of the Earth
and Fault Zones, Plate Tectonics
of an Earthquake
Earthquakes, Seismographs, and Seismograms
– An earthquake which follows a larger earthquake or main shock and originates I or near the rupture zone of the larger earthquake. Generally, major earthquakes are followed by
a larger number of aftershocks, decreasing in frequency with time.
- Amplitude – The maximum
height of a wave crest or depth of a trough.
- Array – An ordered
arrangement of seismometers or geophones, the data from which feeds into a
- Arrival – The
appearance of seismic energy on a seismic record.
- Arrival Time – The time
at which a particular wave phase arrives at a detector.
- Aseismic – Not
associated with an earthquake, as in aseismic slip. Also used to indicate an area with no
record of earthquakes; an aseismic zone.
- Body Wave – A seismic wave
that can travel through the interior of the earth. P-waves and S-waves are body waves.
- Central Angle – An
angle with the vertex at the center of the Earth, with one ray passing
through the hypocenter (and also the epicenter) and the other ray passing
through the recording station.
- Consolidated – Tightly
packed. Composed of particles that
are not easily separated.
- Core – The innermost
layers of the Earth. The inner core
is solid and has a radius of about 1300 kilometers. (The radius of the Earth is about 6371
kilometers.) The outer core is
fluid and is about 2300 kilometers thick.
S-waves cannot travel through the outer core.
- Continental Drift – The
theory, first advanced by Alfred Wegener, that Earth’s continents were originally one
land mass. Pieces of the land mass
split off and migrated to form the continents.
- Crust – The thin out
layer of the Earth’s surface, averaging about 10 kilometers thick under
the oceans and up to about 50 kilometers thick on the continents. This is the only layer of the Earth that
humans have actually seen.
- Earthquake – Shaking of
the Earth caused by a sudden movement of rock beneath its surface.
- Earthquake Swarm – A
series of minor earthquakes, none of which may be identified as the main
shock, occurring in a limited area and time. Frequently associated with imminent
- Elastic Wave – A wave
that is propagated by some kind of elastic deformation, that is, a change
in shape that disappears when the forces are removed. A seismic wave is a type of elastic
- Epicenter – That point
on the Earth’s surface directly above the hypocenter of an earthquake.
- Fault – A weak point in
the Earth’s crust and upper mantle where the rock layers have ruptured and
slipped. Faults are caused by
earthquakes, and earthquakes are likely to reoccur on pre-existing faults.
- First Arrival – The
first recorded signal attributed to seismic wave travel from a source.
- Focus – That point
within the Earth from which originates the first motion of an earthquake
and its elastic waves.
- Foreshock – A small
tremor that commonly precedes a larger earthquake or main shock by seconds
to weeks and that originates in or near the rupture zone of the larger
- Great Earthquake – An earthquake
having a magnitude of 8 or greater on the Richter scale.
- Hazard – A risk. An object or situation that has the
possibility of injury or damage.
- Hypocenter – The
calculated location of the focus of an earthquake.
- Intensity – A measure
of the effects of an earthquake at a particular place on humans,
structures and (or) the land itself.
The intensity at a point depends not only upon the strength of the
earthquake (magnitude) but also upon the distance from the earthquake to
the pint and the local geology at that point.
- Isoseismal Line – A line
connecting points on the Earth’s surface at which earthquake intensity is
the same. It is usually a closed
curve around the epicenter.
- Landslide – An abrupt
movement of soil and bedrock downhill in response to gravity. Landslides can be triggered by an
earthquake or other natural causes.
Undersea landslides can cause tsunamis.
- Latitude – The location
of a point north or south of the equator.
Latitude is shown on a map or globe as east-west lines parallel to
- Leaking Mode – A
surface seismic wave which is imperfectly tapped so that is energy leaks or escapes across a layer boundary
causing some attenuation, or loss of energy.
- Liquefaction – The
process in which a solid (soil) takes on the characteristics of a liquid
as a result of an increase in pore pressure and a reduction in
stress. In other words, solid
ground turns to jelly.
- Lg Wave – A surface wave
which travels through the continental crust.
