Earthquake, shaking of the earth’s surface caused by rapid movement of the
earth’s rocky outer layer. Earthquakes occur when energy stored within the
earth, usually in the form of strain in rocks, suddenly releases. This energy is
transmitted to the surface of the earth by earthquake waves. The study of
earthquakes and the waves they create is called seismology. Scientists who study
earthquakes are called seismologists. (Webster’s p.423) The destruction an
earthquake causes, depends on its magnitude or the amount of shaking that
occurs. The size varies from small imperceptible shaking, to large shocks felt
miles around. Earthquakes can tear up the ground, make buildings and other
structures collapse, and create tsunamis (large sea waves). Many Lives can be
lost because of this destruction. (The Road to Jaramillo p.211) Several hundred
earthquakes, or seismic tremors, occur per day around the world. A worldwide
network of seismographs detect about one million small earthquakes per year.
Very large earthquakes, such as the 1964 Alaskan earthquake, which measured 8.6
on the Richter scale and caused millions of dollars in damage, occur worldwide
once every few years. Moderate earthquakes, such as the 1989 tremor in Loma
Prieta, California (magnitude 7.0), and the 1995 tremor in Kôbe, Japan
(magnitude 6.8), occur about 20 times a year. Moderate earthquakes also cause
millions of dollars in damage and can harm many people. (The Road to Jaramillo
p.213-215) In the last 500 years, several million people have been killed by
earthquakes around the world, including over 240,000 in the 1976 T’ang-Shan,
China, earthquake. Worldwide, earthquakes have also caused severe property and
structural damage. Good precautions, such as education, emergency planning, and
constructing stronger, more flexible structures, can limit the loss of life and
decrease the damage caused by earthquakes. (The Road to Jaramillo p.213-215,263)
AN EARTHQUAKES ANATOMY Seismologists examine the parts of an earthquake, like
what happens to the earth’s surface during an earthquake, how the energy of an
earthquake moves from inside the earth to the surface, and how this energy
causes damage. By studying the different parts and actions of earthquakes,
seismologists learn more about their effects and how to predict ground shaking
in order to reduce damage. (On Shifting Ground p.109-110) Focus and Epicenter
The point within the earth along the rupturing geological fault where an
earthquake originates is called the focus, or hypocenter. The point on the
earth’s surface directly above the focus is called the epicenter. Earthquake
waves begin to radiate out from the focus and follow along the fault rupture. If
the focus is near the surface between 0 and 70 km (0 and 40 mi.) deep shallow
focus earthquakes are produced. If it is deep below the crust between 70 and 700
km (40 and 400 mi.) deep a deep focus earthquake will occur. Shallow-focus
earthquakes tend to be larger, and therefore more damaging, earthquakes. This is
because they are closer to the surface where the rocks are stronger and build up
more strain. (The Ocean of Truth p.76 & The road to Jaramillo p.94-97)
Seismologists know from observations that most earthquakes originate as
shallow-focus earthquakes and most of them occur near plate boundaries areas
where the earth’s crustal plates move against each other. Other earthquakes,
including deep-focus earthquakes, can originate in subduction zones, where one
tectonic plate subducts, or moves under another plate. (The Ocean of Truth
p.54-56) I Faults Stress in the earth’s crust creates faults places where
rocks have moved and can slip, resulting in earthquakes. The properties of an
earthquake depend strongly on the type of fault slip, or movement along the
fault, that causes the earthquake. Geologists categorize faults according to the
direction of the fault slip. The surface between the two sides of a fault lies
in a plane, and the direction of the plane is usually not vertical; rather it
dips at an angle into the earth. When the rock hanging over the dipping fault
plane slips downward into the ground, the fault is called a normal fault. When
the hanging wall slips upward in relation to the bottom wall, the fault is
called a reverse fault or a thrust fault. Both normal and reverse faults produce
vertical displacements, or the upward movement of one side of the fault above
the other side, that appear at the surface as fault scarps. Strike slip faults
are another type of fault that produce horizontal displacements, or the side by
side sliding movement of the fault, such as seen along the San Andreas fault in
California. Strike-slip faults are usually found along boundaries between two
plates that are sliding past each other. (Plate Tectonics p.49-53) II Waves The
sudden movement of rocks along a fault causes vibrations that transmit energy
through the earth in the form of waves. Waves that travel in the rocks below the
surface of the earth are called body waves, and there are two types of body
waves: primary, or P, waves, and secondary, or S, waves. The S waves, also known
as shearing waves, cause the most damage during earthquake shaking, as they move
the ground back and forth. (Plate tectonics p.133) Earthquakes also contain
surface waves that travel out from the epicenter along the surface of the earth.
