Download EARTHQUAKES: Origins and Predictions

EARTHQUAKES:
Origins and Predictions
Aspasia Zerva
Department of Civil and
Architectural Engineering
Drexel University
Philadelphia, PA 19104
Summer 2000
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1. Introduction
“Earthquake passed here at 10 p.m. Tuesday … 3 distinct shocks … the first lasted 3
minutes … a sound like a pattering of stocking feet and then the shock that lasted 5 minutes … then a rolling and rattling sound began … houses swayed badly …’’ (Excerpts
from the Palmetto Post, Port Royal, South Carolina in 1886, reprinted in “Low Country
Quake Tales” by Joyce B. Bagwell).
At 9:50 p.m. on Tuesday, August 31, 1886, a strong earthquake shook Charleston, South
Carolina. In 6 to 8 minutes the earthquake was felt as far as Philadelphia, New York City,
Chicago and St. Louis. In the Charleston area, 100 people died as a result of the earthquake, houses cracked and collapsed, sand beds vented explosively into the air and produced craters with surrounding blankets of ejected sand. The earthquake magnitude is
estimated between 7.0 and 7.7.
Everyday somewhere around the world an earthquake occurs. Most of them are so small,
that we cannot even realize that they happened. Some are large and some are great, and,
depending on where they occur, they can cause very significant damage. With the catastrophic earthquakes of last year in Turkey, Taiwan, Mexico and Greece, people started
believing that the frequency of earthquakes has increased, and this thought started creating panic. But this is not the case: statistically, the number of earthquakes per year and
their magnitude distribution remain the same. Not many people, however, learn about
large earthquakes in uninhabited places. On the other hand, our planet becomes more and
more densely populated and news and pictures travel nowadays at the speed of light. If a
large earthquake strikes in a densely populated area, the damage that it will create will be
significant, and will be viewed by people all around the world. In this way, people know
earthquakes only from their effects and not from their causes; it is, therefore, important to
understand the driving forces behind earthquakes.
2. Origins
Undoubtedly, earthquakes are one of the most feared natural hazards. They are unavoidable, mostly unpredictable, and they produce a feeling of helplessness that cannot be
compared with anything else: it is the earth moving under our feet. It is then only natural
that our forefathers in seismically active regions around the world, who did not have the
information that we have now, believed that it was giant animals or gods who caused the
earth to tremble.
Myths: Many myths were created about the possible origin of earthquakes:
In Japan, the legend has it that the earth is shaken by the movement of a giant catfish hidden in the ground; many Japanese websites still use the word “catfish” in
their url address. In China, people believed that the earth is resting on a giant ox,
and in India, one myth suggests that the earth was held in place by four giant elephants, which were standing on top of a giant turtle, which in turn was standing
on top of a giant cobra; whenever one animal moved, the earth was shaking. The
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ancient Greeks attributed earthquakes to the anger of the gods: when the sea-god
Poseidon became angry, he hit the earth with his trident causing the shaking. This
fearsome phenomenon was attributed to God’s anger as recently as the 18th century: people believed that, when society needed chastening, God was sending
earthquakes. A Romanian myth suggests that three columns support the earth:
Faith, Hope and Charity; whenever people disregarded these virtues, an earthquake occurred.
However, some scientific explanation of earthquakes can be traced as back as the 5th century BC: Archelaus, in ancient Greece, attributed earthquakes to compressed air in underground caverns – a plausible explanation at the time, as earthquakes and volcanoes are, in
many cases, associated, and volcanic eruptions involve the release of compressed gas.
Facts: The facts about the causes of earthquakes were discovered only this last
century, in the 1960’s, with the theory of plate tectonics. The theory suggests that the
earth’s crust consists of “plates” that move relative to one another, and seismic activity is associated mostly with this motion. Most of the earthquake sources are located
along the boundaries of these plates.
3. Plate Tectonics
In order to understand plate tectonics, it is important first to describe the earth’s interior.
Description of the Earth’s Interior: The earth, with a radius of approximately
6,380 km, consists of three layers: the crust, the mantle and the core, as shown in
Figure 1.
