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Earthquakes Earthquakes Describe the frequency, origin and distribution of earthquakes at mid-ocean ridges, ocean basins, subduction zones and continental shields. Earthquakes are happening almost everyday all over the world. Most of the time earthquakes are not strong enough to be felt by people, but the shaking caused by an earthquake is recorded by a seismogram. These are located all over the world at different points. Only occasionally will a larger magnitude earthquake strike and cause damage to the region.

Around the world there are many faults, depending where these faults are plays a major factor in determining where an earthquake will happen. It is these faults that are the reason for earthquakes. The type of fault will also determine how often an earthquake will happen. A mid-ocean ridge occurs under the sea at a divergent boundary. This is where two plates are been pulled apart because of tension.

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This then allows new oceanic crust to be made in the divergent boundary, as magma rises and eventually sets on the sea floor. If the plates on either side of the divergent boundary continue to spread then the ocean slowly becomes larger in width, a process called seafloor spreading. Mid-ocean ridges are characterised by a crack like valley at the divergent boundary. This crack like valley is caused by the tension pulling the plates apart, causing normal faulting to occur a number of times in the divergent boundary. It is these normal faults that are the cause and therefore the origin of earthquakes at divergent boundaries. When the tension pulling apart the two plates becomes too much then the oceanic crust will fracture. This fracturing is caused by many normal faults happening as shown in the diagram.

The normal faults happen because the crust is been extended. When the tension becomes too much the faults slip vertically. They move a large distance in a relatively short space of time, this is the cause of the earthquakes at divergent boundaries. Divergent boundaries mostly occur on the sea floor and therefore the earthquakes that happen at these boundaries are distributed along the boundary. This means that the distributions of earthquakes at divergent boundaries are at shallow depths, where the crust is been pulled apart. The earthquakes happen at shallow depths because the normal faulting occurs near the sea floor, as a result of the tension. The normal faults are the cause of the earthquakes at these divergent boundaries.

The seafloor sees the most intense tectonic activity in the world, meaning that at the sites of mid-ocean ridges the frequency of earthquakes is very high. An example of a mid-ocean ridge is the Mid-Atlantic ridge, there the seafloor is spreading at a rate of about 3cm per year. The frequency of earthquakes at a mid-ocean ridge will depend on how much tension is happening at that point. The more tension means the more seafloor spreading, resulting in a higher frequency of earthquakes at a particular mid-ocean ridge. Four major oceans make up most of the water in the world, The Atlantic (north & south), The Pacific, The Antarctic and The Indian Ocean. Within the basins of these oceans earthquakes can happen without been caused at Mid-ocean ridges, or a Subduction Zones.

When the earth?¦s crust is under tensional forces the crust will become much thinner than normal, if there is no fault. This means that the crust becomes weaker as it is thinner than normal. This can happen to the oceanic crust in the ocean basins, but will only cause an earthquake with a hot spot. A hot spot is an abnormal hot rising area of the mantle that supplies the lava for volcanoes. If at the same time a hot spot is directly below a thinned crust then the magma in the hot spot may hold too much pressure to be held by the thinner weakened crust. If this is the case then the magma can penetrate the lithosphere, and eventually erupt on the surface.

The action of the magma forcing its way up can trigger earthquakes as it breaks through the crust. When its breaks through the crust at the sea bed eventually a volcanic island will be formed in the middle of the ocean. Due to plate movements this can lead to the creation of mid-plate chains of basaltic volcanic islands, e.g. Hawaii. The creation of these islands around the world has happened in other places.

Frequent large earthquakes do not happen along the Hawaiian chain, it is an essentially an asesimic ridge. Therefore the frequency of earthquakes caused in ocean basins by hot spots is very low. The distributions of these earthquakes that do occur happen at shallow depths. This is because the origin is in the crust, which has been thinned because of tension. A subduction zone is where two plates collide and one is forced below the other, they occur at convergent boundaries. They collide because of compression forces, pushing them into each other.

One plate is subducted below the other into the mantle, where it will be recycled. An example of this is shown below with the Pacific plate subducting under the Eurasian plate. The two plates want to travel in opposite directions, they want to go straight into each other. This causes the pressure to build up over a long period of time, as the two plates push at each other. As time progresses one of the plates will start to be bent downward under the other one because of the extreme force, however does not slip, just bends. This is because of the friction between the two plates is enough to allow them to bend, without slipping.

This is a very slow but continuous movement, maybe only a few millimetres every year. Every fraction moved by the plates increases the build-up of elastic strain energy within the rock. The rock continues to store this energy from a few decades to a few thousand years. An earthquake will happen when the strain in the rocks exceeds that of the limit of the rocks. The fault then ruptures, moving a large distance in a short space of time. The plates then snap back into a new position, forcing the already undercutting plate to dive down even further under the other. The collisions of two plates generally produce large forces in the plates.