- Longitude – The
location of a point east or west of the prime meridian. Longitude is shown on a map or globe as
north-south lines left and right of the prime meridian, which passes
through Greenwich, England.
- Love Wave – A major
type of surface wave having a horizontal motion that is shear or
transverse to the direction of propagation (travel). It is named after A. E. H. Love, the
English mathematician who discovered it.
- Low-velocity Zone – Any
layer in the Earth in which seismic wave velocities are lower than in the
layers above and below.
- Magnitude – A measure
of the strength of an earthquake or strain energy released by it, as
determined by seismographic observations.
This is a logarithmic value originally defined by Charles Richter
(1935). An increase of one unit of
magnitude (for example, from 4.6 to 5.6) represents a 10-fold increase in
wave amplitude on a seismogram or approximately a 30-fold increase in the
energy released. In other words, a
magnitude 6.7 earthquake releases over 900 times (30 times 30) the energy
of a 4.7 earthquake – or it takes about 900 magnitude 4.7 earthquakes to
equal the energy released in a single 6.7 earthquake! There is no beginning
nor end to this scale.
However, rock mechanics seems to preclude earthquakes smaller than
about –1 or larger than about 9.5. A magnitude –1.0 event release about 900 times less
energy than a magnitude 1.0 quake.
Except in special circumstances, earthquakes below magnitude 2.5
are not generally felt by humans.
- Major Earthquake – An
earthquake having a magnitude of 7 to 7.99 on the Richter scale.
- Mantle – The layer of
rock that lies between the crust and the outer core of the Earth. It is approximately 2900 kilometers
thick and is the largest of the Earth’s major layers.
- Microearthquake – An earthquake having
a magnitude of 2 or less on the Richter scale.
- Microseism – A more or
less continuous motion in the Earth that is unrelated to an earthquake and
that has a period of 1.0 to 9.0 seconds.
It is caused by a variety of natural and artificial agents.
- Modified Mercalli Scale – Mercalli
intensity scale modified for North American conditions. A scale,
composed of 12 increasing levels of intensity that range from
imperceptible shaking to catastrophic destruction, that is designated by
Roman numerals. It does not have
mathematical basis; instead it is an arbitrary ranking based on observed
- Mohorovicic Discontinuity
(The Moho) – The boundary surface or sharp
seismic-velocity discontinuity (pronounced Mo-ho-ro-vi-chich)
that separates the Earth’s crust from the underlying mantle. Named for Andrija
Mohorovicic, the Croatian seismologist who first
suggested its existence.
- P-Wave – Primary,
longitudinal, irrotational, push, pressure,
dilatational, compressional, or push-pull wave. P-waves are the fastest body waves and
arrive at stations before the S-waves, or secondary waves. The waves carry energy through the Earth
as longitudinal waves, moving particles in the same line as the direction
of the wave. P-waves can travel
through all layers of the Earth.
P-waves are generally felt by humans as a bang or thump.
- Paleomagnetism – The natural magnetic
traces that reveal the intensity and direction of Earth’s magnetic field
in the geologic past. Also, the
study of these magnetic traces.
- Paleoseismology – The study of ancient
- Period – The time
between two successive wave crests.
- Phase – The onset of a
displacement or oscillation on a seismogram indicating the arrival of a
different type of seismic wave.
- Plate – One of the huge
sections which make up the Earth’s crust.
The plates are continuously moving.
- Plate Boundary – The
place where two or more plates in the Earth’s crust meet.
- Plate Tectonics – The
theory that the Earth’s crust and upper mantle (the lithosphere) is broken
into a number of more or less rigid, but constantly moving, segments or
- Rayleigh Wave – A type of
surface wave having a retrograde, elliptical motion at the Earth’s
surface, similar to the waves caused when a stone is dropped into a
pond. These are the slowest, but
often the largest and most destructive, of the wave types caused by an
earthquake. They are usually felt
as a rolling or rocking motion and in the case of major earthquakes, can
be seen as the approach. Named
after Lord Rayleigh, the English physicist who
predicted its existence.
- Recurrence Interval –
The approximate length of time between earthquakes in a specific
seismically active area.
