Waves From An Earthquake example essay topic

POWER OF THE QUAKE Jason Conley Professor Tim Born Earth Science-ES 101020 February, 2002 Power of the quake When humans are under tremendous pressure, they are told that they should try and relieve some stress. The same is true of mother earth when she is under a terrible strain; she has to relieve some stress, only Earths version of blowing off some steam is very deadly and extremely terrifying. An earthquake is a vibration of the Earths surface that follows a release of energy in the Earths crust. There are various ways this energy is released. Manmade explosions, dislocation of segments of the crust or volcanic eruptions are all triggers for an Earthquake; however, most are caused by a slippage of faults.

A fault is a fracture in the Earths crust along which two blocks of the crust have slipped within one another. These faults are grouped into three categories (normal, thrust and strike-slip) and are located all over the Earth. In North America there is a very large fault that runs through California that separates the North American plate and the Pacific plate. The afore mentioned fault is called the San Andreas fault and is approximately 1300 kilometers long and a great source for Earthquakes and activity.

San Andreas is unusual because unlike most faults, which are beneath the Earths surface, the San Andreas is visible on the surface. One of the bigger Earthquakes ever recorded ran along this fault and will be discussed later in the paper. The length and severity of an Earthquake does vary. Much in the way lightning is a preemptive sign that thunder will follow, fore shocks are a preemptive sign that a major Earthquake will follow. Fore shocks are small Earthquakes that last anywhere from years to hours before a major quake.

When fore shocks have been monitored for making predictions it has had mixed results but if nothing else they do allow people to prepare for the potential disaster that may follow. Once that major quake does follow it can last seconds or minutes and the damage can be nil or severe. If severe then the aftershocks that follow can bring down structures that were badly damaged from the major quake. This whole process is repetitious, taking place along faults with powerful quakes once every two years and moderate quakes averaging 40 per year. Scientists' estimate more than 8,000 minor quakes take place each day but do not cause any damage. Fault lines are not limited to the land.

They do run under the water and the strained rocks do set off Earthquakes. Tsunami is seismic sea waves created by these underwater Earthquakes. Gigantic waves of up to 100 feet high that can travel of speeds in excess of 500 miles per hour the Tsunami is one of the fiercest forces of nature there is. The name Tsunami derived from Japan because that is where a lot occur but it was an Earthquake in the "Big Bend" territory that registered an 8.5 on the Richter scale that produced 70-foot waves and was the worst ever to strike South America. Forty percent of all damaging Tsunamis come from South America. Chile and Peru account for more Earthquakes per square mile than anywhere else in the world.

As we have seen so far, Earthquakes can cause massive damage and they are Earths most natural disaster. In 1906, along the San Andreas Fault, the city of San Francisco suffered from an Earthquake that registered an 8.2 on the Richter scale. The significance of this Earthquake was not just the magnitude of the quake but also the damage it caused as the city caught on fire. For 48 seconds the ground trembled as buildings collapsed and communities were destroyed. Thousands were left homeless as the town was virtually destroyed from crumbling buildings and spreading fires. The situation was so bad that martial law took place for the first time ever in that city and the damage caused a total in excess of $150,000,000.

The earthquake itself very rarely kills anyone. It's usually falling debris, fires, collapsed structures or chemical spills that cause the most casualties. Technology does play a role and knowing how to build and where to build can reduce loss of life. By studying areas of previous Earthquakes, fault zones, flood plains and areas that are subjected to landslides or land liquefaction, planners can develop zoning restrictions that can prevent unsafe buildings from being built in Earthquake prone areas. Once builders know where to build they can then plan how they will build.

Engineers have developed a number of different ways to build Earthquake resistant structures. Anywhere from simple to complex designs have been formed and they will vary throughout the various building sizes. Shear walls are support walls reinforced with concrete and steel rods embedded inside. These walls help strengthen structures and help resist rocking forces. Devises called base isolators are made of alternative layers of steel and elastic material, such as synthetic rubber. They absorb some of the sideways motion that can damage a building.

Skyscrapers need special construction to make them Earthquake resistant. They must be anchored deeply and securely into the ground. They need a reinforced framework with stronger joints than an ordinary skyscraper has. This framework makes the skyscraper strong enough and flexible enough to withstand an Earthquake.

Businesses and homes can bolt all heavy appliances, furniture and other equipment down to the floor. Gas and water lines should have flexible lines to prevent breaking thus preventing explosions and reducing fires. Predicting Earthquakes is still a work in progress. There is a lot of pessimism concerning the ability to accurately predict Earthquakes because of the inability of low frequency surface mounted seismic instrumentation to detect the higher frequencies associated with small fractures that occur prior to a large movement of a fault. The higher frequencies associated with these small events are attenuated by the upper mantle and never make it to the surface.

High frequency sensors in deep boreholes can detect small seismic events that precede larger movements of a fault line and it has also been shown that the signal to noise ratio obtainable in a deep borehole is excellent compared to that obtained from surface instruments. The using of these boreholes and the study of uprooting fault lines are helping scientists to develop better technology that is dependable and accurate. Earthquakes are measured by using instruments called seismographs. A weight is freely suspended from a support that is attached to bedrock. When waves from an Earthquake reach the instrument, the inertia of the weight keeps it stationary, while Earth and the support vibrate. The movement is recorded on a rotating drum.

Ground motions are recorded and are called seismogram's and they reveal that seismic waves are elastic energy. Seismogram reveal that two main types of seismic waves are generated by the slippage of rock mass. Surface waves travel on Earths outer layer and body waves travel on the Earths interior. Body waves are further divided into primary and secondary waves. Primary waves will push and pull rocks in the direction the wave is traveling. They can move through liquid, gases and solids unlike secondary waves, which shake particles at right angles in the direction they travel.

This allows the secondary waves to change the shape of the material moving them. This later wave is especially damaging to the foundations of structures. The study and monitoring of these seismic waves allow scientists to determine the location and magnitude of Earthquakes. When people hear that there was an earthquake the first thing that is asked is "how big was it". By asking how big they are referring to the magnitude provided by the Richter scale. In 1935, Charles Richter introduced the concept to describe Earthquake magnitude hence the scale being named Richter scale.

Richter magnitude is determined by measuring the amplitude of the largest wave recorded by the seismogram. The largest Earthquake ever recorded had a magnitude of 8.9. This is equivalent to a billion tons of TNT being detonated. The sheer force of an eruption of this magnitude is mind-boggling. There is a wide variation of intensity on these measurements.

A 2.0 is usually not even felt by humans. Because of these wide variations the logarithmic scale is used to express magnitude, each magnitude means an increase ten fold. The Richter scale is very reliable and very accurate and there are stations all around the world taking measurements. Earthquakes are impossible to stop.

Unfortunately, mankind is at the mercy of Mother Nature's wrath. The intense releasing of energy will be a repetitious cycle resulting in many more violent shakes to come. The more data that is accumulated through the study of fault lines, prior Earthquake regions and the instruments that exist as well as the ones to come with technology, will give scientists a better understanding of Earthquakes, thus enabling more accurate predictions that will save lives. Structures are now being built away from quake zones with material and designs that can withstand the shaking, probably saving thousands of lives.

Humans' ability to cope with natural disaster such as an Earthquake will be an ongoing struggle. The best hope relies on better predictions, better structural design and ultimately no fatalities.

Bibliography

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How Earthquakes Happen (Aerial view of the San Andreas fault in the Carrizo Plain, Central California). Education and Outreach web (1906, April 19).