These forces result in the triggering of the earthquakes within the subduction zones. The frequency of earthquakes in Subduction zones is about the same as that in the mid-ocean ridges. This is because the plates cover the globe, and if they separate in one place then in another place one-plate sinks below another. This means that the triggering of an earthquake at a divergent boundary triggers an earthquake at a convergent plate. Meaning that the frequency of earthquakes at Subduction zones is the same as at Mid-ocean ridges, which is very high. The earthquakes at convergent boundaries are distributed at different points.

The deep focus earthquakes occur along the already subducted plate. Shallow focus earthquakes occur just at the point where one plate starts to be thrust under the other. These earthquakes tend to be more common than the deeper earthquakes. This is shown on the diagram on the left. The red dots show the distribution of earthquakes at a convergent boundary.

Continental shields are extensively flat tectonically stable interiors of the continents, composed of ancient rocks. Most of the stress that builds up by tectonic movements is released in earthquakes at the plate boundaries. However stress can also build up in the interiors of plates. Old fault lines in the plates are weaker than the surrounding rocks, these old fault lines cover many continents, crossing all over each other. The old faults can slip if the stress becomes too much from recent plate movements, which will cause an unexpected earthquake.

This can be a problem as many old fault lines are not known, and many are away from modern plate boundaries that exist today. This is potentially dangerous as many modern settlements may be at risk from earthquakes, even though they are not near modern day faults. The distribution of earthquakes at continental shields is not yet known, as scientists do not know whether these earthquakes will strike the same region within a plate. The strength of these inter-plate earthquakes are relatively small, compared to boundary earthquakes. The frequency is also very small, the last major inter plate earthquake was in Latur-India in 1993.

However they can catch regions totally unexpected because they can affect areas with no previous earthquake history. Also the energy of the earthquake is spread out further without losing as much. Due to the older hard rocks that transmit energy better, than the deformed broken younger rocks. This can cause more damage to a larger region. Earthquakes are common events and are happening all the time.

They can be caused by many different factors within the earth?¦s interior. Depending on the type of area that they happen in will determine the strength of the earthquake, and the frequency of earthquakes within the region. The distribution of earthquakes within an area will much depend upon what caused the earthquake to happen in the first place. We understand today how earthquakes are caused, and we can record where they happen every day of the year. This has helped us to learn and understand about earthquakes in much detail. We now only miss one important factor that we all would like to know, when and where the next one will be.

In truth it must be said that today we are still not close to predicting earthquakes even with all the technology that is available. Bibliography: Understanding Earth 2nd edition by Frank Press and Raymond Siever. Microsoft Encarta Encyclopaedia 1998.


Earthquakes have plagued our lives for as long as people have inhabited
the earth. These dangerous acts of the earth have been the cause of many deaths
in the past century. So what can be done about these violent eruptions that take
place nearly with out warning? Predicting an earthquake until now has almost
been technologically impossible. With improvements in technology, lives have
been saved and many more will. All that remains is to research what takes place
before, during, and after an earthquake. This has been done for years to the
point now that a successful earthquake prediction was made and was accurate.

This paper will discuss a little about earthquakes in general and then about how
predictions are made.

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Earthquake, “vibrations produced in the earth’s crust when rocks in
which elastic strain has been building up suddenly rupture, and then
rebound.”(Associated Press 1993) The vibrations can range from barely noticeable
to catastrophically destructive. Six kinds of shock waves are generated in the
process. Two are classified as body wavesthat is, they travel through the
earth’s interiorand the other four are surface waves. The waves are further
differentiated by the kinds of motions they impart to rock particles. Primary or
compressional waves (P waves) send particles oscillating back and forth in the
same direction as the waves are traveling, whereas secondary or transverse shear
waves (S waves) impart vibrations perpendicular to their direction of travel. P
waves always travel at higher velocities than S waves, so whenever an earthquake
occurs, P waves are the first to arrive and to be recorded at geophysical
research stations worldwide.(Associated Press 1993)
Earthquake waves were observed in this and other ways for centuries, but
more scientific theories as to the causes of quakes were not proposed until
modern times. One such concept was advanced in 1859 by the Irish engineer Robert
Mallet. Perhaps drawing on his knowledge of the strength and behavior of
construction materials subjected to strain, Mallet proposed that earthquakes
occurred either by sudden flexure and constraint of the elastic materials
forming a portion of the earth’s crust or by their giving way and becoming
fractured.(Butler 1995)
Later, in the 1870s, the English geologist John Milne devised a
forerunner of today’s earthquake-recording device, or seismograph. A simple
pendulum and needle suspended above a smoked-glass plate, it was the first
instrument to allow discrimination of primary and secondary earthquake waves.

The modern seismograph was invented in the early 20th century by the Russian
seismologist Prince Boris Golitzyn. His device, using a magnetic pendulum
suspended between the poles of an electromagnet, “ushered in the modern era of
earthquake research.” (Nagorka 1989)
“The ultimate cause of tectonic quakes is stresses set up by movements
of the dozen or so major and minor plates that make up the earth’s
crust.”(Monastersky Oct, 95) Most tectonic quakes occur at the boundaries of
these plates, in zones where one plate slides past anotheras at the San Andreas
Fault in California, North America’s most quake-prone areaor is subducted
(slides beneath the other plate). Subduction-zone quakes account for nearly
half of the world’s destructive seismic events and 75 percent of the earth’s
seismic energy. They are concentrated along the so-called Ring of Fire, a narrow
band about 38,600 km (about 24,000 mi) long, that coincides with the margins of
the Pacific Ocean. The points at which crustal rupture occurs in such quakes
tend to be far below the earth’s surface, at depths of up to 645 km (400 mi).

