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A Century Of Earthquakes: 1906-2006
A CITY BURnS - A SCIEnCE Comes of age
GREAT EARTHQUAKES
Photo courtesy of: San Francisco Public Library
Charles Richter introduced the first earthquake magnitude scale in 1935 for small California
earthquakes. However, the Richter and similar magnitude scales are inadequate for characterizing extremely large earthquakes. Seismologists now use a magnitude scale designed to
1.5 mm
P
S
10 minutes
2004 Sumatra Earthquake (Mw = 9.3)
seismogram (vertical ground motion)
recorded in Kevo, Finland (8850 km
from epicenter).
S
P
10 minutes
Kamchatka,
1923 (8.5)
EURASIAN PLATE
Assam - Tibet,
1950 (8.7)
NORTH
AMERICAN
PLATE
Kuril Islands,
1963 (8.5)
The San Andreas fault in the Carrizo plain in California, seen from the southeast. Note the displacement
of stream gullies as the rocks on the left side of the
fault have moved to the right (north).
In the 1960s, geologists discovered that
the Earth’s outer shell was made up of
about 15 100-km thick tectonic plates
that were smoothly moving by each other
at speeds of a few inches (a few cm) per
year (about the speed that fingernails
grow). Earthquakes primarily occur at the
boundaries where the plates converge,
diverge, or slide past each other. At
spreading centers both plates move away
from the boundary, whereas at subduction zones the underthrusting subducted
plate moves toward the boundary. At
the third boundary type, transform faults
(such as the San Andreas), relative plate
motion is parallel to the boundary.
(b)
(c)
SUNDA
PLATE
Unimak Island,
1946 (8.5)
JUAN
DE FUCA
PLATE
7
5
3
2
1
0
Ecuador,
1906 (8.5)
NAZCA
PLATE
Banda Sea, 1938 (8.5)
Rid
ge
ca
Fu
de
an
Ju
CenturyOfQuakesPoster.indd 1
ge
Rid
Convergent
Boundary
10°N
eF
SRI
LANKA
AFRICA
Subsidence
0°
Displacement
Greatly Exaggerated
Fault
Slip
ped
An earthquake occurs when the accumulated stresses on the fault exceed the fault strength. Beneath Sumatra, the overriding plate that had
been dragged down since the last major earthquake rebounded over the
course of a few minutes and suddenly displaced many cubic kilometers
of ocean, causing the devastating tsunami.
wreaking destruction along seacoasts and causing at least 300,000 deaths.
Tsunami heights grow substantially as they approach the shallow ocean near
the coast, and they can be enormously destructive.
The cause of this event can be traced back over 120 million years ago, when the
subcontinent of India separated from Antarctica and started its steady motion
northward. 50 million years ago it collided with Asia, raising the Himalayas and
forming the Tibetan plateau. The plate collision continues today as the Indian
plate moves northward. Part of the plate boundary extends along the subduction
INDONESIA
10°S
20°S
MADACASCAR
AUSTRALIA
30°S
40°E
60°E
80°E
Mw = 9.3
7 > Mw ≥ 6
Seis
INDIA
THAILAND
bd
Su
ARABIAN
PENINSULA
20°N
uca
Uplift
Global
Arrival Time of First Wave (hrs) 12/26/2004 Indonesian Tsunami
Tsunami
Jua
nd
te
Pacific Pla
Z
ANTARCTIC PLATE
Displacement
Greatly Exaggerated
ked
s
rea
nd
n A ult
Fa
on
ti
uc
e
on
Mendocino
SCOTIA PLATE
Loc
During Earthquakes
1960
1970
1980
1990
2000
100°E
northridge
1994
Loma Prieta
1989
San
Francisco
1906
Sumatra
2004
Mw = 6.7
Mw = 6.9
Average Slip 2 m
Mw = 7.9
Average Slip 4 m
Mw = 9.3
Average Slip 11 m
Average Slip 1 m
Alaska (1964)
Mw = 9.4
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
8 > Mw ≥ 7
Other Mw ≥ 8
Approximate Fault Width
150 km
The fault sizes of more recent damaging earthquakes in California
are small compared to the 1906 earthquake and tiny in comparison to the Sumatra earthquake.
se, Januar
Relea
y 1
t
906
en
m
o
M
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6
M Sumatra (2004) w
Chile (1960)
Mw = 9.5
zone on the west coast of Sumatra, where an oceanic part of the Indian plate subducts beneath the Burma plate, a small
sliver or microplate between the Indian plate and the Sunda plate that includes much of southeast Asia. Every year, about
35 mm of convergence occurs between the Indian and Burma plates. However, the fault is locked, so stress builds up on
it. Eventually the accumulated stress exceeds the strength of the fault, and it slips. During the Sumatra earthquake a huge
area of this plate interface slipped, generating seismic waves that inflicted major damage near the earthquake. Moreover,
because this plate boundary occurs at an underwater trench, the overriding Burma plate that had been dragged down since
the last major earthquake rebounded and displaced many cubic kilometers of ocean, causing the devastating tsunami.