Two types of these surface waves occur: Rayleigh waves, named after British
physicist Lord Rayleigh, and Love waves, named after British geophysicist A. E.
H. Love. Surface waves also cause damage to structures, as they shake the ground
underneath the foundations of buildings and other structures. Body waves, or P
and S waves, radiate out from the rupturing fault starting at the focus of the
earthquake. P waves are compression waves because the rocky material in their
path moves back and forth in the same direction as the wave travels alternately
compressing and expanding the rock. P waves are the fastest seismic waves; they
travel in strong rock at about 6 to 7 km (4 mi.) per second. P waves are
followed by S waves, which shear, or twist, rather than compress the rock they
travel through. S waves travel at about 3.5 km (2 mi.) per second. S waves cause
rocky material to move either side to side or up and down perpendicular to the
direction the waves are traveling, thus shearing the rocks. Both P and S waves
help seismologists to locate the focus and epicenter of an earthquake. As P and
S waves move through the interior of the earth, they are reflected and
refracted, or bent, just as light waves are reflected and bent by glass.
Seismologists examine this bending to determine where the earthquake originated.
(Encarta 98) On the surface of the earth, Rayleigh waves cause rock particles to
move forward, up, backward, and down in a path that contains the direction of
the wave travel. This circular movement is somewhat like a piece of seaweed
caught in an ocean wave, rolling in a circular path onto a beach. The second
type of surface wave, the Love wave, causes rock to move horizontally, or side
to side at right angles to the direction of the traveling wave, with no vertical
displacements. Rayleigh and Love waves always travel slower than P waves and
usually travel slower than S waves. (The Floor of the Sea p.76-78, 112-115) III
CAUSES Most earthquakes are caused by the sudden slip along geologic faults. The
faults slip because of movement of the earth’s tectonic plates. This concept
is called the elastic rebound theory. The rocky tectonic plates move very
slowly, floating on top of a weaker rocky layer. As the plates collide with each
other or slide past each other, pressure builds up within the rocky crust.
Earthquakes occur when pressure within the crust increases slowly over hundreds
of years and finally exceeds the strength of the rocks. Earthquakes also occur
when human activities, such as the filling of reservoirs, increase stress in the
earth’s crust. (Encarta 98) ELASTIC REBOUND THEORY In 1911 American
seismologist Harry Fielding Reid studied the effects of the April 1906
California earthquake. He proposed the elastic rebound theory to explain the
generation of earthquakes that occur in tectonic areas, usually near plate
boundaries. This theory states that during an earthquake, the rocks under strain
suddenly break, creating a fracture along a fault. When a fault slips, movement
in the crustal rock causes vibrations. The slip changes the local strain out
into the surrounding rock. The change in strain leads to aftershocks, which are
produced by further slips of the main fault or adjacent faults in the strained
region. The slip begins at the focus and travels along the plane of the fault,
radiating waves out along the rupture surface. On each side of the fault, the
rock shifts in opposite directions. The fault rupture travels in irregular steps
along the fault; these sudden stops and starts of the moving rupture give rise
to the vibrations that propagate as seismic waves. After the earthquake, strain
begins to build again until it is greater than the forces holding the rocks
together, then the fault snaps again and causes another earthquake. (Plate
tectonics p.56-59) DISTRIBUTION Seismologists have been monitoring the frequency
and locations of earthquakes for most of the 20th century. They have found that
the majority of earthquakes occur along plate tectonic boundaries, while there
are relatively few intraplate earthquakes, that occur within a tectonic plate.
The categorization of earthquakes is related to where they occur, as
seismologists generally classify naturally occurring earthquakes into one of two
categories: interplate and intraplate. Interplate earthquakes are the most
common; they occur primarily along plate boundaries. Intraplate earthquakes
occur within the plates at places where the crust is fracturing internally. Both
interplate and intraplate earthquakes may be caused by tectonic or volcanic
forces. (Naked Earth p.134-135) I Tectonic Earthquakes Tectonic earthquakes are
caused by the sudden release of energy stored within the rocks along a fault.
