An earthquake is also defined as the sudden slip of one part of the Earth's crust, relative to another, along a fault surface.

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Connecting Earthquakes and Faults
An earthquake is also defined as the sudden slip
of one part of the Earth's crust, relative to
another, along a fault surface. A gradual
build-up of mechanical stress in the crust,
primarily the result of tectonic forces, provides
the source of energy for earthquakes sudden
motion along a fault releases it in the form of
seismic waves. It's unclear when the connection
between faults and earthquakes was first made,
but by the late 19th Century most scientists
accepted this association as fact, even if the
mechanisms behind it were still a mystery.
Thrust fault scarp at El Asnam, Algeria.
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Connecting Earthquakes and Faults
Fault research received a tremendous boost in the
aftermath of the great San Francisco earthquake
of 1906. This was one of the first earthquakes
for which both seismographic and fault-rupture
studies were conducted. The fault rupture
occurred in through a very well-surveyed,
developed area.
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Connecting Earthquakes and Faults
Because of this, researchers could not only map
the offset across the fault trace, but also the
amount of displacement between points far removed
from the fault. This work led to the
formulation of the elastic rebound theory of
fault rupture by Princeton geologist Harry F.
Reid.
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Connecting Earthquakes and Faults
As technology improved, seismic networks grew,
and research into the mechanism of fault rupture
increased, new methods arose that helped quantify
the link between earthquakes and faults. One
important find helped link magnitude (energy)
with the severity of fault rupture. The seismic
moment (MO) of an earthquake, which can be
estimated from analysis of seismic waves, was
discovered to be directly proportional to the
extent of the actual fault rupture.
1999 Chi-Chi earthquake, Taiwan
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Earthquake Magnitude
How big is an earthquake? Depends on how big a
patch of the fault breaks. If the patch that
breaks is a few square miles, you may have a
magnitude five earthquake. If it's up to a
couple hundred square miles, you have a magnitude
seven. If it's a couple of thousand square miles,
you get a M 7.8, 1906 San Francisco quake."
1999 Chi-Chi earthquake, Taiwan
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EARTHQUAKE SOURCE PARAMETERS Magnitude, fault
area, fault slip, stress drop, energy release
the big one
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EARTHQUAKE MAGNITUDE
Earliest measure of earthquake size Dimensionless
number measured various ways, including ML
local magnitude mb body wave magnitude Ms surface
wave magnitude Mw moment magnitude Easy to
measure Empirical - except for Mw, no direct tie
to physics of faulting
Note not dimensionally correct
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Connecting Earthquakes and Faults
The seismic moment is the product of the area of
fault surface that ruptures, the average
displacement along that surface, and a constant
-- a measure of the elastic property of rock
(i.e. how easily it can be stretched) called the
modulus of rigidity. Moment magnitude (MW) is
based upon the seismic moment, and represents a
kind of bridge between the seismological and
geological views of an earthquake.
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Connecting Earthquakes and Faults
The seismic moment is the product of the area of
fault surface that ruptures, the average
displacement along that surface, and a constant
-- a measure of the elastic property of rock
(i.e. how easily it can be stretched) called the
modulus of rigidity. Moment magnitude (MW) is
based upon the seismic moment, and represents a
kind of bridge between the seismological and
geological views of an earthquake.
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Connecting Earthquakes and Faults
Seismic moment The seismic moment is a measure
of the size of an earthquake based on the area of
fault rupture, the average amount of slip, and
the force that was required to overcome the
friction sticking the rocks together that were
offset by faulting. Seismic moment can also be
calculated from the amplitude spectra of seismic
waves.
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Seismic moment a better measure of EQ size
A more consistent measure of big earthquakes
nowadays is the magnitude calculated on the basis
of seismic moment (MO), called Moment Magnitude
(MW). Because fault geometry and displacement
are a part of the MO, moment is a more consistent
measure of earthquake size than is magnitude, and
more importantly, moment does not have an upper
bound. Moment does not tend to saturate as
Richter magnitude does. The seismic moment is
related to the faulting process.
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Earthquake size and the area of slip
The size of the area that slips during an
earthquake is increases with earthquake size. The
largest earthquakes generally rupture the entire
depth of the fault, which is controlled by
temperature. The temperature increases with
depth to a point where the rocks become plastic
and no longer store the elastic strain energy
necessary to fail suddenly.
The shaded regions on the fault surface are the
areas that rupture during different size events
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Seismic Moment
Seismologists have more recently developed a
standard magnitude scale that is completely
independent of the type of instrument. It is
called the moment magnitude, and it comes from
the seismic moment. To get an idea of the
seismic moment, go back to the concept of torque.
A torque is a force that changes the angular
momentum of a system. It is defined as the force
times the distance from the center of rotation.