(Monastersky Dec, 95) Alaska’s disastrous Good Friday earthquake of 1964 is an
example of such an event.

Seismologists have devised two scales of measurement to enable them to
describe earthquakes quantitatively. “One is the Richter scale named after the
American seismologist Charles Francis Richterwhich measures the energy released
at the focus of a quake. It is a logarithmic scale that runs from 1 to 9; a
magnitude 7 quake is 10 times more powerful than a magnitude 6 quake, 100 times
more powerful than a magnitude 5 quake, 1000 times more powerful than a
magnitude 4 quake, and so on.”(Associated Press 1992)
The other scale, introduced at the turn of the 20th century by the
Italian seismologist Giuseppe Mercalli, measures the intensity of shaking with
gradations from I to XII. (Associated Press 1992) Because seismic surface
effects diminish with distance from the focus of the quake, the Mercalli rating
assigned to the quake depends on the site of the measurement. Intensity I on
this scale is defined as an event felt by very few people, whereas intensity XII
is assigned to a catastrophic event that causes total destruction. Events of
intensities II to III are roughly equivalent to quakes of magnitude 3 to 4 on
the Richter scale, and XI to XII on the Mercalli scale can be correlated with
magnitudes 8 to 9 on the Richter scale.( Associated Press 1992)
Attempts at predicting when and where earthquakes will occur have met
with some success in recent years. At present, China, Japan, Russia, and the U.S.

are the countries most actively supporting such research. In 1975 the Chinese
predicted the magnitude 7.3 quake at Haicheng, evacuating 90,000 residents only
two days before the quake destroyed or damaged 90 percent of the city’s
buildings. One of the clues that led to this prediction was a chain of low-
magnitude tremors, called foreshocks, that had begun about five years earlier in
the area. (Day 1988) Other potential clues being investigated are tilting or
bulging of the land surface and changes in the earth’s magnetic field, in the
water levels of wells, and even in animal behavior. A new method under study in
the U.S. involves measuring the buildup of stress in the crust of the earth. On
the basis of such measurements the U.S. Geological Survey, in April 1985,
predicted that an earthquake of magnitude 5.5 to 6 would occur on the San
Andreas fault, near Parkfield, California, sometime before 1993.(Day 1988) Many
unofficial predictions of earthquakes have also been made. In 1990 a zoologist,
Dr. Iben Browning, warned that a major quake would occur along the New Madrid
fault before the end of the year. Like most predictions of this type, it proved
to be wrong. Groundwater has also played an important part in earthquake
predictions. A peak in radon in the groundwater at Kobe, Japan 9 days before the
7.2 earthquake cause quite a stir. Radon levels peaked 9 days before the quake,
then fell below the normal levels 5 days before it hit.(Monastersky July, 95)
In North America, the series of earthquakes that struck southeastern
Missouri in 1811-12 were probably the most powerful experienced in the United
States in historical time. The most famous U.S. earthquake, however, was the one
that shook the San Francisco area in 1906, causing extensive damage and taking
about 700 lives.(Nagorka 1989)
The whole idea behind earthquake predicting is to save lives. With the
improvement in technology, lives have been saved. New ideas and equipment is
starting to prove to be very helpful in predicting were and when an earthquake
will strike. The time and research put into earthquake predicting has already
started to pay off. It is only a matter of time before earthquakes will no
longer be a threat to us.

Associated Press 1992, “The Big One: Recent Tremors May Be a `Final Warning'”;
SIRS 1993 Earth Science, Article 12, Aug. 30, 1992, pg. J1+.

Associated Press 1993, “Predicting the Effects of Large Earthquakes”; SIRS 1994
Applied Science, Article 17, Sept./Oct. 1993, pg. 7-17.

Butler, Steven 1995, “Killer Quake”; SIRS 1995 Earth Science, Article 47, Jan.

30, 1995, pg. 38-44.

Day, Lucille, 1988, “Predicting The Big One”; SIRS 1989 Earth Science, Article 5,
Summer 1988, pg. 34-41.

Monastersky, R. 1995, “Electric Signals May Herald Earthquakes”; Science News, v.

148, Oct. 21 ,1995, pg. 260-1.

Monastersky, R. 1995, “Quiet Hints Preceded Kobe Earthquake”; Science News, v.

148, July 15, 1995, pg. 37.

Monastersky, R. 1995, “Radio Hints Precede a Small U.S. Quake”; Science News, v.

148, Dec. 23;30, 1995, pg. 431.

Nagorka, Jennifer 1989, “Earthquakes: Predicting Where Is Easy–It’s When
That’s Tough”; SIRS 1990 Earth Science, Article 26, Oct.29, 1989, pg. E1-2.
Category: Science


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