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

1. 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.
Connecting Earthquakes and Faults
Thrust fault scarp at El Asnam,
Algeria.

2. 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.
Connecting Earthquakes and Faults

3. 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.
Connecting Earthquakes and Faults

4. 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.
Connecting Earthquakes and Faults
1999 Chi-Chi earthquake, Taiwan

5. 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."
Earthquake Magnitude
1999 Chi-Chi earthquake, Taiwan

6. EARTHQUAKE
SOURCE
PARAMETERS
Magnitude, fault
area, fault slip,
stress drop, energy
release
“the big one”

7. 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

11. 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.
Connecting Earthquakes and Faults

12. 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.
Connecting Earthquakes and Faults

13. 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.
Connecting Earthquakes and Faults

14. 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.
Seismic moment = a better measure of EQ size

15. 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
Earthquake size and the area of slip

16. 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.
Seismic Moment

17. 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.
Seismic Moment
What is a dyne?
1 gram of mass an acceleration of a
cm/s2
1 dyne = 1 gram x cm/sec2

18. 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.

19. fault length: 100 km Seismic moment and Moment Magnitude
fault depth: minimum of 12 km
average slip: 6±2 m
average shear modulus (µ): 3x1011
dyne/cm2
Mo = µ SA: where S = avg slip, A = fault area; µ = shear modulus
Mo = (3x1011
dyne/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 = (3x1011
dyne/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

20. 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
Doesn’t saturate
Stein & Wysession, 2003

21. 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.

22. 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

23. 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

24. 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

25. THREE
EARTHQUAKES IN
NORTH AMERICA -
PACIFIC PLATE
BOUNDARY ZONE
Tectonic setting affects
earthquake size
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.
Stein & Wysession, 2003

26. LARGER EARTHQUAKES GENERALLY HAVE LONGER
FAULTS AND LARGER SLIP
M7, ~ 100 km long, 1 m slip; M6, ~ 10 km long, ~ 20 cm slip
Important for earthquake source physics and hazard estimation
Wells and
Coppersmith, 1994

27. Most Destructive Known Earthquakes on Record in the World
(> 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)

28. 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

29. 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

30. 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 M> 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 > 6.5) earthquakes.
2004 Indonesia