The released energy is produced by the strain on the rocks due to movement
within the earth, called tectonic deformation. The effect is like the sudden
breaking and snapping back of a stretched elastic band. (The Ocean of truth
p.122) II Volcanic Earthquakes Volcanic earthquakes occur near active volcanoes
but have the same fault slip mechanism as tectonic earthquakes. Volcanic
earthquakes are caused by the upward movement of magma under the volcano, which
strains the rock locally, and leads to an earthquake. As the fluid magma rises
to the surface of the volcano, it moves and fractures rock masses and causes
continuous tremors that can last up to several hours or days. Volcanic
earthquakes occur in areas that are associated with volcanic eruptions, such as
in the Cascade Mountain Range of the Pacific Northwest, Japan, Iceland, and at
isolated hot spots such as Hawaii. (Plate tectonics p.74) LOCATIONS
Seismologists use global networks of seismographic stations to accurately map
the focuses of earthquakes around the world. After studying the worldwide
distribution of earthquakes, the pattern of earthquake types, and the movement
of the earth’s rocky crust, scientists proposed that plate tectonics, or the
shifting of the plates as they move over another weaker rocky layer, was the
main underlying cause of earthquakes. The theory of plate tectonics arose from
several previous geologic theories and discoveries. Scientists now use the plate
tectonics theory to describe the movement of the earth's plates and how this
movement causes earthquakes. They also use the knowledge of plate tectonics to
explain the locations of earthquakes, mountain formation, deep ocean trenches,
and predict which areas will be damaged the most by earthquakes. It is clear
that major earthquakes occur most frequently in areas with features that are
found at plate boundaries: high mountain ranges and deep ocean trenches.
Earthquakes within plates, or intraplate tremors, are rare compared with the
thousands of earthquakes that occur at plate boundaries each year, but they can
be very large and damaging. (On shifting ground p.17-19) Earthquakes that occur
in the area surrounding the Pacific Ocean, at the edges of the Pacific plate,
are responsible for an average of 80 percent of the energy released in
earthquakes worldwide. Japan is shaken by more than 1000 tremors greater than
3.5 in magnitude each year. The western coasts of North and South America are
very also active earthquake zones, with several thousand small to moderate
earthquakes each year. (U.S.G.S.) Intraplate earthquakes are less frequent than
plate boundary earthquakes, but they are still caused by the internal fracturing
of rock masses. The New Madrid, Missouri, earthquakes of 1811 and 1812 were
extreme examples of intraplate seismic events. Scientists estimate that the
three main earthquakes of this series were about magnitude 8.0 and that there
were at least 1500 aftershocks. (The ocean of truth p.67-69) EFFECTS Ground
shaking leads to landslides and other soil movement. These are the main damage
causing events that occur during an earthquake. Primary effects that can
accompany an earthquake include property damage, loss of lives, fire, and
tsunami waves. Secondary effects, such as economic loss, disease, and lack of
food and clean water, also occur after a large earthquake. (On shifting ground
p.47) Ground Shaking and Landslides Earthquake waves make the ground move,
shaking buildings and structures and causing poorly designed or weak structures
partially or totally collapse. The ground shaking weakens soils and foundation
materials under structures and causes dramatic changes in fine-grained soils.
During an earthquake, water-saturated sandy soil becomes like liquid mud, an
effect called liquefaction. Liquefaction causes damage as the foundation soil
beneath structures and buildings weakens. Shaking may also dislodge large earth
and rock masses, producing dangerous landslides, mudslides, and rock avalanches
that may lead to loss of lives or further property damage. (The road to
Jaramillo p.43-45) REDUCING DAMAGE Earthquakes cannot be prevented, but the
damage they cause can be greatly reduced with communication strategies, proper
structural design, emergency preparedness planning, education, and safer
building standards. In response to the tragic loss of life and great cost of
rebuilding after past earthquakes, many countries have established earthquake
safety and regulatory agencies. These agencies require codes for engineers to
use in order to regulate development and construction. Buildings built according
to these codes survive earthquakes better and ensure that earthquake risk is
reduced. (On shifting ground p.56) Tsunami early-warning systems can prevent
some damage because tsunami waves travel at a very slow speed. Seismologists
immediately send out a warning when evidence of a large undersea earthquake
appears on seismographs. Tsunami waves travel slower than seismic P and S waves
in the open ocean, they move about ten times slower than the speed of seismic
waves in the rocks below. This gives seismologists time to issue tsunami alerts
so that people at risk can evacuate the coastal area as a preventative measure
to reduce related injuries or deaths. Scientists radio or telephone the
information to the Tsunami Warning Center in Honolulu and other stations.(The
floor of the sea p.59) Engineers minimize earthquake damage to buildings by
using flexible, reinforced materials that can withstand shaking in buildings.