- Reflect – To bounce
back from a surface.
- Refract – To bend or
- Richter Scale – The system used to measure the strength of an
earthquake. Developed by Charles
Richter in 1935 as a means of categorizing local earthquakes. It is a collection of mathematical
formulas; it is not a physical device.
- Rupture Zone – The area
of the Earth through which faulting occurred during an earthquake. For very small earthquakes, this zone
could be the size of a pinhead, but in the case of a great earthquake, the
rupture zone may extend several hundred kilometers in length and tens of
kilometers in width.
- S-Wave – Shear,
secondary, rotational, tangential, equivoluminal,
distortional, transverse, or shake wave.
These waves carry energy through the Earth in very complex patterns
of transverse (crosswise) waves.
These waves move more slowly than P-waves, but in an earthquake
they are usually bigger. S-waves
cannot travel through the outer core because these waves cannot exist in
fluids, such as air, water, or molten rock.
- Seiche – A free or standing
wave oscillation of the surface of water in an enclosed basin that is
initiated by local atmospheric changes, tidal currents, or
earthquakes. Similar to water
sloshing in a bathtub.
- Seismic Belt – An
elongated earthquake zone, for example, circum-Pacific, Mediterranean, Rocky Mountain. About 60% of the world’s earthquakes
occur in the circum-Pacific seismic belt.
- Seismic Constant – In
building codes dealing with earthquake hazards, an arbitrarily-set
acceleration value (in units of gravity) that a building must withstand.
- Seismicity – Earthquake
- Seismic – Of or having
to do with earthquakes.
- Seismic Sea Wave – A tsunami
generated by an undersea earthquake.
- Seismic Zone – A region
in which earthquakes are known to occur.
- Seismogram – A written record
of an earthquake, recorded by a seismograph.
- Seismograph – An
instrument that records the motions of the Earth, especially earthquakes.
- Seismograph Station – A
site at which one or more seismographs are set up and routinely monitored.
- Seismologist – A
scientist who studies earthquakes.
- Seismometry – The instrumental
aspects of seismology.
- Signal-to-Noise Ratio –
The comparison between the amplitude of the seismic signal and the
amplitude of noise caused by seismic unrest and (or) the seismic instruments.
- Spread – The layout of
seismometer or geophone groups from which data from a single shot (the
explosive charge) are recorded simultaneously.
- Spreading Center – An
elongated region where two plates are being pulled away from each
other. New crust is formed as
molten rock is forced upward into the gap.
Examples of spreading centers include the Mid-Atlantic Ridge and
the East African Rift.
- Subduction – The
process in which one lithospheric plate collides with and is forced down
under another plate and drawn back into the Earth’s mantle.
- Subduction Zone – An
elongated region along which a plate descends relative to another plate,
for example, the descent of the Nazca plate beneath the South American
plate along the Peru-Chile Trench.
- Surface of the Earth –
The value given is the depth below the surface of the mean spheroid. The mean spheroid is a uniform
approximation to the true shape of the Earth. No adjustment is made to the depth due
to any differences between the true Earth and the mean spheroid. For example, the minimum depth that will
be given is 0 kilometers, even though a quake directly under Mount Everest (elevation 8848
meters) could legitimately have a depth of –6 kilometers and still be 2
kilometers underground. On the
other hand, a depth of 10 kilometers would actually be more than 1
kilometer above the ocean floor of Challenger Deep (elevation –11,033
meters) in the Marianas Trench of the Pacific Ocean.
- Surface Waves – Waves
that move over the surface of the Earth.
Rayleigh waves and Love waves are surface
- Teleseism – An earthquake that
is distant (usually more than 20 degrees) from the recording station.
- Tidal Wave – A term
that seismologist hate. The correct
word for the big waves people often call “tidal waves”
is tsunami. True “tidal waves” – or
waves caused by tides – are the ordinary waves people see on the ocean.
- Travel Time – The time
required for a wave train to travel from its source to a point of
- Tsunami – One or a
series of huge sea waves caused by earthquakes or other large-scale
disturbance of the ocean floor.
