Earthquakes

I chose to do my assignment on earthquakes because I have always wondered about what causes something that can be so destructive to man and nature alike.

One of the papers I chose is titled 'Comparison of Short-Term and Time-Independent Earthquake Forecast Models for Southern California' by Agnes Helmstetter, Yan Kagan, and David Jackson. They discuss how they initially developed a time-independent forecast for the southern California area. They can reasonably predict magnitude 5 or greater quakes by studying small magnitude 2 or less earthquakes. Many large earthquakes are preceded by and probably triggered by smaller ones. They use these models to issue daily earthquake forecasts for southern California.

The other paper I read is titled 'Anomalous Seismic Amplitudes Measured in the Los Angeles Basin Interpreted as a Basin-Edge Diffraction Catastrophe' by Allen Husker, Monica Kohler, and Paul Davis. In this paper they examine data from the Los Angeles Basin Passive Seismic Experiment (LABPSE), which is an array of eighteen seismic stations along a line crossing the Los Angeles basin from the foothills of the San Gabriel Mountains through the Puente Hills to the coast. They have a much higher resolution than the permanent network of stations in southern California because they are spaced 3-5 km apart. They found that certain areas in the basin had a good chance of a diffraction catastrophe, which could amplify ground shaking by a factor up to five.

Now I will move on to explain earthquakes in much greater detail. Earthquakes have been recorded as early as 526 A.C., but seismology is comparatively new. Until the 18th century, few accurate descriptions of earthquakes were recorded, and little was known about what caused them. When seismology was introduced it was learned that many earthquakes are the result of sea floor spreading, but most are caused by volcanic eruptions and plate tectonics. The plate tectonic theory explains that the earth is made up of 20 different plates that are always moving slowly past each other. This action pulls and compacts the plates, creating intense forces that cause the plates to break. This, in turn, causes earthquakes. The energy released then travels along fault lines in seismic waves.

An earthquake can be likened to the effect observed when a stone is thrown into water. After the stone hits the water a series of concentric waves will move outwards from the center. The same events occur in an earthquake. There is a sudden movement within the crust or mantle, and concentric shock waves move out from that point. The origin of the earthquake is the focus. Since this is often deep below the surface and difficult to map, the location of the earthquake is often referred to as the point on the Earth surface directly above the focus. This point is called the epicenter.

There are three types of waves that are created when stress is released as energy in earthquakes. Primary waves, also known as P-Waves, are identical in character to sound waves. They are high frequency, short-wavelength, longitudinal waves which can pass through both solids and liquids. The ground is forced to move forwards and backwards as it is compressed and decompressed. This produces relatively small displacements of the ground. Secondary Waves (S-Waves) travel more slowly than P-Waves and arrive at any given point after the P-Waves. Like P-Waves they are high frequency, short-wavelength waves, but instead of being longitudinal they are transverse. They move in all directions away from their source, at speeds, which depend upon the density of the rocks through which they are moving. They cannot move through liquids. On the surface of the Earth, S-Waves are responsible for the sideways displacement of walls and fences, leaving them 'S' shaped. S-waves move particles at 90o to the wave's direction. Surface Waves (L-Waves) are low frequency transverse vibrations with a long wavelength. They are created close to the epicenter and can only travel through the outer part of the crust. They are responsible for the majority of the building damage caused by earthquakes. This is because L-Waves have a motion similar to that of waves in the sea. The ground is made to move in a circular motion, causing it to rise and fall as visible waves move across the ground. Together with secondary effects such as landslides, fires and tsunami these waves account for the loss of approximately 10,000 lives and over $100 million per year. L-waves move particles in a circular path.

The four major layers of the earth are the crust, mantle, outer core and inner core. The earth's core was formed very early in earth's evolution as heavier molten iron sand toward the center of the plane. The deepest part of the earth, 1512 miles deep is a solid that contains both iron and nickel. It is because of this that the center of the earth is a magnet, like a compass. It generates a magnetic field that protects the earth from flying out of orbit. The inner core is very hot. Intense pressure from the weight of the rest of the earth prevents the materials of the inner core from becoming a liquid. This sets up the electric current that generates the earth's magnetic field. The outer core is liquefied iron and nickel. The molten outer core flows at a very slow rate of several kilometers per year.

Above the core is the mantle; it is a rocky layer that is about 3,000 kilometers thick. It is made of hot, iron-rich silicate rocks. The mantle is not as rigid as a solid but not able to flow as a true fluid. There is two parts to the mantle, the first is the lower mantle, it extends from earth's outer core most of the way to earth's surface. The upper mantle extends from the crust-mantle boundary down to about 350 kilometers. The upper mantle is also divided. The lower