Geology - Earthquakes

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, rayleigh waves 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 liquidfied 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 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