On December 26, 2004 the world saw yet again how
the stress built up over hundreds of years by slow
and almost imperceptible motions of tectonic plates
can be released with devastating effect. A great
earthquake beneath the Indonesian island of Sumatra
generated a massive sea wave, or tsunami, that raced
across the Indian Ocean at the speed of a jet plane,
Sa
ch
Mendocino Fault
Divergent Boundary
Black dots represent earthquakes below Mw 8.5
Uplift
1950
The constants in the equation allow the moment magnitude scale
to describe great earthquakes while
matching other magnitude scales at
smaller magnitudes. An earthquake
with Mw greater than or equal to 8.0,
Chile, 1960 (9.5)
All Earthquakes Mw 8.5 and above for 1906-2006
Between Earthquakes
1940
Mw = (2/3) x (log10Mo - 9.1)
SOUTH
AMERICAN
PLATE
AUSTRALIAN
PLATE
Shortening
n
Tre
Juan de
Fuca Ridge
n Zone
Pacific Plate
Subductio
Transform
Fault
Cascadia
Earthquakes are studied using global
seismometer networks that produce data
about their locations, times, sizes and the
nature of faulting. Because earthquakes
generally result from the motions of the
plates, knowledge of the direction and
amount of motion is important for understanding plate motions and the forces
that cause them. Such studies are key to
assessing the societal hazards posed by
earthquakes.
Juan de Fuca
Plate
1930
The rigidity is a number that characterizes the stiffness of rocks near
the fault. Fault area and fault slip
can be estimated from the analysis
of seismograms. For a seismic moment expressed in units of Newton-meters (which is a unit of force
times distance), the corresponding
moment magnitude, Mw, is:
PLATE
THE KILLER WAVE
North
American
Plate
1920
2005
ber
cem
Because plate boundaries extend for a total
of more than 150,000 km, and some earthquakes occur in plate interiors, earthquakes
occur frequently on Earth. An earthquake of
magnitude 7 or greater occurs roughly every
month, and an earthquake of magnitude 6
or greater occurs on average every three
days. Earthquakes of a given magnitude
occur about ten times less frequently than
earthquakes one magnitude smaller.
1910
Mo = rigidity x fault area x fault slip
PACIFIC PLATE
100 Years of Great Earthquakes:
Chile (1960)
4
De
The San Francisco earthquake occurred on the San Andreas fault in northern California, part
of the boundary along which the Pacific plate moves northward relative to the North American
plate. Studies using the Global Positioning System satellites show that the two plates move
by each other at a speed of about 4.5 cm/yr. Most parts of the San Andreas fault are “locked”
most of the time, but slip several meters in a large earthquake every few hundred years. A simple calculation (earthquake slip/slip per year) suggests that such an earthquake with a 4-m slip
should occur on average about 90 years.
The real interval is not uniform, for reasons
that are unclear, and longer, because some
of the motion occurs on other faults.
Alaska (1964)
6
blend with the original Richter scale, but based on the seismic moment, which relates directly to three key physical properties of the fault: stiffness of rock, fault area, and fault slip. Seismic moment is a quantity used by seismologists to measure the amount of energy released
by an earthquake. The seismic moment generated by a slipping fault is:
CARIBBEAN
PLATE
Andreanof ISLAnDs,
COCOS
1957 (8.6)
Sumatra,
2004 (9.3)
Sumatra,
2005 (8.7)
8
mic
Motion of a fence crossing the San Andreas fault before and
during an earthquake. The elastic rebound model of earthquakes
assumes that between earthquakes, material on the two sides
of a fault undergoes relative motion. Because the fault is locked,
features across it that were linear at time (a), such as a fence, are
slowly deformed with time (b). Finally the strain becomes so great
that the fault breaks in an earthquake, offsetting the features (c).
Sumatra (2004)
9
Between 1906 and 2006 about half of all global moment release occurred in just three great earthquakes: Sumatra
(2004), Alaska (1964) and Chile (1960).
