Download Scientific level COMPUTER | EARTHQUAKE SIMULATION THE

Scientific level
COMPUTER | EARTHQUAKE SIMULATION
THE CLIP
Instead of building structures and seeing if they can withstand an earthquake or not, computer simulations are
being used to simulate the effect of earthquakes on buildings, bridges, roads and other such structures. So first,
construction engineers have to create a small model of the structure they would like to build, in this case a
bridge. Then they have to simulate an earthquake by using shake tables, which are large motors and computer
controlled systems that try to precisely simulate earthquake movements. They place sensors on the most critical
parts of the structure which are also connected to a computer. During a simulated tremor the data from the
sensors is automatically inputted into the computer. The engineers can then simulate different types and
magnitudes of earthquakes on the computer, observe their effects on the bridge and change parts of it
accordingly before building the real thing.
THIS DEMONSTRATES ....
The importance of using computer simulations of the effects of earthquakes on structures before they are built.
THE CAUSES OF EARTHQUAKES
An earthquake is a vibration that travels through the earth’s crust and affects a fairly large area, such as an
entire city. Earthquakes can be caused by:
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Volcanic eruptions
Meteor impacts
Underground explosions (ex. an underground nuclear test)
Collapsing structures (ex. a collapsing mine)
But the majority of naturally-occurring earthquakes are caused by movements of the earth’s plates.
Earthquakes have caused a great deal of property damage over the years, and they have claimed many lives. In
the last hundred years alone, there have been more than 1.5 million deaths due to earthquakes. Usually, it's not
the shaking ground itself that claims lives - it's the associated destruction of manmade structures and the
instigation of other natural disasters, such as tsunamis, avalanches and landslides.
DAMAGE CAUSED BY AN EARTHQUAKE
SLIDING PLATES
The biggest scientific breakthrough in the history of seismology - the study of earthquakes - came in the middle
of the 20th century, with the development of the theory of plate tectonics. Scientists proposed this idea to explain
a number of peculiar phenomenon on earth, such as the apparent movement of continents over time, the
clustering of volcanic activity in certain areas and the presence of huge ridges at the bottom of the ocean.
The basic theory is that the surface layer of the earth -- the lithosphere - is comprised of many plates that slide
over the lubricating athenosphere layer. At the boundaries between these huge plates of soil and rock, three
different things can happen:
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plates can move apart,
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plates can push together,
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plates can slide against each other.
Where these plates meet, faults are formed. Faults are breaks in the earth’s crust where the blocks of rock on
each side are moving in different directions. Earthquakes are much more common along fault lines than they are
anywhere else on the planet.
PLATE BOUNDARIES
FAULTS
There are different types of faults however in all types of faults, the different blocks of rock push very tightly
together, creating a good deal of friction as they move. If this friction level is high enough, the two blocks
become locked, that is, the friction keeps them from sliding against each other. When this happens, the forces in
the plates continue to push the rock, increasing the pressure applied at the fault.
If the pressure increases to a high enough level, then it will overcome the force of the friction, and the blocks will
suddenly snap forward. Some fault shifts create visible changes at the earth's surface, but other shifts occur in
rock well under the surface, and so don't create a surface rupture.
The initial break that creates a fault, along with these sudden, intense shifts along already formed faults, are the
main sources of earthquakes.
FAULTS
SEISMIC WAVES
When a sudden break or shift occurs in the earth's crust, the energy radiates out as seismic waves, just as the
energy from a disturbance in a body of water radiates out in wave form. In every earthquake, there are different
types of seismic waves - body waves and surface waves.
Body waves move through the inner part of the earth, while surface waves travel over the surface of the earth.
Surface waves are responsible for most of the damage associated with earthquakes, because they cause the
most intense vibrations. Surface waves originate from body waves that reach the surface.
RATING MAGNITUDE AND INTENSITY OF AN EARTHQUAKE
The Richter scale is a standard scale used to compare earthquakes. It is measured through a seismograph. It is
a logarithmic scale, meaning that the numbers on the scale measure factors of 10. So, for example, an
earthquake that measures 4.0 on the Richter scale is 10 times larger than one that measures 3.0. On the Richter
scale, anything below 2.0 is undetectable to a normal person and is called a microquake. Microquakes occur
constantly. Moderate earthquakes measure less than 6.0 or so on the Richter scale. Earthquakes measuring
more than 6.0 can cause significant damage. The maximum quake rating ever measured was about 8.9.
A SEISMOGRAM
Richter ratings only give you a rough idea of the actual impact of an earthquake. An earthquake's destructive
power varies depending on the composition of the ground in an area and the design and placement of manmade
structures. The extent of damage is rated on the Mercalli Scale. Mercalli ratings, which are given as Roman
numerals, are based on largely subjective interpretations. A low intensity earthquake, one in which only some
people feel the vibration and there is no significant property damage, is rated as a II. The highest rating, an XII,
is applied only to earthquakes in which structures are destroyed, the ground is cracked and other natural
disasters, such as landslides or Tsunamis, are initiated.