Figure 1: Cross-cut along the earth’s interior; the left sketch – shown to scale –
indicates how thin the crust is compared to the earth’s size (figure courtesy of
USGS, http://pubs.usgs.gov/publications)
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The uppermost layer, the crust, is only up to 100 km thick: it is as thin as 5 km
underneath the oceans, but much denser (heavier) than that underneath the continents, around 30 km underneath the continents, and reaches its maximum thickness underneath the large mountain ranges of the earth. Below this thin crust lies
the mantle – up to a depth of 2,900 km. The mantle consists of several layers itself, with its uppermost solid part, that, together with the crust, form the lithosphere, and, right below it, the semi-solid asthenosphere. The core of the earth
consists of a 2,200 km thick liquid layer (outer core), and the center of the earth
(inner core), with a radius of 1,300 km, is solid.
The lithosphere is not continuous; it is broken up in irregularly shaped slabs that float on
the slowly moving asthenosphere. These irregularly shaped slabs are called tectonic
plates, and the theory that describes their motion plate tectonics. Figure 2 presents the
seven major tectonic plates of the world, and many of the smaller ones. In many cases the
boundaries of the plates are not well defined, as, e.g., between the African and Eurasian
plates, where many smaller plates exist.
Figure 2: The major tectonic plates of the world; the shape of the continents is also shown
(figure courtesy of USGS, http://pubs.usgs.gov/publications)
The development of the theory of plate tectonics is owed to a great degree to Alfred L.
Wegener, Harry H. Hess and J. Tuzo Wilson. Their contributions that mark the evolution
of our present understanding of plate tectonics are highlighted in the following:
The Theory of Continental Drift: In 1914, Alfred Wegener, a German meteorologist, developed his theory of continental drift that formed the basis of plate tectonics. He postulated that the earth continents, as we know them now, formed a
supercontinent, termed Pangaea (meaning “the entire earth” in Greek), that started
to break apart and drift away about 200 million years ago. Figure 3 presents the
evolution of the surface of the earth from Pangaea to present. Wegener’s theory
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was based on the intriguing similarity of the borders of the South American and
African continents that appear as if they were fitted together, and the similarity in
ancient plant and animal fossils that were discovered in these two continents that
are now separated by the Atlantic Ocean.
Figure 3: Evolution of continents on the earth’s surface from Pangaea (about 225200 million years ago) to present according to the theory of continental drift (figure courtesy of USGS, http://pubs.usgs.gov/publications)
Wegener’s continental drift theory was never accepted during his lifetime. The basic
question of the theory’s critics was: “what is the force that is actually driving the motion
of these plates?” It took many years and many additional discoveries and developments
until the theory of plate tectonics was formalized.
Seafloor spreading: Ocean floor mapping revealed some intriguing features: contrary to common beliefs, the ocean floor is very rough with high “ridges” and deep
“trenches”. In particular, the mountain ridges seem to surround the continents: In Figure 2, the ridge starts (north) between the North American and Eurasian plates, and
continues between the South American and African plate; this ridge is called the MidAtlantic ridge. The ridge continues to the east between the African and Australian
plates, with a diversion to the north between the African and Arabian and Indian
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plates, continues further between the Pacific and Antarctic plates and then north again
between the Pacific plate and the Nazca, Cocos and part of the Juan de Fuca plates,
with diversions north and South of the Nazca plate.
In 1959, Harry Hess, a professor of geology at Princeton University, came up with
the hypothesis of seafloor spreading. He postulated that, along these ridges, the ocean
floor is opening up, and magma (molten rock containing minerals and gases), from
underneath the lithosphere, is surfacing up and cooling. This process is repeated, so
that the ocean floor is moving away from the ridges as new ocean floor is surfacing at
the top of these mountains. This hypothesis was confirmed with the discovery of
magnetic stripes, parallel to the ridges, that showed magnetic polarity consistent with
the reversals of the earth’s magnetic field over the ages, and measurements of the age
of the formations, that revealed that their age was becoming younger as their distance
from the ridges was becoming shorter.