(Referred to incorrectly by many as a tidal wave, but these waves
have nothing to do with tides.) The
word tsunami is Japanese, meaning “harbor wave.”
- Unconsolidated – Loosely
arranged, not cemented together, so particles separate easily.
- UTC – Coordinated
Universal Time. The time scale
based on the atomic second but correct every now and again to keep it in
approximate sync with the earth’s rotation. The corrections show up as the leap
seconds put into UTC – usually on New Year’s Eave. In the most common usage, the terms GMT
and UTC are identical.
Structure of the Earth
size of the Earth -- about 12,750 kilometers (km) in diameter-was known by the
ancient Greeks, but it was not until the turn of the 20th century that
scientists determined that our planet is made up of three main layers: crust,
mantle, and core. This layered structure can be compared to that of a boiled
egg. The crust, the outermost layer, is rigid and very thin compared with the
other two. Beneath the oceans, the crust varies little in thickness, generally
extending only to about 5 km. The thickness of the crust beneath continents is
much more variable but averages about 30 km; under large mountain ranges, such
as the Alps or the Sierra Nevada,
however, the base of the crust can be as deep as 100 km. Like the shell of an
egg, the Earth's crust is brittle and can break.
Cutaway views showing the internal structure of the Earth. Below: This view drawn to
scale demonstrates that the Earth's crust literally is only skin deep. Below right: A view not drawn to scale to
show the Earth's three main layers (crust, mantle, and core) in more detail
the crust is the mantle, a dense, hot layer of semi-solid rock approximately
2,900 km thick. The mantle, which contains more iron, magnesium, and calcium
than the crust, is hotter and denser because temperature and pressure inside
the Earth increase with depth. As a comparison, the mantle might be thought of
as the white of a boiled egg. At the center of the Earth lies the core, which
is nearly twice as dense as the mantle because its composition is metallic
(iron-nickel alloy) rather than stony. Unlike the yolk of an egg, however, the
Earth's core is actually made up of two distinct parts: a 2,200 km-thick liquid
outer core and a 1,250 km-thick solid inner core. As the Earth rotates, the
liquid outer core spins, creating the Earth's magnetic field.
surprisingly, the Earth's internal structure influences plate tectonics. The
upper part of the mantle is cooler and more rigid than the deep mantle; in many
ways, it behaves like the overlying crust. Together they form a rigid layer of
rock called the lithosphere (from lithos, Greek for
stone). The lithosphere tends to be thinnest under the oceans and in
volcanically active continental areas, such as the Western United States. Averaging at least 80 km
in thickness over much of the Earth, the lithosphere has been broken up into
the moving plates that contain the world's continents and oceans. Scientists
believe that below the lithosphere is a relatively
narrow, mobile zone in the mantle called the asthenosphere
(from asthenes, Greek for weak). This zone is
composed of hot, semi-solid material, which can soften and flow after being
subjected to high temperature and pressure over geologic time. The rigid
lithosphere is thought to "float" or move about on the slowly flowing
Plate Tectonics, Faults, and Fault Zones
who has ever stared at a map or globe must have noticed how well the eastern
coast of South America and the western coast of Africa would fit together much
like two pieces in a puzzle. Were they
once together or is this just a coincidence?
If they were once together, why are they now an ocean apart?
concept that the two continents were once joined is not a very old
concept. Only recently in scientific
history have scientists come to realize that these continents, as well as
others, were once very different than they are today. The theory of continental drift was the
concept of German scientist Alfred Wegener in
1912. Not only did Wegener
notice the puzzle “fit” of several continents, he was able to make the
connection by other means – fossils.
tectonics refers to the study of the formation and movement of large pieces of
rigid, solid land masses that move or “float” across the surface of the
earth. The Earth’s surface is divided
into a dozen major plates and several minor plates. Some of the plates are moving toward each
other, and some are moving apart. For
example, the South American plate and the African plate are moving apart as are
the North American plate and the Eurasian plate.
plates, however, are not spreading apart.
