Geology – Earthquakes I chose to do my assignment on earthquakes, because I have been through many growing up in California, and I have always wondered about them. There are many types of earthquakes. The most common are tectonic, which my paper will be about. There are also volcanic, collapse and explosion earthquakes. All earthquakes create waves that travel through the Earth's interior. An earthquake is the sudden shifting of the earth's natural means of releasing stress. When the shifting occurs, it creates vibrations, called seismic waves. The speed of seismic waves depends on the elasticity of the materials through which they are traveling. Rocks have elastic properties. When forces are applied to rocks, such as pulling, pushing, twisting, or compression, they change their shape. Rocks like other materials with elastic properties have an elastic limit,any additional force will deform the object's shape. The elasticity of a material is controlled by its density and stiffness. There are three types of waves that are created when stress is released as energy in earthquakes. Primary waves or P-waves, is the fastest, and the first detected by seismographs. They are able to move through both liquid and solid rock. P-waves are like sound waves; they are compressed and expand matter as they move through it. Secondary waves or S-waves, are the waves directly following the P-waves, they are transverse waves. As they move, S-waves cut the rock they travel through sideways at right angles to the direction of motion. S-waves can't travel through liquid, and they are the most dangerous type of waves, because they are larger than P-waves, and produce vertical and horizontal motion in the ground surface. Another name for P and S waves is body waves. There are also two types of surface waves, rayleighwaves and love waves. Rayleigh waves move both horizontally and vertically in a vertical plane pointed in the direction of travel. Love waves; move like S-waves but only horizontally. They both move slower than P and S waves. The way these waves travel has helped scientists to see what's going on in the earth's interior and layers. 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 liquidfiediron 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 part of the upper mantle is called the asthenoshpere, which is the layer of the earth, found 100-500 kilometers below the earth's surface that yields to persistent stresses. It behaves like a plastic. The asthensosphere flows gradually because convection currents carry warmer material upward as colder, denser material sinks. Above the asthenosphere is the lithosphere, which is the solid, rocky outer part of the earth. It's about 50 miles thick, comprised of the crust and the solid portion of the mantle. The lithosphere is more rigid, brittle and doesn't flow. The motions of the material beneath it in the asthenosphere carry it along. The top of the lithosphere is the crust, which is earth's outermost surface. The crest is typically about 25 miles thick beneath continents, and about 6.5 miles thick beneath oceans. It's made of dense, basaltic rocks. Continental masses are mostly composed of granite rocks. So, when the earth is releasing stress, the plates move against each other, stress is put on the lithosphere. When this stress is great enough, the lithosphere breaks or shifts. When the break occurs, the stress is released as energy, which moves through the earth in form waves, which we feel. A fracture or zone of fractures in rock along which the two sides have been displaced relative to each other is called a fault. Earthquakes can happen anywhere on earth, but they usually happen along these faults. One well-known fault is the San Andreas Fault in California, (which was about 5 miles from where I grew up). A lot of major earthquakes occur here. This particular fault is also known as a normal fault. Stress in rocks can occur by tension. Tension causes rocks in a hanging wall to drop downward along the fault plane relative to those in the adjacent footwall. The most common scale used to measure earthquakes is the Richter scale, devised in 1935 by Dr. Charles Richter. The Richter scale is calculated by using a standard seismometer to measure the maximum amplitude of shaking. It factors in the distance of seismometer from the epicenter, and then uses mathematical relationships to convert the number to a standard scale. There are common myths about earthquakes for example, changes in animal behavior before earthquakes have been observed and documented in different parts of the world. Dogs, cats, snakes, and horses have been known to behave strangely before quakes, but that is still being studied. Some people sense that an earthquake is about to happen. There is no scientific explanation for the symptoms that some people have before an earthquake hits. California will not fall into the ocean, but Los Angeles and San Francisco could be adjacent one day. The San Andreas Fault plates are moving horizontally past one another. The last one, there is no such thing as earthquake weather. Aristotle proposed that earthquakes were caused by winds trapped in subterranean caves. This was just a theory. There is no connection with the wind, but from my personal experience, the air does seem still with no wind, and no sounds of nature, before an earthquake occurs. It's kind of eerie. I have been in many earthquakes and tremors growing up in California. I've been in the 1971 San Fernando quake, which cracked our living room ceiling; of course I was only 2 years old. I remember a quake or tremor in 1987; it felt like my bedroom on the second story, swung out to the street. I've been in the 1989 San Francisco quake, only about 45 minutes away, it knocked out our power and phone lines. Earthquakes are scary, but after awhile you get use to them. I haven't been in one since 1989, so I'm sure I would still freak out, if one occurred.