The question that arises then is: if the ocean floor is spreading, and the earth’s surface size doesn’t change significantly, what happens to the excessive oceanic lithosphere? The ocean floor mapping revealed not only high ridges, but also very deep
trenches. In Figure 2, the plate boundaries between the Pacific plate and the Philippine and North American plates are such trenches. There the ocean floor is subducted
underneath the continental floor and disappears. As it enters the warmer and deeper
parts of the earth, it melts again, so that the oceanic floor is, essentially, recycled.
These subduction zones are termed Hadadi-Benioff zones, in honor of the Japanese
and U.S. seismologists who first recognized them.
Thus, the theory of plate tectonics was shaped. Still, the question of what is driving the
motion of these plates is unanswered. But, it appears that what is happening in the mantle
is very similar to what is happening in a pot of boiling liquid: The hotter liquid from the
bottom of the pot, where the source of heat is located, moves up, whereas the colder liquid from the surface moves down, thus forming convection cells. As the hotter material
from the bottom of the mantle moves up towards the lithosphere, the colder material from
the surface moves downwards forming convection cells with shape similar to those forming in the pot of hot liquid, but of much greater scale. Where the hotter mantle material
approaches the earth’s surface, we have the formation of the mid-ocean ridges, and where
the colder material starts to move downwards, we have the trenches.
The association of plate tectonics and earthquakes is that the vast majority of earthquakes
occur along the boundaries of these tectonic plates.
4. Earthquakes and Tectonic Plate Boundaries
The plate boundaries and the earthquakes associated with them are then as follows:
Divergent boundaries: The boundaries along the ocean ridges are called
divergent, as the lithosphere is “opening up”. These divergent boundaries are lo-
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cated mostly in the ocean floor. An exception is the island of Iceland (see Figure
2), where the Mid-Atlantic ridge surfaces up and cuts the island in two. Divergent
boundaries are associated with low seismic activity that occurs at shallow depths,
because the lithosphere is weak and stresses cannot build up. These boundaries
are also associated with volcanic activity.
Convergent boundaries: The plate boundaries where one plate is submerged underneath another or thrust against another are called convergent
boundaries. As already indicated, such boundaries exist along the north and
northwest border of the Pacific plate (Figure 2). They also exist between the
Nazca and South American plates, where the subduction of the Nazca plate resulted in the uplift of the South American plate and the formation of the Andes.
Convergent boundaries are also associated with volcanic activity. Earthquakes
there can be of shallow, intermediate or large depth.
Convergent boundaries also occur between continents. In this case, however, because the continental lithosphere is light and resists downward motion, the
crust is moved either up or sideways. This is the case of the Himalayas and the
Tibetan Plateau. The journey of the Indian plate over millions of years can be observed in Figure 3: In Permian, India was located right north of what is now Antarctica and South-East of Africa. In Cretaceous, India, rotating slightly, has already reached the Equator, and 40-50 million years ago it collided with the Eurasian plate pushing up the Himalayans and the Tibetan Plateau. These convergent
boundaries are also associated with shallow, intermediate or deep earthquakes.
Transform Boundaries: These are plate boundaries, connecting ridges and
trenches, where one plate moves horizontally relative to another. It was J. Tuzo
Wilson, a Canadian geophysicist, who first visualized their existence. The most
well-known transform boundary is the San Andreas fault in California. The San
Andreas fault is part of the boundary between the Pacific and North American
plates. The Pacific plate moves northwest relative to the North American plate in
the perpetual motion of the earth’s lithosphere, causing offsets of the two sides of
the boundary. These boundaries are associated with shallow earthquakes but not
associated with volcanic activity.
Plate Boundary Zones: In many cases, the plate boundaries are not as
clearly defined as shown in Figure 2. One such example is the Mediteranean sea,
where many microplates form a wide zone. In this case, the African plate is converging north underneath the Eurasian plate (and in millions of years the Mediteranean sea will seize to exist), the Arabian plate is moving north to north-west, and
the smaller Anatolian plate (between the African, Arabian and Eurasian plates in
Figure 2) is compressed and moves westwards, causing large earthquakes in
Greece and Turkey. The large, catastrophic earthquakes in Turkey last year
(1999) occurred along the Northern Anatolian fault, which is the boundary between the Anatolian and Eurasian plates. Many smaller plates exist within the
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area of the Anatolian plate, with complex motions that cause additional seismic
activity.