Since the Earth is finite in size, plates that are spreading apart, must be colliding with another plate on its opposite
side. Some plates slide past each other
instead of colliding. For example, in California, the Pacific plate and
North American plate are sliding past each other. However, in South America, the South American plate
is moving westward directly colliding with the Nazca plate. In this case, the westward moving South
American plate is riding over the Nazca plate, which is in turn forced downward. The leading edge of the South American plate
is marked all along the South American continent by the Andes Mountains. The trailing edge of the Nazca plate is
marked by a deep underwater trench where it is being forced downward by the
advancing South American plate. An
example of this action can be viewed on the right hand side of the diagram
Where plates collide, the result is usually a mountain
range formed along the leading edge of the advancing plate; whereas, a trench
forms where the second plate is being forced downward. Where plates spread apart, a feature known as
a “ridge” is formed. A ridge can extend
for nearly the length of the earth from pole to pole.
of the best known ridges is the Mid-Atlantic Ridge. The southern half of this ridge is bounded by
the South American plate to the west and the African plate to the east. Heated material (magma) due to convection
currents below the boundary rises to the surface causing the sea floor to
spread outward to the east and west.
This is the primary engine for the basis of continental drift.
Plate boundaries can be classified into the categories of
diverging boundaries, sliding boundaries, converging boundaries collision, and
converging boundaries subduction.
Diverging boundaries, as previously discussed, are plate boundaries that
are spreading apart.
In some cases, plate boundaries are sliding past each
other. The North American plate and the
Pacific plate is a prime example of a sliding boundary. It is denoted by a break or crack in the
Earth’s crust known as a fault. The
southwestern part of California
and the Pacific plate are moving to the northwest with respect to the rest of
the United States
and the North American plate.
to popular belief, California will not fall into
the Pacific. Instead, San Diego and Los Angeles will continue to move ever
so slowly to the northwest toward the San Francisco/Oakland area. Perhaps millions of years from now, these
cities will be cross-town neighbors!
plates converge, two possible results can occur. One result, the collision boundary, causes
the two plates to merge and is usually forced upward to form mountain
ranges. An example is the collision
between the Indian subcontinent with Asia. The result is the Himalayan Mountains. Other similar collisions caused the Ural Mountains to form 300 million years
ago when Europe collided with Siberia. The Appalachian Mountains formed when the North
American plate collided with northern Africa. Of course, these continents are now spreading
other result of converging plates is a subduction boundary. A subduction boundary is formed when one
plate plunges down under another overriding plate. The plunging plate is said to be subducting
beneath the overriding plate. The result
of subduction is usually a deep-sea trench.
The collision of South America with the Nazca plate is an
example of subduction.
The three basic types of
faults are determined by how a plate is interacting with its neighbor. When plates are pulling apart, a fault known
as a normal fault is formed. A normal
fault (extensional) occurs when the rocks on one side of the fault plane drop
down with respect to the rocks on the other side. The Mid-Atlantic Ridge is an example of this
type of faulting.
plates converge, a reverse fault (compressional) is typically found. Reverse faults result from the stresses that
push toward each other. Reverse faults
are important in mountain building because they allow the crust to be shortened
as the plates collide.
third type of fault is the strike-slip fault (transform). The San Andreas Fault in California is an example of this type
of fault. It is formed when two plates
slide horizontally past each other.
As the interactions of plates form different types of
faults, so do they produce different types of earthquakes.
In 1969, Muawia Barazangi
and James Dorman published the locations of all earthquakes which occurred from
1961 to 1967. Most of the earthquakes are confined to narrow belts and these
belts define the boundaries of the plates. The interiors of the plates
themselves are largely free of large earthquakes, that is, they are aseismic.