Richter scale ratings are determined soon after an earthquake, once scientists can compare the data from
different seismograph stations. Mercalli ratings, on the other hand, can't be determined until investigators have
had time to talk to many eyewitnesses to find out what occurred during the earthquake. Once they have a good
idea of the range of damage, they use the Mercalli criteria to decide on an appropriate rating.
DEALING WITH EARTHQUAKES
Scientists can make general guesses when they come to predict earthquakes but they still have to do more
research and learn more about earthquakes in order to be able to say with precision when one is going to occur.
So the major advances over the past 50 years have been in preparedness - particularly in the field of
construction engineering. In 1973, the Uniform Building Code, an international set of standards for building
construction, added specifications to fortify buildings against the force of seismic waves. This includes
strengthening support material as well as designing buildings so they are flexible enough to absorb vibrations
without falling or deteriorating. It's very important to design structures that can take this sort of punch, particularly
in earthquake-prone areas.
Another way of avoiding too much damage during earthquakes is to educate the public on how to prepare one’s
house for the possibility of an earthquake and also what to do when an earthquake hits.
EARTHQUAKE CONSTRUCTION
Earthquake construction is a branch of architectural engineering concerned with making sure structures
withstand as severe an earthquake shock as possible given the materials available.
When the structure in question is a human habitation, the questions of surviving earthquake damage become
much more serious. Examples of inhabited structures collapsing during earthquakes abound and are sadly all
too frequent. Areas of the world frequently hit by fatal earthquake damage include Japan, Turkey and Algeria.
Earlier in mankind's history man used to live in tents, which can withstand earthquakes quite well. He then
moved on to more comfortable structures made from timber, mud brick, limestone, stacked rubble and other
more sturdy materials.
Some of these materials can be used to form solid, earthquake resistant structures. The important point is to use
them wisely and with an understanding of how earthquakes really apply stresses to structures in practice. A
structure might have all the appearances of stability, yet offer nothing but danger when an earthquake strikes.
The crucial fact is that for safety, earthquake resistant construction techniques are as important as using the
correct materials.
The specific mode of failure in an earthquake for most structures is the lateral (sideways) shaking. It frequently
collapses walls, or moves them enough that the roof displaces and falls in. Both of these effects, obviously, can
be deadly to any occupants.
DEVELOPMENT OF EARTHQUAKE CONSTRUCTION TECHNIQUES
People living in frequently shaken areas like Japan started early to develop earthquake resistant buildings based
on scientific study. Other countries likewise have studied, and continue to study intensely, how to make their
citizens safer by understanding the problems posed by earthquakes more accurately.
Until the last 75 years or so, the only way to run "frequent tests" was to build on a fault and hope. Even then,
earthquakes may only happen at any given spot every couple of hundred years, and construction techniques
may not therefore take account of earthquake concerns. Modern shake tables have helped this; large motors
and computer control systems try to precisely simulate earthquake movements.
Modern materials like concrete and reinforced concrete can help, but they also must withstand the same lateral
(sideways) forces.
Good earthquake construction pays careful heed to lateral forces. Proper concrete construction involves
significant use of steel reinforcing bar (rebar). All the joints, where beams meet the columns, are carefully tied in
with rebar. The concrete is of very high quality, and high strength. Brick infill is avoided for the walls.
Most countries have a building code that specifies lateral strength, but however, these codes are reliant on strict
enforcement.
MODERN TECHNIQUES
Modern construction techniques for earthquake zones involve designing structures that fail in predictable ways
at predictable energy levels based on quantified earthquake severities. Many historic buildings have been
subjected to a seismic retrofit.
In residential structures, buildings are designed to have the roof fall right in the middle of a room, but stay up
near the walls. People are always urged to take refuge in doorways and away from the middle of the room, and
are therefore safe in these buildings. The structure of a residence may also be attached to the foundation with
bolts to prevent the building from sliding off the foundation during shaking and collapsing.
GROUND STABILIZATION
Another failure mode of a structure in an earthquake involves the soil underneath the structure. In a strong
enough seismic event, the soil can be shaken hard enough that it will break up, sometimes leading to the
collapse of the structure sitting upon it. The most common method of protecting a structure against this failure
mode is to flow cement into the soil beneath the structure. This method provides marginal support for the
structure as the cement may not set evenly.
An alternative method to infusing the ground with cement is under study. The method involves using a bacterium
that secretes a viscous, sticky polymer that binds the soil together. In an experiment, the bacteria,
Flavobacterium johnsoniae, was mixed with sand and was given several days to colonize the sand. The friction
coefficient of the sand was measured and compared against sand without the bacteria colony. The sand
colonized with the bacteria was almost twice as solid as the sand without the polymer producing bacteria.
In the future, improvements in prediction and preparedness should further minimize the loss of life and property
associated with earthquakes. But it will be a long time, if ever, before we'll be ready for every substantial
earthquake that might occur. Just like severe weather and disease, earthquakes are an unavoidable force
generated by the powerful natural processes that shape our planet. All we can do is increase our understanding
of the phenomenon and develop better ways to deal with it.
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References:
How Stuff Works [Online],
http://www.howstuffworks.com/, 24 Feb, 2006.
Wikimedia Foundation 2006, Wikipedia - The Free Encyclopedia [Online],
wikipedia.org/wiki/Main_Page, 28 Feb, 2006.