The description of divergent, convergent and transform boundaries clearly defines one
type of motion. The boundaries themselves, however, may include more than one form;
for example, transform boundaries cross the Mid-Atlantic ridge, which is basically a divergent boundary.
Although the majority of earthquakes occur along the plate boundaries, there is additional
seismic activity within the plates, but it is not yet fully clear why these earthquakes occur.
Examples of these intra-plate earthquakes are: The Charleston earthquake, described in
the Introduction of this manuscript, occurred within the North American plate, and is
thought to be caused by activity in the Mid-Atlantic ridge. There are many small faults
underneath Manhattan, some of which may be active; in 1985, a 4.0 magnitude shook the
island in New York City. Earthquakes occur in Pennsylvania as well; the Reading, Pennsylvania, 1994 earthquakes of 4.0 and 4.5 magnitude caused some minor damage. The
earthquakes in these areas, as well as those in New England, are thought to occur along
ancient, weak zones of plate boundaries. Another example is the three New Madrid, Missouri, earthquakes, of estimated magnitude above 8.0, that occurred in 1811 and 1812.
These earthquakes were so strong, that formed two lakes (Lake St. Francis and Reelfoot
Lake) on the west and east of the Mississippi River, and were felt over an area of
1,000,000 square miles. It is thought that this activity is caused by a system of faults, that
are the remnants of an aborted attempt for the formation of a plate boundary in the area.
The reason why these earthquakes are felt over large areas has to do with the properties
of the rock formations: because earthquakes are not frequent in these regions, the rock
formations are not cracked and fractured and, thus, do not absorb much seismic energy.
On the other hand, in California, where earthquakes are frequent and the rock formations
are cracked and fractured, the seismic energy is absorbed quickly.
5. Earthquakes and Fault Rupture
Let us now formalize the earthquake terminology, which we have, to some degree used
already. First of all, fault is a rough surface where shear fracture occurs; the rocks on
one or both sides of this fracture surface have slipped along each other. The strike of a
fault is the compass direction of its trace on the earth surface, and its dip is the angle (less
than or equal to 90o) between the earth surface and the fault surface measured perpendicular to the strike.
We have encountered the San Andreas fault in California and the Northern Anatolian
fault in Turkey. The two sides of these faults move horizontally relative to one another,
i.e., parallel to the strike of the fault. These faults are termed strike-slip faults; transverse
boundaries are strike-slip faults. The two sides of a fault may also slip along the dip of
the fault (dip-slip faults). When the rocks on either side of the fault are pulled apart (tension), as is the case in divergent boundaries, the faults are termed normal faults; shear
rupture at normal faults increases the size of the earth crust. When, on the other hand, the
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rocks on either side of the fault are squeezed (compression), as is the case in convergent
boundaries, the faults are termed reverse or thrust faults; shear rupture at reverse faults
reduces the size of the earth crust. It is important to note that strike-slip faults may also be
associated with some dip-slip activity and vice-versa. Furthermore, it is important to note
again, that the plate boundary zones may include more than one form of fault rupture: We
have seen that the San Andreas fault is a transform boundary (strike slip fault). However,
the 1994 Northridge, California, earthquake occurred along a blind thrust fault, south of
the San Andreas fault; the word “blind” indicates that the fault does not break the earth
surface.
An earthquake then is caused by the sudden slip along a fault. How and why does this
happen? We have seen that the tectonic plates move perpetually on top of the aesthenosphere. This means that the Pacific side of the San Andreas fault in California (Figure 2)
moves northwest relative to the North American side. But the rocks adjacent to the fault
over extended lengths do not. Why is that? Rock is, basically, elastic material, which
means that it can deform under the action of forces, but, when the forces are removed, it
returns to its original position. Now, when the plates move, the rock adjacent to the fault
cannot: faults are rough surfaces and large forces are required to overcome friction. So
the rock adjacent to the fault is stuck, but, further away from the fault, moves along with
the plate motion. The transitional part of rock between the “stuck” part and the “moving”
part deforms elastically by bending, accumulates strain and stores energy. Eventually,
however, enough stress is built up in the rock, so that the friction forces are overcome,
and the rock adjacent to the fault rebounds elastically, meaning that it snaps to a position
consistent with the plate motion, causing the fault to slip and the earthquake to occur.