Since the 1960s, scientists and engineers have greatly improved earthquake
resistant designs for buildings that are compatible with modern architecture and
building materials. They use computer models to predict the response of the
building to ground shaking patterns and compare these patterns to actual seismic
events, such as in the 1994 Northridge, California, earthquake and the 1995 Kôbe,
Japan, earthquake. They also analyze computer models of the motions of buildings
in the most hazardous earthquake zones to predict possible damage and to suggest
what reinforcement is needed. (Martin Alfred p.62) Structural Design Geologists
and engineers use risk assessment maps, such as geologic hazard and seismic
hazard zoning maps, to understand where faults are located and how to build near
them safely. Engineers use geologic hazard maps to predict the average ground
motions in a particular area and apply these predicted motions during
engineering design phases of major construction projects. Engineers also use
risk assessment maps to avoid building on major faults or to make sure that
proper earthquake bracing is added to buildings constructed in zones that are
prone to strong tremors. They can also use risk assessment maps to aid in the
retrofit, or reinforcement, of older structures. (The ocean of truth p.23) In
urban areas of the world, the seismic risk is greater in non-reinforced
buildings made of brick, stone, or concrete blocks because they cannot resist
the horizontal forces produced by large seismic waves. Fortunately,
single-family timber-frame homes built under modern construction codes resist
strong earthquake shaking very well. Such houses have laterally braced frames
bolted to their foundations to prevent separation. Although they may suffer some
damage, they are unlikely to collapse because the strength of the strongly
jointed timber-frame can easily support the light loads of the roof and the
upper stories even in the event of strong vertical and horizontal ground
motions.(On shifting groung p.73) Emergency Preparedness Plans Earthquake
education and preparedness plans can help significantly reduce death and injury
caused by earthquakes. People can take several preventative measures within
their homes and at the office to reduce risk. Supports and bracing for shelves
reduce the likelihood of items falling and potentially causing harm. Maintaining
an earthquake survival kit in the home and at the office is also an important
part of being prepared. (On shifting ground p.97) In the home, earthquake
preparedness includes maintaining an earthquake kit and making sure that the
house is structurally stable. The local chapter of the American Red Cross is a
good source of information for how to assemble an earthquake kit. During an
earthquake, people indoors should protect themselves from falling objects and
flying glass by taking refuge under a heavy table. After an earthquake, people
should move outside of buildings, assemble in open spaces, and prepare
themselves for aftershocks. They should also listen for emergency bulletins on
the radio, stay out of severely damaged buildings, and avoid coastal areas in
the event of a tsunami. (The floor of the sea p.46) In many countries,
government emergency agencies have developed extensive earthquake response
plans. In some earthquake hazardous regions, such as California, Japan, and
Mexico City, modern strong motion seismographs in urban areas are now linked to
a central office. Within a few minutes of an earthquake, the magnitude can be
determined, the epicenter mapped, and intensity of shaking information can be
distributed via radio to aid in response efforts.(The floor of the sea p.18)
STUDYING EARTHQUAKES Seismologists measure earthquakes to learn more about them
and to use them for geological discovery. They measure the pattern of an
earthquake with a machine called a seismograph. Using multiple seismographs
around the world, they can accurately locate the epicenter of the earthquake, as
well as determine its magnitude, or size, and fault slip properties. (Alfred
Wegener & encarta 98) I Measuring Earthquakes An analog seismograph consists
of a base that is anchored into the ground so that it moves with the ground
during an earthquake, and a spring or wire that suspends a weight, which remains
stationary during an earthquake. In older models, the base includes a rotating
roll of paper, and the stationary weight is attached to a stylus, or writing
utensil, that rests on the roll of paper. During the passage of a seismic wave,
the stationary weight and stylus record the motion of the jostling base and
attached roll of paper. The stylus records the information of the shaking
seismograph onto the paper as a seismogram. Scientists also use digital
