.9g]
XI. Most buildings collapse. Some bridges are destroyed. Large cracks appear in the ground. Underground pipelines are
destroyed. Railroad tracks are badly bent. [v>100, a>1g]
XII. Almost everything is totally destroyed. Objects are thrown into the air. The ground moves in waves or ripples.
Magnitude - a unique measure of an earthquake's energy released or size. Many Magnitudes Scales are used, using
body waves, surface waves, felt area, or scalar moment, but each EQ only has one magnitude.
The (Charles) Richter Magnitude scale ML (1935) expresses the seismic energy released by shallow earthquakes
(typical of those of CA) using the amplitude of 0.1-2 sec period S-waves on a seismogram (30-600 km from the EQ); it
does not work well for distant or very large EQ. [Unitless]
Magnitude values given by different seismometer stations for an event may vary (typical uncertainty  0.3). Richter
showed that the larger the energy of the earthquake, the larger the amplitude of ground motion at a given distance.
The equation for Richter local Magnitude is: ML = log10A + f(D), where A is the amplitude, in millimeters, measured
directly from the paper record of the Wood-Anderson seismometer. To determine the Richter magnitude, you measure the
shear wave amplitude, take its logarithm, and correct it using the f(D) value, incorporating the distance separating the
seismometer and earthquake, based on the S-P time difference, (∆t in sec). See nomogram in text Fig 3.24.
Other magnitude scales are based on wave amplitude, period, and seismometer distance from focus
mb = Body wave magnitude [Unitless] [uses P-waves with short (1-10 sec) periods]
MS = Surface wave magnitude [Unitless] [uses Rayleigh waves with 18-22 sec periods].
mN, mf = Felt area magnitude [Unitless] [formulas using surface area in which shaking was felt]
Seismic moment, Mo, [Energy Units of N-m=Joules)] also measures the size of an earthquake.
The seismic moment measures the energy radiated out from the entire fault, and is defined as:
Mo = ( * A * s) (units of Newton-meters), where  is the shear modulus (crust-mantle: ~5x1010 N/m2),
A is the area of the fault (m2); s is the average slip on the fault (m).
Empirical studies show log10 Mo ~ 11 + 1.1 * MS , MS saturates at ~7.3 and mb saturates at ~6 [Unitless]
Moment Magnitude, Mw = 2/3 * log10 Mo – 6 (unitless)
Magnitude and Maximum Intensity are broadly correlated:
Moment
Max MM
# EQ global
EQ
Magnitude Intensity
per year
Name & Effects
1.0-1.9
I
~3,000,000
Tiny EQ-recorded if instrument is close
2.0-2.9
II
~350,000
Very Minor EQ-Generally not felt indoors, but recorded.
3.0-3.9
III
~50,000
Minor EQ-Generally felt indoors.
4.0-4.9
IV-V
~6,000
Light EQ-Often felt, but rarely causes damage
5.0-5.9
VI-VII
~800
Moderate EQ-Slight to major damage to buildings
6.0-6.9
VII-VIII
~300
Strong EQ-Can be destructive within ~80 km
7.0-7.9
IX-X
~20
Major EQ-Can cause serious damage
>8.0
XI-XII
~1
Great EQ-Can cause very serious damage
[both 1964 AK & 1906 SF EQ have M L=8.3 but other scales indicate they should be ML=9.2 and 7.8]
The Moment Magnitude scale is not a simple logarithmic scale (factor of ten) with respect to energy-
MW
Energy Ratio
4--------5--------6--------7--------8
48
43
39
35; so in terms of energy, each MW=6 EQ ≈ 2050 * MW=4 EQ
Seismic Energy Both the magnitude and the seismic moment are related to the amount of energy that is radiated by an
earthquake. Charles Richter and Beno Gutenberg developed a relationship between magnitude and energy:
log10(ES) = 4.8 + 1.5*MS , where the energy, ES, is in Joules (=N-m) and MS=surface wave magnitude.
Note that ES is not the total “intrinsic” energy of the earthquake transferred from sources such as gravitational energy or
to sinks such as heat energy. It is only the amount radiated from the earthquake as seismic waves, which is a small
fraction of the total energy transferred during the earthquake process.
Earthquake
Loma Prieta 1989
San Francisco 1906
Alaska 1964
Chile 1960
mb
6.2
-6.2
--
MS
7.1
7.8
8.4
8.3
(km) ave. offset (m)
40 x 15
1.7
450 x 10
4
500 x 300
7
800 x 200
21
Fault slip area
Mo (N-m)
19
3.0x10
5.4x1020
5.2x1022
2.4x1023
Mw
6.9
7.8
9.1
9.5
Energy Comparisons of Volcanoes & Earthquakes
Moment
Magnitude
-6
-3
-2
-1
0
1
2
3
4
5
6
7
8
9
10
11
12
Energy
(Joules)
6.3e-5
2.0e00
6.3e01
2.0e03
6.3e04
2.0e06
6.3e07
2.0e09
6.3e10
2.0e12
6.3e13
2.0e15
6.3e16
2.0e18
3.0e19
2.0e20
7.0e21
TNT
examples
Volcanic examples
(tons)
Magnitude
1.5e-14
7 mm long fault in windshield; 0.7 m offset
4.8e-10
5 hours burning 100W light bulb
1.5e-08
drop brick 1 m; 70 cm long fault, 0.07 mm offset
4.8e-07
break a rock with a sledge hammer
1.5e-05
7 m long fault, 0.7 mm offset
4.8e-04
2 ton truck traveling at 75 mph; 22 m, 2.2 mm
1.5e-02
large quarry or mine blast
4.8e-01
lightning bolt; 220 m long fault, 2.2 cm offset
1.5e01
average tornado; 700 m long fault, 7 cm offset
4.8e02
electrical energy of a typical thunderstorm
1.5e04
7 km long fault, 0.7 m offset
Landers
CA
1992
4.8e05
Annual Global Lightning; 22 km fault, 2.2 m offset
1.5e07 San Fran. CA 1906
~4
Augustine AK 1986; 70 km fault, 7 m offset
4.8e08
Alaska 1964
~5
Mt. St. Helens WA 1980; ave. 10 day hurricane
7.2e09 SAF circling Earth
~6
Vesuvius 79, Krakatau 1883, Pinatubo 1991
4.8e10
~7
Tambora 1815
1.7e12
Fault Earth in
~9
Toba 73,000 BC
half through
M V=
log10 (erupted mass, kg)-7
Volcanic magnitude of 1=108 kg ; 5=1012 kg
center
Meteor Crater impact (@20 km/sec) (1.2 km) MW ~ 8.5; (10 km) MW ~ 12.5
Y2K US Energy Consumption~1e20J: MW ~ 10.8 (~ global annual EQ energy)
~Earth’s annual internal heat flow MW ~ 11
Daily Solar Energy striking Earth MW ~ 11.5