This, in simple words, is the elastic rebound theory developed by H.F. Reid, after the
1906 San Francisco 8.3 magnitude earthquake. However, not all fault movement results
in large earthquakes. At some locations, like at the Calaveras fault near Hollister, California, the fault creeps slowly with time; small earthquakes do occur along these faults,
but not major ones, because not enough stress is built up in the material.
The location where rupture is initiated on the fault surface is called the hypocenter or focus of the earthquake and its projection on the ground surface the epicenter. The rupture
at the fault is a complex process that is very much dictated by the frictional characteristics and the shape of the fault surface. The rupture (slip) originates at the hypocenter and
may propagate (spread) in one or more directions (unilateral, bilateral rupture), and be
arrested in rougher spots along the fault called asperities.
6. What is Felt on the Earth’s Surface
The energy that is released with the rupture at the fault is transmitted throughout the earth
in the form of waves. These waves initiate at the hypocenter and start propagating away
from the fault, and continue being initiated as rupture spreads along the fault. The process
is similar to that of waves forming circles on the surface of the sea when we throw in
pebbles, but much more complicated. It is much more complicated because we have
many different types of waves and a complex medium – the entire earth.
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The waves can be classified into two general types: body waves and surface waves. Body
waves propagated within the body of the earth, and surface waves along its surface, as
depicted in Figure 4. There are two types of body waves: compressional (or primary or
P-) waves and shear (or secondary or S-) waves. Compressional waves travel at great
speeds and ordinarily reach first the surface of the earth; they displace the earth material
particles along their direction of propagation. Shear waves travel slower, but are stronger
than the P-waves and displace the earth material particles at right angles to the direction
of their propagation. When a P-wave encounters a discontinuity in material properties
within the earth, it is reflected back as a P- and an S-wave, and refracted forward as a Pand an S-wave. Similarly, at discontinuities, the S-wave is reflected back as a P- and an
S-wave, and refracted forward as a P- and an S-wave. However, only P-waves can propagate in the liquid outer core of the earth.
Figure 4: Propagation of waves from the origin of the earthquake through the earth’s
body and along its surface(figure courtesy of USGS, http://pubs.usgs.gov/publications)
Surface waves propagate on the ground surface. They are also of various types: Rayleigh
waves are generated along the earth surface and displace the earth material particles in an
elliptical, horizontal and vertical, pattern along their direction of propagation. Love
waves are generated at layered media, when the surface layer is of lower density than that
beneath it; these waves displace earth material particles parallel to the earth surface and
perpendicular to their direction of propagation.
A seismogram (recording of the superposition of all the waves) is presented in Figure 5.
We can recognize the direct P-wave at the onset of the ground shaking, the arrival of the
stronger S-waves, around 8 sec in the plot, and, after these, the longer duration surface
waves. This seismogram was recorded by an array of seismographs at the SMART-1 array (Strong Motion ARray in Taiwan), located in Lotung, Taiwan. The earthquake –
called Event 5 – stroke on January 29, 1981, had a magnitude of 6.3 and occurred at a
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distance of 30 km from Lotung. The seismographs record ground motions in three directions: two horizontal (North-South and East-West) and the vertical. The North-South
component of the motions is shown in the figure.
1.2
0.9
acceleration time history (m/sec2)
0.6
0.3
0
−0.3
−0.6
−0.9
−1.2
0
5
10
15
20
time (sec)
25
30
35
40
Figure 5: Accelerogram recorded at station I12 of the SMART-1 array in the North-South
direction during Event 5.