seismographs, computerized seismic monitoring systems that record seismic
events. Digital seismographs use re-writeable, or multiple-use, disks to record
data. They usually incorporate a clock to accurately record seismic arrival
times, a printer to print out digital seismograms of the information recorded,
and a power supply. Some digital seismographs are portable; seismologists can
transport these devices with them to study aftershocks of a catastrophic
earthquake when the networks upon which seismic monitoring stations depend have
been damaged. (Plate Tectonics p.56-58, 64) There are more than 1000 seismograph
stations in the world. One way that seismologists measure the size of an
earthquake is by measuring the earthquake’s seismic magnitude, or the
amplitude of ground shaking that occurs. Seismologists compare the measurements
taken at various stations to identify the earthquake’s epicenter and determine
the magnitude of the earthquake. This information is important in order to
determine whether the earthquake occurred on land or in the ocean. It also helps
people prepare for resulting damage or hazards such as tsunamis. When readings
from a number of observatories around the world are available, the integrated
system allows for rapid location of the epicenter. At least three stations are
required in order to triangulate, or calculate, the epicenter. Seismologists
find the epicenter by comparing the arrival times of seismic waves at the
stations, thus determining the distance the waves have traveled. Seismologists
then apply travel-time charts to determine the epicenter. With the present
number of worldwide seismographic stations, many now providing digital signals
by satellite, distant earthquakes can be located within about 10 km (6 mi.) of
the epicenter and about 10 to 20 km (6 to 12 mi.) in focal depth. Special
regional networks of seismographs can locate the local epicenters within a few
kilometers. (the Ocean of truth) . All magnitude scales give relative numbers
that have no physical units. The first widely used seismic magnitude scale was
developed by the American seismologist Charles Richter in 1935. The Richter
scale measures the amplitude, or height, of seismic surface waves. The scale is
logarithmic, so that each successive unit of magnitude measure represents a
tenfold increase in amplitude of the seismogram patterns. This is because ground
displacement of earthquake waves can range from less than a millimeter to many
meters. Richter adjusted for this huge range in measurements by taking the
logarithm of the recorded wave heights. So, a magnitude 5 Richter measurement is
ten times greater than a magnitude 4; while it is 10 x 10, or 100 times greater
than a magnitude 3 measurement. (The floor of the sea p.89-91) Today,
seismologists prefer to use a different kind of magnitude scale, called the
moment magnitude scale, to measure earthquakes. Seismologists calculate moment
magnitude by measuring the seismic moment of an earthquake, or the
earthquake’s strength based on a calculation of the area and the amount of
displacement in the slip. The moment magnitude is obtained by multiplying these
two measurements. It is more reliable for earthquakes that measure above
magnitude 7 on other scales that refer only to part of the seismic waves,
whereas the moment magnitude scale measures the total size. The moment magnitude
of the 1906 San Francisco earthquake was 7.6; the Alaskan earthquake of 1964,
about 9.0; and the 1995 Kôbe, Japan, earthquake was a 7.0 moment magnitude; in
comparison, the Richter magnitudes were 8.3, 8.6, and 6.8, respectively for
these tremors. (U.S.G.S.) Earthquake size can be measured by seismic intensity
as well, a measure of the effects of an earthquake. Before the advent of
seismographs, people could only judge the size of an earthquake by its effects
on humans or on geological or human-made structures. Such observations are the
basis of earthquake intensity scales first set up in 1873 by Italian
seismologist M. S. Rossi and Swiss scientist F. A. Forel. These scales were
later superseded by the Mercalli scale, created in 1902 by Italian seismologist
Guiseppe Mercalli. In 1931 American seismologists H. O. Wood and Frank Neumann
adapted the standards set up by Guiseppe Mercalli to California conditions and
created the Modified Mercalli scale. Many seismologists around the world still
use the Modified Mercalli scale to measure the size of an earthquake based on
its effects. The Modified Mercalli scale rates the ground shaking by a general
description of human reactions to the shaking and of structural damage that
occur during a tremor. This information is gathered from local reports, damage
to specific structures, landslides, and peoples’ descriptions of the damage.
