Transcript Document

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.
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.
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.
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
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
EARTHQUAKE
SOURCE
PARAMETERS
Magnitude, fault area,
fault slip, stress drop,
energy release
“the big one”
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
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.
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.
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.
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.
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
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.
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
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.
fault length: 100 km
Seismic moment and Moment Magnitude
fault depth: minimum of 12 km
average slip: 6±2 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
COMPARE
EARTHQUAKES
USING SEISMIC
MOMENT M0
Magnitudes, moments (dyncm), 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
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.
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
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
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
THREE EARTHQUAKES
IN NORTH AMERICA PACIFIC PLATE
BOUNDARY ZONE
Tectonic setting affects
earthquake size
Stein & Wysession, 2003
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.
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
Most Destructive Known Earthquakes on Record in the World
(> 50,000 deaths)
Date
Location
(Listed in order of greatest number of deaths)
Deaths
M
January 23, 1556
October 11, 1737
July 27, 1976
December 26, 2007
China, Shansi
India, Calcutta**
China, Tangshan
Indonesia
830,000
300,000
255,000*
225,000
August 9, 1138
May 22, 1927
December 22, 856+
December 16, 1920
March 23, 893+
September 1, 1923
December 28, 1908
September, 1290
November, 1667
November 18, 1727
November 1, 1755
December 25, 1932
May 31, 1970
1268
January 11, 1693
May 30, 1935
Syria, Aleppo
China, near Xining
Iran, Damghan
China, Gansu
Iran, Ardabil
Japan, Kwanto
Italy, Messina
China, Chihli
Caucasia, Shemakha
Iran, Tabriz
Portugal, Lisbon
China, Gansu
Peru
Asia Minor, Silicia
Italy, Sicily
Pakistan, Quetta
230,000
200,000
200,000
200,000
150,000
143,000
70,000
100,000
80,000
77,000
70,000
70,000
66,000
60,000
60,000
30,000
February 4, 1783
June 20, 1990
Italy, Calabria
Iran
50,000
50,000
Comments
8.0
9.3
Large tsunami
8.3
Large fractures.
8.6
Major fractures, landslides
8.3
7.5
Great Tokyo fire
Earthquake & tsunami (100,000)
8.7
7.6
7.8
Great tsunami
7.5
Quetta almost completely
destroyed (~60,000)
7.7
Landslides
Great rock slide and flood
* 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)
TEN LARGEST EARTHQUAKES IN THE UNITED STATES
Magnitude Date (UTC)
Location
9.2
8.8
8.7
8.3
8.3
8.2
8.2
8.0
7.9
7.9
7.9
7.9
7.9
7.9
7.5
03/ 28/1964
03/09/1957
02/04 /1965
11/11/1938
07/10/1958
10/10/1899
10/4/1899
05/7/1986
11/3/2002
02/7/1812
01/9/1857
04/3/1868
10/9, 1900
11/30/1987
03/18/1906
Prince William Sound, Alaska
Andreanof Islands, Alaska
Rat Islands, Alaska
East of Shumagin Islands, Alaska
Lituya Bay, Alaska
Yakutat Bay, Alaska
Near Cape Yakataga, Alaska
Andreanof Islands, Alaska
South central Alaska
New Madrid, Missouri
Fort Tejon, California
Ka'u District, Island of Hawaii
Kodiak Island, Alaska
Gulf of Alaska
Length Duration
(km)
(sec)
340
360
130
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
A longer fault produces a bigger earthquake that lasts a longer time.
Magnitude
Date
Location
Length
(km)
Duration
(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
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 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.