There are notable exceptions to this. An obvious one is the 1811-1812
earthquakes at New Madrid, Missouri, and another is the 1886 earthquake at Charleston,
South Carolina. As yet there is no
satisfactory plate tectonic explanation for these isolated events;
consequently, we will have to find alternative mechanisms.
tectonics confirms that there are four types of seismic zones. The first
follows the line of mid-ocean ridges. Activity is low, and it occurs at very
shallow depths. The point is that the lithosphere is very thin and weak at
these boundaries, so the strain cannot build up enough to cause large
earthquakes. Associated with this type of seismicity is the volcanic activity
along the axis of the ridges (for example, Iceland, Azores, Tristan da Cunha).
second type of earthquake associated with plate tectonics is the shallow-focus event
unaccompanied by volcanic activity. The San Andreas fault is a good example of this, so is the Anatolian
fault in Northern Turkey. In these faults, two mature plates are scraping by
one another. The friction between the plates can be so great that very large
strains can build up before they are periodically relieved by large
earthquakes. Nevertheless, activity does not always occur along the entire
length of the fault during any one earthquake. For instance, the 1906 San Francisco event was caused by breakage
only along the northern end of the San Andreas
third type of earthquake is related to the collision of oceanic and continental
plates. One plate is thrust or subducted under the other plate so that a deep
ocean trench is produced. In the Philippines, ocean trenches are
associated with curved volcanic island arcs on the landward plate, for example
the Java trench. Along the Peru - Chile trench, the Pacific plate
is being subducted under the South American plate which responds by crumpling
to form the Andes. This type of earthquake can be shallow, intermediate, or deep,
according to its location on the down-going lithospheric slab. Such inclined
planes of earthquakes are know as Benioff
fourth type of seismic zone occurs along the boundaries of continental plates.
Typical of this is the broad swath of seismicity from Burma to the Mediterranean, crossing the Himalayas, Iran, Turkey, to Gibraltar. Within this zone, shallow
earthquakes are associated with high mountain ranges where intense compression
is taking place. Intermediate- and deep-focus earthquakes also occur and are
known in the Himalayas and in the Caucasus. The interiors of continental plates are very complex, much more so
than island arcs. For instance, we do not yet know the full relationship of the
Alps or the East African rift
system to the broad picture of
can plate tectonics help in earthquake prediction? We have seen that
earthquakes occur at the following three kinds of plate boundary: ocean ridges
where the plates are pulled apart, margins where the plates scrape past one
another, and margins where one plate is thrust under the other. Thus, we can
predict the general regions on the Earth's surface where we can expect large
earthquakes in the future. We know that each year about 140 earthquakes of
magnitude 6 or greater will occur within this area which is 10 percent of the
on a worldwide basis we cannot say with much accuracy when these events will
occur. The reason is that the processes in plate tectonics have been going on
for millions of years. Averaged over this interval, plate motions amount to a
several millimeters per year. But at any instant in geologic time, for example,
the year 1977, we do not know exactly where we are in the worldwide cycle of
strain buildup and strain release. Only by monitoring the stress and strain in
small areas, for instance, the San Andreas fault, in great detail can we
hope to predict when renewed activity in that part of the place tectonics arena
is likely to take place.
summary, plate tectonics is a blunt, but, nevertheless, strong tool in
earthquake prediction. It tells us where 90 percent of the Earth's major
earthquakes are likely to occur. It cannot tell us much about exactly when they
will occur. For that, we must study in detail the plate boundaries themselves.
Perhaps the most important role of plate tectonics is that it is a guide to the
use of finer techniques for earthquake prediction.
History of Earthquakes
scientific study of earthquakes is comparatively new. Until the 18th century,
few factual descriptions of earthquakes were recorded, and the natural cause of
earthquakes was little understood. Those who did look for natural causes often
reached conclusions that seem fanciful today; one popular theory was that
earthquakes were caused by air rushing out of caverns deep in the Earth's
earliest earthquake for which we have descriptive information occurred in China in 1177 B.C. The Chinese earthquake
catalog describes several dozen large earthquakes in China during the next few
thousand years. Earthquakes in Europe are mentioned as early as 580 B.C., but the
earliest for which we have some descriptive information occurred in the
mid-16th century. The earliest known earthquakes in the Americas were in Mexico in the late 14th century
and in Peru in 1471, but descriptions
of the effects were not well documented. By the 17th century, descriptions of
the effects of earthquakes were being published around the world - although
these accounts were often exaggerated or distorted.
most widely felt earthquakes in the recorded history of North America were a series that occurred
in 1811-1812 near New Madrid, Missouri. A great earthquake, whose magnitude is
estimated to be about 8, occurred on the morning of December
16, 1811. Another great earthquake occurred on January
23, 1812, and a third, the strongest yet, on February
7, 1812. Aftershocks were nearly continuous between these great earthquakes and
continued for months
afterwards. These earthquakes were felt by people as far away as Boston and Denver. Because the most intense
effects were in a sparsely populated region, the destruction of human life and
property was slight. If just one of these enormous earthquakes occurred in the
same area today, millions of people and buildings and other structures worth
billions of dollars would be affected.