(a)
INDIAN
PLATE
10
Year
PHILIPPINE
PLATE
AFRICAN
PLATE
Alaska, 1964 (9.4)
Kamchatka,
1952 (9.0)
EARTHQUAKES, FAULTS, AnD PLATES
As part of the investigation, H.F. Reid proposed
the elastic rebound theory of earthquakes, where
materials at distance on opposite sides of the
fault move smoothly relative to each other, but
friction on the fault “locks” each side and prevents it from slipping. Eventually the accumulated
forces (stress) are more than the fault can endure,
and the fault slips in an earthquake.
Rat Islands,
1965 (8.7)
x 1023
Cumulative Moment (newton-meters)
0.2 mm
1906 San Francisco Earthquake
(Mw = 7.9) seismogram (N-S ground
motion) recorded in Göttingen,
Germany (9080 km from epicenter).
Approximate Fault Length
On April 18, 1906 at 5:12 AM the city of
San Francisco suffered a major earthquake with a magnitude of about 7.9. After a minute of strong shaking, much of
the city lay in ruins. Fires broke out and
raged for three days, causing considerably more damage than the earthquake.
It is estimated that 3000 deaths and
$400 million in damage resulted. Scientific study of the earthquake showed
that the ground had moved along a
300-km long zone, a segment of the San
Andreas fault. Displaced features like
fences showed that material on one side
of the fault suddenly moved up to 3-6
m relative to material on the other side.
Moreover, the fault showed signs of previous movements, showing that this was
not the first such earthquake.
San Francisco
(1906)
Mw = 7.9
which on average occurs about every 1.5 years,
is classified as a great earthquake. The total energy release on Earth is dominated by the largest of
the great earthquakes; between 1906 and 2006
nearly half of all global moment release occurred
in just three great earthquakes: Sumatra (2004),
Alaska (1964) and Chile (1960).
Compared to the 1906 San Francisco earthquake,
the 2004 Sumatra earthquake had roughly three
times the average slip on a fault with about 60
The energy release of the 1906 earthquake is barely
times the area, which corresponds to an increase
visible in the above plot of seismic energy released
in moment magnitude of about 1.4. As seen in
during the past century.
the formula above, fault area, slip and rigidity of
the rock constrain how large an earthquake can be. Rock type and depth account for how rigid
the rocks are and how much stress can build up. The more stress that builds up, the more energy
can be released when the rocks slip. In California, rocks that are deeper than 20 km are weak and
do not let stress build up over time. In contrast, earthquakes in subduction zones have large surface areas in zones of stronger rocks, which allow more stress to be built up over time. Because
of these factors, about 80% of the global seismic
moment is released by subduction
zone earthquakes.
We need to understand great
str
on
earthquakes and live with
g(
w
bri
them. In fact, we can’t live
ea
ttle
k(
)
ma
without them. Great earthquakes
lle
ab
are a result of plate tectonics, which
le)
keeps the Earth habitable by maintaining
our atmosphere. We have not yet learned
to predict when great earthquakes will happen. The best we can do is build our societies
to minimize the damage from earthquakes. This
can be done by earthquake resistant construction, systems to warn of imminent tsunami waves,
and other measures. Deciding how to do this is a
challenging and important task.
lt
au
ef
n
cal
zo
verti
n
tio
fault
uc
d
b
su
Earthquakes that
have a vertical dip, like the
1906 San Francisco earthquake,
generally have a smaller fault area and less
of it in the area where stress builds. In contrast,
subduction zone earthquakes occur on faults that
dip shallowly into the earth, which can have much
larger fault areas with more of it capable of building
stress. Compare the yellow (brittle) section of the
vertical and subduction zone faults in the diagram.
Global earthquake data provided by the IRIS/USGS Global Seismographic Network, Meredith Nettles (Harvard University) and Lynn Sykes (Lamont-Doherty
Earth Observatory). 1906 seismogram provided by Dave Wald (USGS). Sumatra seismogram provided by the IRIS/USGS Global Seismographic Network.
Juan de Fuca map provided by Bill Schultz (Institute for the Application of Geospatial Technology). Additional figures derived after NOAA and Hyndman &
Wang. Produced by Richard Aster (New Mexico Institute of Mining and Technology),
Seth Stein and Emile Okal (Northwestern University), Michael Wysession (Washington University), John Taber, Gayle Levy (IRIS Consortium). Designed by Jason
Mallett (IRIS Consortium). IRIS Consortium; www.iris.edu
2/28/06 4:12:35 PM