On October 17, 1989, as my family and I were preparing to watch game 3 of the World Series, a special news break announced that there had been a devastating earthquake that had struck the San Francisco bay area. News cameras quickly began
showing the devastation that this magnitude 6.9 earthquake had wrought. Buildings were on the verge of collapse and the two tier bay bridge had partially collapsed trapping hundreds of motorists. 63 people lost their lives due to the earthquake, 3,500 were injured and over 100,000 buildings were damaged (bbc.co.uk, 2005). Earthquakes are notorious throughout history as devastating phenomena, but what causes them?
In order to understand what makes the earth tremble so violently, you have to delve deep under the Earth’s surface. At one time, scientists thought that the earth’s crust or Lithosphere was continuous without any breaks or cracks but in the 1960’s, new research showed that the lithosphere was actually a fluid puzzle of irregular segments, or plates. These plates are made up of cool, solid rock that is four to forty miles thick. These enormous blocks of Earth’s crust vary in size and shape and cut through continents and oceans.
There are nine major plates. Six of them are named for the continents they are embedded in: the North American, South American, Eurasian, African, Indo-Australian, and the Antarctic. The other three are oceanic plates called the Pacific, Nazca and Cocos plates. These plates are in constant motion and it is along their boundaries that tremendous energy is released changing the earth’s surface. There are three types of plate boundaries: divergent, convergent, and transform. Although the plates appear to be moving independently of each other they are connected and a plate cannot move without affecting the other plates. The plates are either pulling apart or coming together. For example as the South American plate pulls away from the African plate, making the Atlantic Ocean larger in a divergent boundary, the pacific sea floor is being consumed in deep subduction trenches or a convergent boundary.
The “Ring of Fire” around the margins of the Pacific Ocean is caused by a convergent boundary or subduction zones found around the edge of the Pacific plate. These are plate margins where one plate is overriding another, thereby forcing the other plate in to the mantle beneath it. All the old oceanic crust is going into these systems as new crust is formed at the spreading centers. In this way the Earth recycles its self. And it is why crust older than the Cretaceous cannot be found in any ocean basin because it has already been destroyed by the process of subduction. Subduction zones are the location of very strong earthquakes, which occur because one plate is overriding the other.
The third type of boundary or margin is called a transform boundary because plate material is neither created nor destroyed, rather the plates slide past each other along faults. The San Andreas Fault in California was responsible for the World Series Earthquake, and it occurs as the North American plate and the Pacific plates move past each other. There are many earthquakes along this fault which are caused by the accumulation and the release of the strain as the two plates slide past each other.
Faults are surfaces along margins in which rocks have fractured and been displaced. The tectonic stresses caused by plate motions build up over time and eventually cause breaks in the crust of the Earth along which the rocks grind past one another. When this happens, an earthquake occurs. The stresses that are released along these faults are what cause the movement we feel when the Earth trembles during an earthquake.
Earthquakes are measured and detected by using seismographs. Inside a seismograph used to measure horizontal motion, a weight is freely suspended. As waves from the earthquakes reach the seismograph the weight stays in the same place, while the ground and support move around it. This movement is recorded on magnetic tape by a pen attached to the weight or mass. In a seismograph designed to measure vertical motion, the weight is connected to a spring, so as the ground and support move up and down, the pen on the weight measures the vertical motion. The metal tape which the motion is recorded on is marked with lines that correspond to one minute intervals. When motion is recorded, a seismogram is created, which tells about the waves, how big they were and how long they lasted. Using information from the seismogram, the epicenter and focus of the earthquake can be determined. The focus is the point on the fault at which the first movement occurred. The epicenter is the point directly above the focus.
Waves, or the motion that seismographs pickup are created when stress is released as energy in earthquakes. There are three types of waves. The P wave, or primary way, is the fastest of the three waves and the first detected by seismographs. P waves are able to move through both liquid and solid rock. P waves are compressional waves which mean that they compress and expand matter as they move through it. S waves or secondary waves are the waves that directly follow the P waves. As S waves move they cut the rock they travel through sideways at right angles to the direction of the motion. S waves are the most dangerous type of wave because they are larger than P waves and produce vertical and horizontal motion in the ground surface. Both S and P waves are called body-waves because they move within the Earth’s interior. The speed of these waves varies depending on the density and the elastic properties of the material they pass through, and they are amplified as they reach the surface. The third type of wave is called the surface wave. This type of wave moves close to or on the surface of the ground.
Earthquakes are measured in several ways. Intensity is the measure, in terms of degrees, of damage to the surface and the effects on humans. Intensity records observations of effects on the curst, not actual ground motion or wave amplitudes which can be recorded by instruments. The second type of measurement is called the Magnitude of the earthquake. Magnitude is measured by wave amplitude and distance. Magnitude is determined using mathematical formulae and information from seismograms.
The Richter scale is a magnitude scale that is logarithmic, which means each step in magnitude is greater than the last. To determine the Richter magnitude, a seismograph is used. Using a seismogram, the time difference between the recording of the P wave and the S wave is determined and matched to a corresponding distance value. The single maximum amplitude recorded on the seismogram is calculated and a line is drawn between the amplitude scale and the distance scale. The line crosses another scale, which corresponds to the magnitude.
A new type of measurement, called GPS, allows scientists to monitor the movement of the Earth’s crust all over the world, between and during earthquakes. From these measurements, maps and models can be created to show how fast and in what direction the crust is moving due to both plate and fault movement. Through using GPS technology, it will be possible in the future to reduce the risk of damage caused by earthquakes.
Although scientists have gained a lot of knowledge regarding earthquakes and the other forces that shape our planet, they are still unable to predict impending earthquakes the way they are able to predict other natural disasters such as hurricanes, or tornados. There is still much to learn. Only through research and the development of new technologies, will man finally be able to understand the destructive forces that are churning away beneath our feet.
References
bbc.co.uk. (2005, October 17). 1989:Earthquake hits San Francisco. Retrieved March 1, 2008, from On This Day: http://news.bbc.co.uk/onthisday/hi/dates/stories/october/17/newsid_2491000/2491211.stm.
Earthquake. (2008). In Encyclopedia Britannica. Retrieved March 1, 2008, from Encyclopedia Britannica Online: http://www.search.eb.com.ezproxy.apollolibrary.com/eb/article-9106195