There are some special features, associated with the earthquake phenomenon, that require
special attention. They are:
Site Effects: Sediment basins, depending on their properties, have significant effects on the seismic motions resulting on their surface. These local site conditions
tend to amplify the motions, trap waves within the basin and generate long duration
surface waves. The effects are, in many cases, devastating. The damage reported in
Mexico City during the 8.1 earthquake of 1985 is an example: The earthquake source
was some 350 km away from the city, but much of the city is built over the sediments
filling an old lake. As a result, the seismic waves incident to this basin were amplified
and their duration increased with catastrophic consequences for the city. It was a bad
coincidence that the long duration of the seismic waves had a period of 2 sec between
wave crests, which was close to the period of oscillation of many multistory structures in the city. When the period of the seismic motions coincides with the period of
oscillation of a structure (a phenomenon called resonance), the oscillation of the
structure is greatly amplified, so that even well-built structures can collapse.
Liquefaction: Liquefaction is the phenomenon when soil deposits loose their
strength to such degree that they appear to flow as fluids. When liquefaction of soils
occurs underneath a structure, the soil can no longer support the structure, and the latter invariably collapses. A spectacular example of liquefaction occurred during the
1964 Niigata earthquake in Japan, that had a magnitude of 7.5. In an apartment building complex, some structures remained intact and standing, some tilted severely from
their vertical position, and some, without fully collapsing, tilted so much, that they
“lied” on the ground surface. Liquefaction occurs only in saturated soils and is mostly
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observed near water masses. Associated with the phenomenon of liquefaction is that
of lateral spreading, that is characterized by incremental displacements during the
seismic shaking. Some dramatic bridge failures occurred due to lateral spreading during the Good-Friday, 1964 Alaskan earthquake of magnitude 9.2. Landslides can also
be caused by liquefaction as well as failure of earth slopes that were marginally stable
before the earthquake. Although the scale of landslides is, in many cases, small, it can
also be devastating: During the 1970 Peruvian earthquake, an entire village was buried by a landslide that involved 50,000,000 cubic meters and covered, eventually, an
area of 8,000 square kilometers; 25,000 people were killed by this landslide.
Tsunamis: A major threat that can be caused by subduction earthquakes is the
creation of tsunamis. The sudden motion of the oceanic floor during a major earthquake displaces large masses of water that travel at the speed of commercial jetliners
across the ocean. At open seas the effect of the tsunami wave is not significant. It becomes however devastating when it reaches shallow waters – i.e., the ocean floor.
There the tsunami wave reaches enormous heights and its effect on the coastline is
devastating. The islands of Hawaii, in the middle of the Pacific ocean, are particularly
susceptible to tsunami waves. Tsunami waves, as well as earthquakes, are also said to
be the cause of the destruction of the Minoan civilization in Crete in 1450 B.C.; the
waves were created from the eruption of the volcano on the island of Thira (Santorini)
and the massive collapse of a large part of the island in the Aegean sea.
7. Earthquake Measurements
The size of an earthquake is truly measured by the energy that is released from the fault.
However, before modern instruments were developed, the severity of an earthquake was
characterized qualitatively from its effects.
Intensity: Intensity is the qualitative measure of earthquake damage and
human reaction observed at a specific location. The most commonly used intensity scale in the Western world is the Modified Mercalli Intensity scale, developed
by the Italian seismologist Giuseppe Mercalli near the end of the 19th century, and
was later (1931) modified by Charles Richter in the U.S. The scale, shown in Table 1, uses roman numerals, so that it is distinguished from the earthquake magnitude. The numerals range from I (1) to XII (12), with I referring to earthquake
events that are “not felt except by a very few under especially favorable circumstances”, and XII to events where “damage is total; waves seen on ground surface;
lines of sight and level distorted; objects thrown upward into the earth”. Additional intensity scales are the Medveded-Spoonheuer-Karnik (MSK) scale used in
Central and Eastern Europe, and the one developed by the Japanese Meteorological Agency (JMA).
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Table 1: Intensity Scale
The Modified Mercalli Scale of Intensity of Ground Shaking
I
Not felt except by very few under especially favorable circumstances.
II
Felt only by a few persons at rest, especially on upper floors of buildings.
Delicately suspended objects may swing.
III
Felt quite noticeably indoors, especially on upper floors of buildings, but many
people do not recognize it as an earthquake. Standing motor cars may rock
slightly. Vibration like passing of truck. Duration estimated.
IV
During the day felt indoors by many, outdoors by few. At night, some awakened.
Dishes, windows, doors disturbed; walls made cracking sound. Sensation like
heavy truck striking building. Standing motor cars rocked noticeably.
