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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: 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: plates can move apart, plates can push together, 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. ___________________________________________________________________ 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.