San Francisco earthquakes of 1906 was one of the most destructive in the
recorded history of North America - the earthquake and the fire that followed
killed nearly 700 people and left the city in ruins. The great 1906 San Francisco earthquake and fire
destroyed most of the city and left 250,00 people
Alaska earthquake of March 27,
was of greater magnitude than the San Francisco earthquake; it released perhaps twice as much energy and was felt over an area of
almost 500,000 square miles. The ground
motion near the epicenter was so violent that the tops of some trees were
snapped off. One hundred and fourteen people (some as far away as California) died as a result of this
earthquake, but loss of life and property would have been far greater had Alaska been more densely
Anatomy of an Earthquake
have looked at plate tectonics, types of faults, and the history of
earthquakes. But what is an earthquake
and what is associated with them?
previously discussed, earthquakes are the direct result of continental plates
sliding past each other. If you have
ever taken two rocks and rubbed them together, you have no doubt noticed they
do not smoothly slide past each other.
Instead, it is a rough jerky type of motion. Solid rock has high friction properties.
tectonic plates behave in a similar fashion.
They do not move smoothly past each other. Instead, they “lock up” building up
tremendous amounts of stress. When the stress becomes too high, a portion of the plates “snap”
past each other releasing copious amounts of energy in the form of an
an earthquake fault ruptures, it causes two types of deformation: static and dynamic. Static deformation is the permanent
displacement of the ground due to the event.
The earthquake cycle progresses from a fault that is not under stress,
to a stressed fault as the plate tectonic motions driving the fault slowly
proceed, to rupture during an earthquake and a newly-relaxed but deformed
Typically, someone will build a straight reference line
such as a road, railroad, pole line, or fence line across the fault while it is
in the pre-rupture stressed state. After
the earthquake, the formerly straight line is distorted into a shape having
increasing displacement near the fault, a process known as elastic rebound.
The second type of deformation, dynamic
motions, are essentially sound waves radiated from the earthquake as it
ruptures. While most of the
plate-tectonic energy driving fault ruptures is taken up by static deformation,
up to 10% may dissipate immediately in the form of seismic waves.
The mechanical properties of the rocks
that seismic waves travel through quicklyorganize the
waves into two types.
Compressional waves, also known as primary or P waves,
travel fastest, at speeds between 1.5 and 8 kilometers per second in the
Earth’s crust. Shear waves, also known
as S waves travel more slowly, usually at 60% to 70% of the speed of P waves.
P waves shake the ground in the direction they are
propagating, while S waves shake perpendicularly or transverse to the direction
Although wave speeds vary by a factor of ten or more in
the Earth, the ratio between the average speeds of a P wave and of its
following S wave is quite constant. This
fact enables seismologists to simply time the delay between the arrival of the
P wave and the arrival of the S wave to get a quick and reasonably accurate
estimate of the distance of the earthquake from the observation station. Just multiply the S-minus-P (S-P) time, in
seconds, by the factor 8km/s to get the approximate distance in kilometers.
The dynamic, transient seismic waves from any substantial
earthquake will propagate all around and entirely through the Earth. Given a sensitive enough detector, it is
possible to record the seismic waves from even minor events occurring anywhere
in the world an any other location on the globe. Nuclear test-ban treaties in effect today
rely on our ability to detect a nuclear explosion anywhere equivalent to an
earthquake as small as Richter Magnitude 3.5.
An earthquake generates a series of waves that penetrate
the entire Earth and travel at and through its surface. Each wave has a characteristic time: each has its own move of travel. They are quite complex, but a few basic facts
will explain how they travel through the Earth and how an earthquake’s
epicenter can be determined from seismograph records.