V
Felt by nearly everyone; many awakened. Some dishes, windows, etc., broken;
a few instances of cracked plaster; unstable objects overturned. Disturbance of
trees, poles, and other tall objects sometimes noticed. Pendulum clocks
may stop.
VI
Felt by all; many frightened and run outdoors. Some heavy furniture moved; a
few instances of fallen plaster or damaged chimneys. Damage slight.
VII Everybody runs outdoors. Damage negligible in buildings of good design and
construction; slight to moderate in well-built ordinary structures; considerable
in poorly built or badly designed structures; some chimneys broken. Noticed
by persons driving motor cars.
VIII Damage slight in specially designed structures; considerable in ordinary
substantial buildings with partial collapse; great in poorly built structures. Panel
walls thrown out of frame structures. Fall of chimneys, factory stacks, columns,
monuments, walls. Heavy furniture overturned. Sand and mud ejected in small
amounts. Changes in well water. Persons driving motor cars disturbed.
IX
Damage considerable in specially designed structures; well designed frame
structures thrown out of plumb; great in substantial buildings with partial collapse.
Buildings shifted off foundations. Ground cracked conspicuously. Underground
pipes broken.
X
Some well-built wooden structures destroyed; most masonry and frame structures
destroyed with foundations; ground badly cracked. Rails bent. Landslides
considerable from river banks and steep slopes. Shifted sand and mud. Water
splashed (slopped) over banks.
XI
Few if any (masonry) structures remain standing. Bridges destroyed. Broad
fissures in ground. Underground pipelines completely out of service. Earth
slumped and land slipped in soft ground. Rails bent greatly.
XII
Damage total. Waves seen on ground surface. Lines of sight and level distorted.
Objects thrown upward into the air.
Seismology, however, was revolutionized with the development of the seismograph, the
instrument that records the seismic motion, and produces the seismogram, an example of
which is presented in Figure 5. The seismograph consists of three basic components: a
seismometer, which responds to ground shaking; a chronograph or timing system; and a
recording device. Since their initial use in England in 1840 by the English scientist James
Forbes, seismographs have been significantly improved.
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Magnitude: The use of seismographs made possible the development of
another, more appropriate scale, to measure earthquakes. This “scale”, termed
magnitude, was developed by the American seismologist, Charles Richter, in
1935. In its initial conception, magnitude (which is now known as local magnitude, ML) is the logarithm (base 10) of the maximum trace amplitude (in micrometers) recorded on a Wood-Anderson seismograph located 100 km from the epicenter of the earthquake. It was found, however, that this magnitude scale was not
appropriate for earthquake measurements, because it did not distinguish between
the different types of waves that exist in a seismogram. In 1936, Gutenberg and
Richter at the California Institute of Technology, presented a world-wide magitude scale, the surface wave magnitude, MS, that is based on the amplitudes of
Rayleigh waves with period of 20 sec. Later on, in 1945, Gutenberg developed
another world-wide magnitude scale, the body mave magnitude, mb, that is based
on the amplitudes of the first few cycles of the P-wave. These different magnitude
scales produce similar (but not identical) numbers for earthquakes with magnitudes in the range between 4 and 6, but start deviating significantly for magnitudes higher than 6, with mb lower than ML, and ML lower than Ms. It is, therefore,
important, to know which magnitude is used in the description of an earthquake.
An additional problem of the aforementioned magnitude scales is that they become “saturated” with the size of the earthquake: as the earthquakes become bigger, the recorded ground motion characteristics become less sensitive to the earthquake size. Hiroo Kanamori at Caltech in 1977, and a few years later, Kanamori
and Thomas Hanks developed the moment magnitude scale, Mw, that does not become saturated with earthquake size. This magnitude is directly related to the
source rupture by means of the following equations:
Mw =
log M O
− 10.7
1.5
where M0 is the seismic moment given in (dyne – cm) by the following equation:
M0 = µ A D
with µ being the rupture strength of the material along the fault, A the rupture area
and D the average amount of slip.