Earthquakes are caused by internal torques, from
the interactions of different blocks of the earth
on opposite sides of faults. The moment of an
earthquake is simply expressed by
The moment of an earthquake, is fundamental to
our understanding of how dangerous faults of a
certain size can be.
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Seismic Moment
Seismic Moment µ S A µ shear modulus
3x1011 dyne/cm2 in continental crust A Length
x Width fault area S average displacement or
slip during fault rupture.
What is a dyne? 1 gram of mass an acceleration of
a cm/s2 1 dyne 1 gram x cm/sec2
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MO µSA Mw (2/3)log(µSA)-10.7 or MW
(2/3)log(MO) - 10.7 µ is the shear strength of
the faulted rock A is the area of the fault S is
the average slip or displacement on the fault.
These factors have led to the definition of a
new magnitude scale MW. It is based on seismic
moment, where MW 2/3 log10(MO) - 10.7. (MO is
in dyne/centimeter) MW close approximates MS up
to magnitude 7.0, but continues to rise without
saturation to values as large as 9.5 for the 1960
Southern Chile earthquake.
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fault length 100 km Seismic moment
and Moment Magnitude fault depth minimum of 12
km average slip 62 m average shear modulus
(m) 3x1011 dyne/cm2   Mo m SA where S avg
slip, A fault area m shear modulus Mo
(3x1011dyne/cm2)(6 m 100 cm/m)(100 km 100,000
cm/km x 12 km 100,000 cm/km) 2.16 x 1027 Mw
2/3logMo-10.7 7.5 Mo (3x1011dyne/cm2)(8 m
100 cm/m)(100 km 100,000 cm/km x 12 km
100,000 cm/km) 2.88 x 1027 Mw 2/3logMo -
10.7 7.6
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COMPARE EARTHQUAKES USING SEISMIC MOMENT M0
Magnitudes, moments (dyn-cm), fault areas, and
fault slips for several earthquakes Alaska San
Francisco differ much more than Ms implies M0
more useful measure Units dyne-cm or
Nt-M Directly tied to fault physics Doesnt
saturate
Stein Wysession, 2003
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EARTHQUAKE SOURCE PARAMETER ESTIMATES HAVE
CONSIDERABLE UNCERTAINTIES FOR SEVERAL
REASONS - Uncertainties due to earth's
variability and deviations from the mathematical
simplifications used. Even with high-quality
modern data, seismic moment estimates for the
Loma Prieta earthquake vary by about 25, and Ms
values vary by about 0.2 units. - Uncertainties
for historic earthquakes are large. Fault length
estimates for the San Francisco earthquake vary
from 300-500 km, Ms was estimated at 8.3 but now
thought to be 7.8, and fault width is
essentially unknown and inferred from the depths
of more recent earthquakes and geodetic data. -
Different techniques (body waves, surface waves,
geodesy, geology) can yield different
estimates. - Fault dimensions and dislocations
shown are average values for quantities that can
vary significantly along the fault Hence
different studies yield varying and sometimes
inconsistent values. Even so, data are sufficient
to show effects of interest.
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Moment magnitude Mw Magnitudes saturate No
matter how big the earthquake mb never exceeds
6.4 Ms never exceeds 8.4 Mw defined from moment
so never saturates
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THREE EARTHQUAKES IN NORTH AMERICA - PACIFIC
PLATE BOUNDARY ZONE Tectonic setting
affects earthquake size
San Fernando earthquake on buried thrust fault in
the Los Angeles area, similar to Northridge
earthquake. Short faults are part of an oblique
trend in the boundary zone, so fault areas are
roughly rectangular. The down-dip width
controlled by rocks deeper than 20 km are weak
and undergo stable sliding rather than accumulate
strain for future earthquakes.
Stein Wysession, 2003
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THREE EARTHQUAKES IN NORTH AMERICA - PACIFIC
PLATE BOUNDARY ZONE Tectonic setting
affects earthquake size
San Francisco earthquake ruptured a long segment
of the San Andreas with significantly larger
slip, but because the fault is vertical, still
had a narrow width. This earthquake illustrates
approximately the maximum size of continental
transform earthquakes.
Stein Wysession, 2003
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Alaska earthquake had much larger rupture area
because it occurred on shallow-dipping
subduction thrust interface. The larger fault
dimensions give rise to greater slip, so the
combined effects of larger fault area and more
slip cause largest earthquakes to occur at
subduction zones rather than transforms.