There are four basic types of seismic waves; two
preliminary body waves that travel through the Earth and two that travel only
at the surface (L waves). Combinations, reflections, and diffractions produce an infinity of other types, but body waves are the main
interest in this discussion.
Body waves are composed of two principal types; the P
(primary) wave, comparable to sound waves, which compresses and dilates the
rock as it travels forward through the Earth; and the S (secondary) wave, which
shakes the rock sideways as it advances at barely more than half the P-wave
2. Travel-time curves with idealized seismograms (earthquake records
The P wave is designated the primary preliminary wave
because it is the first to arrive at a seismic station after an earthquake. It
travels at a speed usually less than 6 kilometers per second in the Earth's
crust and jumps to 13 kilometers per second through the core.
The S wave is the secondary preliminary wave to be
recorded. It follows paths through the Earth quite similar to those of the
P-wave paths, except that no consistent evidence has yet been found that the S
wave penetrates the Earth's core.
The lines labeled P, S, and L in the curves shown on
figure 2 represent the travel time required for each phase at distances of 0 to
1300 kilometers from the earthquake's epicenter. They mark the points on the
record at which these waves first arrive at the station.
The simplest method of locating an earthquake on a globe
is to find the time interval between the P- and S-wave arrivals at several
seismograph stations. The distance to the earthquake from each station is then
determined from standard travel-time tables and travel-time curves.
Great-circle arcs are drawn on the globe
using the distance of the
earthquake to the station as a radius. All the arcs should intersect at a
common point - the epicenter.
Another method of locating an earthquake is to use the
P-wave arrival-time minus origin-time (P - O) interval instead of distance.
This method is more common because the time can be taken directly from surface
focus travel-time tables assuming an origin of 00 hours. This method, however,
requires that travel-time tables be available for various depths of focus. For
locating a deep shock, one 700 kilometers deep, for example, travel-time tables
and travel-time curves for that depth have to be used to calculate the origin
time and distances.
Other wave types can be generated inside the Earth by P
and S waves, as shown in figure 3 (following page). As many as five different
wave groups or phases can emerge when a P or S wave encounters a discontinuity
or interface within the Earth.
3. Propagation paths of combinations of P, S, and L waves from an
Seismographs, and Seismograms
Seismographs are based on the principal of inertia and
the differential motion between a free mass (which tends to remain at rest) and
a supporting structure anchored in the ground (which moves with the vibrating
Earth). Seismographs can be constructed
to record both horizontal and vertical motion.
The most common seismograph is the horizontal seismograph.
The seismograph consists of a large aluminum plate with a
support along the rear of the plate which holds the pendulum. A horizontal pendulum rests against the
bottom of a screw on the rear support.
The pendulum is supported by guitar string. Near the end of the pendulum is a lead weight
used for mass as well as a coil of copper wire.
The pendulum is carefully balanced and has a naturally
occurring period of around 16 seconds.
Paddles dipped in oil prevent the pendulum from swinging excessively
(damping). Two large magnets are positioned
so that the coil fits between them with a clearance of around ¼” on either
Since the base is in contact with the “Earth” and the
pendulum is suspended by a string, any lateral motion in the Earth moves the
base and the magnets leaving the pendulum to remain in place. The displacement between the coil at the end
of the pendulum and the magnets generates very small
electrical currents which in turn is interpreted by a computer as
More sophisticated seismographs, such as those at the
United States Geological Survey, record the information on drums as well as on
computers. The seismograph can measure
earthquakes within the continental United States
as small as magnitude 4.5 should the quake occur along the west coast. Smaller, closer quakes can be measured such
as those along the New Madrid fault zone in southeastern Missouri
and northeast Arkansas; however,
no such events have occurred since the seismograph has gone online.
Quakes of 5.5 or higher can usually be detected in the
western hemisphere with larger quakes of 6.5 or higher being detected anywhere
in the world. Recorded earthquakes will
be entered into the Public Seismic Network’s database and displayed on the
internet along with other postings from around the world.