(The road to Jaramillo p.122) II Predicting Earthquakes Seismologists try to
predict how likely it is that an earthquake will occur, with a specified time,
place, and size. Earthquake prediction also includes calculating how a strong
ground motion will affect a certain area if an earthquake does occur. Scientists
can use the growing catalogue of recorded earthquakes to estimate when and where
strong seismic motions may occur. They map past earthquakes to help determine
expected rates of repetition. Seismologists can also measure movement along
major faults using global positioning satellites (GPS) to track the relative
movement of the rocky crust of a few centimeters each year along faults. This
information may help predict earthquakes. Even with precise instrumental
measurement of past earthquakes, however, conclusions about future tremors
always involve uncertainty. This means that any useful earthquake prediction
must estimate the likelihood of the earthquake occurring in a particular area in
a specific time interval compared with its occurrence as a chance event. (The
ocean of truth p.29) The elastic rebound theory gives a generalized way of
predicting earthquakes because it states that a large earthquake cannot occur
until the strain along a fault exceeds the strength holding the rock masses
together. Seismologists can calculate an estimated time when the strain along
the fault would be great enough to cause an earthquake. As an example, after the
1906 San Francisco earthquake, the measurements showed that in the 50 years
prior to 1906, the San Andreas fault accumulated about 3.2 meters (10 feet) of
displacement, or movement, at points across the fault. The maximum 1906 fault
slip was 6.5 meters (21 feet), so it was suggested that 50 years x 6.5
meters/3.2 meters, about 100 years, would elapse before enough energy would
again accumulate to produce a comparable earthquake. (Plate Tectonics)
Scientists have measured other changes along active faults to try and predict
future activity. These measurements have included changes in the ability of
rocks to conduct electricity, changes in ground water levels, and changes in
variations in the speed at which seismic waves pass through the region of
interest. None of these methods, however, has been successful in predicting
earthquakes to date. (U.S.G.S) Seismologists have also developed field methods
to date the years in which past earthquakes occurred. In addition to information
from recorded earthquakes, scientists look into geologic history for information
about earthquakes that occurred before people had instruments to measure them.
This research field is called paleoseismology. Seismologists can determine when
ancient earthquakes occurred. (The floor of the sea p.118) Seismology,
basically, the science of earthquakes, involving observations of natural ground
vibrations and artificially generated seismic signals, with many theoretical and
practical ramifications. A branch of geophysics, seismology has made vital
contributions to understanding the structure of the earth’s interior.
(Webster’s) SEISMIC PHENOMENA Different kinds of seismic waves are produced by
the deformation of rock materials. A sudden slip along a fault, for example,
produces both longitudinal push-pull (P) and transverse shear (S) waves.
Compressional trains of P waves, set up by an quick push or pull in the
direction of wave propagation, cause surface formations to shake back and forth.
Sudden shear displacements move through materials with slower S-wave velocity as
vertical planes shake up and down. When P and S waves encounter a boundary such
as Mohorovièiæ discontinuity (Moho), which lies between the crust and the
mantle, they are partly reflected, refracted, and transmitted, breaking up into
several other types of waves as they pass through the earth. Travel times depend
on compressional and S-wave velocity changes as they pass through materials with
different elastic properties. Crustal granitic rocks typically show P-wave
velocities of 6 km/sec, where as underlying mafic and ultramafic rocks show
velocities of 7 and 8 km/sec. In addition to P and S waves—body-wave
types—two surface seismic waves are the Love waves, named for the British
geophysicist Augustus E. H. Love, and Rayleigh waves, named after the British
physicist John Rayleigh. These waves travel fast and are guided in their
propagation by the earth’s surface. (Plate Tectonics p.142) INTRAMENTS OF
STUDY Longitudinal, transverse, and surface seismic waves cause vibrations at
points where they reach the earth’s surface. Seismic instruments have been
designed to detect these movements through electromagnetic or optical methods.
The main instruments, called seismographs, were perfected following the
development by the German scientist Emil Wiechert of a horizontal seismograph
about the turn of the century. (Naked Earth p.36-42) Some instruments, such as
the electromagnetic pendulum seismometer, employ electromagnetic recording; that
is, induced tension passes through an electric amplifier to a galvanometer. A
photographic recorder scans a rapidly moving film, making sensitive
time-movement registrations. Refraction and reflection waves are usually
recorded on magnetic tapes, which are readily adapted to computer analysis.
