Earthquakes Essay, Research Paper
Geology is the study of the earth’s landmasses. The earth is constantly changing. Often changes are to slow to be able to see them in a human lifetime. Forces cause different things to happen on the surface of the earth (Such as Mountains growing and eroding). Scientists believe that the earth was formed by very dense elements such as iron and nickel which are in the core of the earth. The earth’s crust covers the mantle. The crust is made up of over two thousand different compounds called minerals. The crust is the outer most and thinnest layer. It ranges between four and seven kilometers. The mantel is sits right below the crust and is about two thousand nine hundred kilometers thick. As you go deeper into the mantel is becomes hotter and more pressurized.
The upper part of the mantel and the crust make up the lithosphere. The lithosphere is the dense, solid layer that surrounds the earth; we call it the ground. There are giant plates that sit on top of the earth’s mantle. These plates are very large rigid slabs of crustal rock that float on top of the mantel. The lighter plates carry the most landmass and the more dense plates carry less landmass. These plates are like large sheets of ice siting on top of a pond and like the ice they do not sit still. Scientist believe that the plates are driven by convection currents in the mantle. There are nine major plates. These plates have boundaries. There are different types of boundaries. A divergent boundary is when two plates move apart from one another. A convergent boundary is when two plates move towards each other. A transform boundary is when two plates slide horizontally past each other these boundaries are also called faults.
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. Stress in the earth’s crust creates faults-places where rocks have moved and slip, resulting in earthquakes. The properties of an earthquake depend strongly on the type of fault slip, or movement along the fault. Geologists categorize faults according to the direction of the fault slip. The surface between the two sides of a fault lies in a plane, 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 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.
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. 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 (P & 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 (about 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 (about 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. On the surface of the earth, Rayleigh waves cause rock particles to move forward, up, backward, and down in the direction of the wave travels. 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.
There have been many major earthquakes in the bay area. In all of the major earthquakes there have been deaths as a direct result of building design and earthquake safety laws. On October 21, 1868 there was one of the most destructive earthquakes San Francisco has had. Know as the “Great San Francisco Earthquake”. It ruptured from the southern end of the Hayward fault. It had a magnitude of 7.0. This Quake killed over twenty people and injuring over sixty people; it destroyed $300,000 dollars of property – a lot of money back then. On April 18, 1906, the San Andreas Fault ruptured with a magnitude of 7.7. This earthquake caused over three thousand deaths and near 225,000 injuries. The total destruction cost of this earthquake was over 400 million dollars.
Earthquake design in the Bay Area has become a pressing issue because we have seen the damage an earthquake can cause and we are trying to prepare for the next one We now know the importance of earthquake safety in the bay area. The San Francisco Bay Area is part of a very complex plate boundary system between the pacific and the northern American plates. Near Hollister, the Calaveras fault branches off from the San Andreas Fault towards the north. The Hayward fault branches off from the Calaveras towards the northwest. At a much smaller scale many thrust faults run parallel and cross the San Andreas Fault. Although most of the present day seismic activity in the Bay Area comes from the major faults (San Andreas, Hayward-Mission creek, Concord-Calaveras, and the Antioch faults) and an additional ten percent happens in the minor and unmapped faults. 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.
Many factors influence the strength of earthquake shaking at a site as well, including the earthquake’s magnitude and the site’s proximity to the fault. In addition, soft soil always amplifies shear waves. If an earthquake is strong enough and close enough to cause damage, the damage will usually be more severe on soft soils.
Seismologists have observed that some districts tend to repeatedly experience stronger seismic shaking than others do. This is because the ground under these districts is relatively soft. Soft soils amplify ground shaking. If you live in an area that in past earthquakes suffered shaking stronger than that felt in other areas at comparable distance from the source, you are likely to experience relatively strong shaking in future earthquakes as well. An example of this effect was observed in San Francisco, where many of the same neighborhoods were heavily damaged in both the 1906 and 1989 earthquakes. The influence of the underlying soil on the local amplification of earthquake shaking is called the site effect.
In urban areas of the Bay Area, 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- 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. 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 this.
The design of current buildings in California is highly complicated. Current laws state that new buildings built in an earthquake zone must have up to date earthquake design. There are many types of earthquake design in the bay area. Technology includes using seismic isolation, where the building sits on top of a huge rubber bearing with steel sheets. The rubber bearings are connected to the ground and the building. When the building is involved in an earthquake the rubber bearings act as dampeners and allow the building to move with the earthquake. The most commonly used, and cost effective method, is the lateral load resting system where there are many types of lateral load system. Such as shear wall and braced frames.
A shear wall design is when there are vertical walls that are on the outside of the building, which supports the weight of the building. When you build a house of cards you are building a shear wall structure. Where the outside walls must be place at a certain angle to one another or the house will collapse. Just like the house of cards these outside walls must be placed a particular way to support the inside structure.
A braced frame building is more for the larger type building such as the Pixar building in Emeryville. This design puts major steel frames though out the building to support the heavy load of the building. When these large steel rods are connected properly the hold the building together thorough virtually anything. The only thing wrong with this design is if the brace frames are put under an earthquake with a magnitude of 8.2 or above the steel frames will collapse from the vibration.
The future of are children clearly is at stake. As we have seen earthquakes are not going to go away anytime soon. They are just getting stronger, as we have seen in San Francisco. If we continue to strive for the perfectly safe earthquake building then we are giving are children not only a better world but also a better chance of living. If we stop currently improving building old and new than we are doing nothing to help the future. We are only hurting our self. Millions of people have died from lack of earthquake building design. The future of not only the Bay Area is in play but the future of the world. Our generation is not only getting more technologically advance but smarter. If we put are resources together than we can guarantee our children a safe future.
I think that these major corporations should put more money into the design of major building. If we develop build last will last though the earthquake that the earth has then we are giving the future something to work with. When more time and money is put into anything you can see a difference take the Metreon is San Francisco it took the 2 years just to develop the building. This means that they are planing for this building to be around for a long time. By these major corporations doing this they are only protecting their investment. When we put more money and more time into building earthquake design we get greater results and better designs.
It is not just Major Corporation that needs to improve earthquake design. I believe that city need to make their earthquake design laws a little more advanced. Sure people would complain about the extra cost of building but what they don’t understand is it is for the better of the human race.
Earthquake design for buildings is at the very minimal at this time and ages we need to educate are young ones so that they can further develop technology that we are using. Maybe they could correct some of the flaws that we have had. By educating the future generation we will be getting more people interested in earthquake design. When kids find something interesting they tell there parents and then they parents become interested and so on.
While looking at earthquake design I discovered that many people do not know actually what to do during an earthquake. We should further educate society so that that instead of every one running around like chickens with their heads cut of there will be some type of game plan. This step would only help the common person during an earthquake. If we educated people then, that would greatly reduce deaths during earthquakes. I feel that there is no amount of money that could be traded for they lives of people so why not spend they money and save lives. The whole point of earthquake design is to save lives but all that does not help if every body runs outside the buildings that were not retrofitted or designed to withstand an earthquake falls on them.
If the next big earthquake hit maybe a 9.0 or higher I believe that are current earthquake design of buildings would be ineffective. When they do testing for earthquake design in the lab they reenact past earthquake. I think they should not reenact past earthquake but may simulate more sever earthquakes. By doing this we are preparing for the future.
After doing this report I have found that there are major flaws in our earthquake design system. People now do not care about the future they only care about present day and how many shortcuts they can take to save money on construction. I say let’s put more money into the system and demand better earthquake design laws.
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