THREE EARTHQUAKES IN NORTH AMERICA - PACIFIC
PLATE BOUNDARY ZONE Tectonic setting
affects earthquake size
Stein Wysession, 2003
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LARGER EARTHQUAKES GENERALLY HAVE LONGER FAULTS
AND LARGER SLIP
Wells and Coppersmith, 1994
M7, 100 km long, 1 m slip M6, 10 km long,
20 cm slip Important for
earthquake source physics and hazard estimation
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Most Destructive Known Earthquakes on Record in
the World (gt 50,000 deaths) (Listed in order of
greatest number of deaths) Date
Location Deaths M
Comments January 23, 1556 China, Shansi
830,000 October 11, 1737
India, Calcutta 300,000 July 27, 1976
China, Tangshan
255,000 8.0 December 26, 2007
Indonesia 225,000 9.3
Large tsunami August 9, 1138
Syria, Aleppo 230,000 May 22,
1927 China, near Xining
200,000 8.3 Large
fractures. December 22, 856 Iran, Damghan
200,000 December 16, 1920 China,
Gansu 200,000 8.6
Major fractures, landslides March 23, 893
Iran, Ardabil
150,000 September 1, 1923 Japan, Kwanto
143,000 8.3 Great Tokyo
fire December 28, 1908 Italy, Messina
70,000 7.5 Earthquake
tsunami (100,000) September, 1290
China, Chihli 100,000
November, 1667 Caucasia, Shemakha
80,000 November 18, 1727 Iran, Tabriz
77,000 November 1, 1755
Portugal, Lisbon 70,000 8.7
Great tsunami December 25, 1932
China, Gansu 70,000
7.6 May 31, 1970 Peru
66,000 7.8
Great rock slide and flood 1268
Asia Minor, Silicia
60,000 January 11, 1693 Italy, Sicily
60,000 May 30, 1935
Pakistan, Quetta 30,000
7.5 Quetta almost completely
destroyed (60,000) February 4, 1783
Italy, Calabria 50,000 June 20,
1990 Iran
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)
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TEN LARGEST EARTHQUAKES IN THE UNITED STATES
Magnitude Date (UTC)
Location Length Duration (km)
(sec) 9.2 03/ 28/1964 Prince William
Sound, Alaska 8.8 03/09/1957
Andreanof Islands, Alaska 8.7 02/04
/1965 Rat Islands, Alaska 8.3
11/11/1938 East of Shumagin Islands, Alaska
8.3 07/10/1958 Lituya Bay, Alaska
8.2 10/10/1899 Yakutat Bay, Alaska
8.2 10/4/1899 Near Cape Yakataga,
Alaska 8.0 05/7/1986 Andreanof
Islands, Alaska 7.9 11/3/2002
South central Alaska
340 7.9 02/7/1812 New Madrid,
Missouri 7.9 01/9/1857 Fort
Tejon, California 360 130 7.9
04/3/1868 Ka'u District, Island of
Hawaii 7.9 10/9, 1900 Kodiak
Island, Alaska 7.9 11/30/1987 Gulf
of Alaska 7.5 03/18/1906 San Francisco,
California (downgraded from M 8) For comparison,
the largest earthquake ever recorded was a moment
magnitude 9.5 in Chile on May 22, 1960. The
largest earthquake ever recorded in the United
States was in Alaska on March 27, 1964, with
moment magnitude 9.2
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A longer fault produces a bigger earthquake that
lasts a longer time. Magnitude Date
Location Length Duration
(km) (sec) 7.8 January 9,
1857 Fort Tejon 360
130 7.7 April 18,
1906 San Francisco 400 110 7.5
July 21, 1952 Kern County 75 27
7.3 June 28, 1992 Landers 70
24 7.0 October 17, 1989 Loma
Prieta 40 7
6.9 May 18, 1940 Imperial Valley
50 15 6.7 February 9, 1971
San Fernando 16 8 6.7
January 17, 1994 Northridge
14 7 6.6 November 24, 1987
Superstition Hills 23
15 6.5 April 9, 1968
Borrego Mountain 25 6 6.4
October 15, 1979 Imperial Valley 30
13 6.4 March 10, 1933 Long Beach
15 5 6.1 April 22, 1992
Joshua Tree 15 5 5.9
July 8, 1986 North Palm
Springs 20 4 5.9 October
1, 1987 Whittier Narrows 6 3
5.8 June 28, 1991 Sierra Madre
5 2
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2004 Indonesia
Earthquakes of a given magnitude are 10 times
less frequent than those one magnitude smaller.
An M7 earthquake occurs approximately monthly,
and an earthquake of Mgt 6 about every three days.
Magnitude is proportional to the logarithm of
the energy released, so most energy released
seismically is in the largest earthquakes. An M
8.5 event releases more energy than all other
earthquakes in a year combined. Hence the hazard
from earthquakes is due primarily to large
(typically magnitude gt 6.5) earthquakes.