The total seismic energy released during an earthquake, E, is estimated
from a relation derived by Gutenberg and Richter, and later shown by Kanamori
to be valid for the moment magnitude as well; it is:
log E = 11.8 + 1.5 M S
where E is expressed in ergs. This relation shows that if Ms is increased by one
unit, then the released seismic energy is increased by 101.5, which means that the
energy released by an earthquake of Ms =6 is abut 32 times bigger than the one re-
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leased by an earthquake of Ms =5, and about 1000 times bigger than the one released by an event with Ms =4.
8. Predictions
Given that we have all the aforementioned information regarding plate tectonics, the most
important question that arises is whether we can predict where and when the next earthquake will occur, so that people can be warned and loss of life and property can be reduced to a minimum.
Presently, the available approaches for estimating the potential of the occurrence of an
earthquake at a site are based on probabilistic methods and strain measurements. The
probabilistic methods are based on historical evidence of large earthquakes in a region; if,
for example, five earthquakes of magnitude 6 have occurred in the last 200 years (or
every 200 years in known times) at a fault, then it is reasonable to assume that a 6 magnitude earthquake will occur in the next forty years, i.e., its return period is forty years. A
problem with this “prediction” is that fault systems are interrelated: when strain is released at one part of the fault, it may increase or decrease elsewhere, so that the strict
probabilistic evaluation of the return period may not be valid. Strain measurements are
also performed along faults, and, in particular, the San Andreas fault. The evaluation of
strain measurements, how much time has passed since the last earthquake and how much
energy was released during the last event give scientists an indication of when the next
event is going to take place. These evaluations, however, are still subject to the interactions of the particular fault with additional faults in its vicinity, and, also, these elaborate
measurements and evaluations are not performed on every known fault. Additional approaches for the prediction of earthquakes have been proposed by scientists around the
world, but they are still in their development and justification stages.
The safest approach, for the time being, for the reduction of loss of life and property during an earthquake is the evaluation of the proper seismic hazard for the region, and the
design and construction of structures capable to withstand this hazard. Seismic hazard is
the quantitative description of ground shaking caused at a particular site, and can be analyzed deterministically or probabilistically. The probabilistic approach considers uncertainties in the earthquake size, the fault, where the earthquake will occur, and the return
period of earthquakes, among other parameters. The deterministic analysis considers a
particular earthquake scenario at the site of interest. In this case, the controlling earthquake (the one that will produce the strongest shaking at the site) is identified from all
existing faults and their shortest distance to the site. Based on the controlling earthquake,
structural design parameters are identified, and structures are designed and built. Additional considerations in seismic hazard evaluations are the special features of the earthquake phenomenon, such as the site effects highlighted earlier.
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9. Conclusions
We have learned what are the driving forces behind earthquakes, how tectonic plates
move on the earth’s hotter interior, what their boundaries are, and how the rupture at the
fault causes an earthquake. We have also seen that, what we know now, is the accumulated effort of scientists over the last thirty to forty years, which is a miniscule amount of
time compared to the amount of time that was required to make the earth surface be what
it is now. Actual seismic prediction, however, is yet to come; let us hope that its
achievement will be feasible and in the not-too-far future.
10. Assignment
A. Select an earthquake from the following regions of the earth:
U.S. (Alaska, California (North), California (South), Central US, Hawaii, Pennsylvania, South Carolina), Chile, China, Greece, Iceland, Italy, Japan, Mexico,
New Zealand, Peru, Taiwan, Turkey.
B. Fully document the earthquake:
1. Location, faulting, and magnitude
2. Damage description
C. Associate the earthquake with historic seismic activity in the area and plate tectonics.
D. Recommend seismic hazard for your selected area.
Additional Reading / Bibliography
D.S. Brumbaum, Earthquakes. Science and Society, Prentice Hall, NJ, 1999.
S.L. Kramer, Geotechnical Earthquake Engineering, Prentice Hall, NJ, 1996.
C.F. Richter, Elementary Seismology, W.H. Freeman, San Francisco, CA, 1958.
W.J. Kious and R.I. Tilling, This Dynamic Earth: Theory of Plate Tectonics, U.S. Department of the Interior/ U.S. Geologic Survey.
U.S. Geologic Survey, http://pubs.usgs.gov/publications.
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