Strain seismographs, employing electronic measurement of the change in distance
between two concrete pylons about 30 m (100 ft.) apart, can detect compressional
and extensional responses in the ground during seismic vibrations. The Benioff
linear strain seismograph detects strains related to tectonic processes, those
associated with propagating seismic waves, and tidal yielding of the solid
earth. Still more recent inventions used in seismology include rotation
seismographs; tiltmeters; wide frequency band, long-period seismographs; and
ocean bottom seismographs. (Alferd Wegener p.118-120) Similar seismographs are
deployed at stations around the world to record signals from earthquakes and
underground nuclear explosions. The World Wide Standard Seismograph Network (WWSSN)
incorporates some 125 stations. (U.S.G.S.) Richter Who? Richter, Charles
(1900-1985), American seismologist who wrote fundamental seismology texts, and
who established an earthquake magnitude scale with German-American seismologist
BenoGutenberg. (Encarta 98) Richter was born in Ohio but moved to Los Angeles as
a child. He attended Stanford University and received his undergraduate degree
in 1920. In 1928 he began work on his Ph.D. in theoretical physics from the
California Institute of Technology (Caltech), but before he finished it, he was
offered a position at the Carnegie Institute of Washington. At this point, he
became fascinated with seismology. After he worked at the new Seismological
Laboratory in Pasadena, under the direction of Beno Gutenberg. In 1932 Richter
and Gutenberg developed a standard scale to measure the relative sizes of
earthquake sources, called the Richter scale. In 1937 he returned to Caltech,
where he spent the rest of his career, eventually becoming professor of
seismology in 1952. Richter and Gutenberg also worked to locate and catalog
major earthquakes and used them to study the deep interior of the earth.
Together they wrote a very influential textbook, published in 1954, called
Seismicity of the Earth. In 1958 Richter published the textbook, Elementary
Seismology, which many consider his greatest contribution to the field. Richter
visited Japan on a Fulbright Fellowship in 1959-1960. (Encarta 98) Richter was
also involved in public awareness and safety issues surrounding earthquakes,
taking a sensible stance rather than using scare tactics. He was devoted to his
work in science and learned several languages in order to read the global
earthquake literature. Richter was so interested in earthquakes, he even
installed a seismograph in his living room of his Los Angeles home. He
influenced Los Angeles building codes that city officials credited with saving
many lives in the 1971 earthquake in San Fernando, California. After retirement
he continued to work on earthquake safety design. (Encarta 98) (PUT MONTH)
EARTHQUAKE FINDINGS During the month of march we charted all of the bigger
earthquakes that occurred . We charted the earthquakes measuring from 4 to 7 on
the Richter scale. We plotted this data to see where most of the earthquakes
would occur. Also to see how high most of the quakes would be on the scale.
According to our analyses most of the earthquakes occurred around the plate
boundaries. Especially in South America along the South American plate and
Mexico along the North American plate. Yet, to our surprise there weren’t many
earthquakes whatsoever, along the boundary between the Eurasian plate and the
African plate. We also found Seismic activity in some unusual areas like the
arctic region above Europe and the Antarctic region. Most of the quakes we
recorded were not generally large either. Most of them were recorded at 4 on the
Richter scale. There were not many large earthquakes in the month of March. The
largest quake we recorded was 6.8 in Xizang-India border region. We also found
that there were an unusually high number of earthquakes in the month of March.
From the data that we collected we noticed that earthquakes can also occur in
the middle of the ocean. In conclusion from the data we have constructed we came
to find out that large earthquakes are rare and far in between. We have come to
realize how devastating earthquakes can really be to people and their
surroundings.
Bibliography
Kidd, J.S. & Kidd, R. A. (1997). On shifting ground “the story of
continental drift”. New York: Facts on File, Inc. Erickson, J. (1992). Plate
Tectonics. New York: Facts on File, Inc. Glen, W. (1982). The Road to Jaramillo.
Stanford, California: The Stanford University Press. Menarld, H.W. (1986). The
Ocean of Truth. Princeton, N.J.: The Princeton University Press Suhwartzbach, M.
(1986). Alfred Wegener. Madison, Wisconsin.: Science Teck Inc. Vogel, S. (1992).
Naked Earth. New York: Dutton Books. Wertenbacher, W. (1974). The Floor of the
Sea. Boston Massachusetts.: Little Brown and Co. Internet. (1999).
wwwneic.cr.usgs.gov/neis/bulletin.html. Computer source.: Internet explorer.
Apsell, P. S. (Producer). (1990). Nova Earthquake. [Video Tape]. Western Video