Plate Tectonics and Earthquakes

Like floating pieces of a 3D jigsaw puzzle, the Earth's plates that form the crust of the Earth are not locked in place. On the contrary, the plates are always in motion and it is not always a smooth ride. We may not see the plates as they move against each other, but we can sometimes feel the result in the form of earthquakes. This paper will discuss the three types of plate boundaries, their motion, and earthquakes.

The lithosphere or outermost shell of the Earth is approximately 100 kilometers think. It is composed of the crust and the uppermost part of the mantle. There are seven main plates and numerous smaller plates that make up the lithosphere. Where these plates come together, at plate margins or boundaries, is where the bulk of the Earth's volcanic and seismic activity occurs.

The Earth's plates come together in one of three ways. The first type of boundary is a divergent plate boundary. "Divergent" means that the two plates are separating or spreading as new material coming up from the mantle is generated. This is evident along mid-ocean ridges such as in the Atlantic Ocean. As the new material comes up, it becomes part of the rigid plates (Fowler, 1990).

The surface area of the Earth does not change so that means as new crust is formed at the divergent plate boundaries, plates are being destroyed somewhere else. This process takes place at convergent place boundaries. As two plates come together and converge, one of the plates is forced under the other and is destroyed. Some convergent boundaries occur along oceanic trenches while others can occur on land. The Japanese islands and the Himalaya Mountains are examples of products of convergent plate boundaries (Fowler, 1990).

The third type of plate boundary is the conservative or transform fault boundary. They are called conservative because plates are neither added to nor destroyed at their margins. The plates slide past each other laterally (Fowler, 1990). One prime example of a transform fault boundary is that of the San Andreas Fault in California. The myth that someday California will fall into the Pacific Ocean is impossible because the Pacific plate is sliding north relative to the North American Plate. The only change we could see is that the land to the west of the fault will keep creeping to the north.

Earthquakes generally occur at the plate margins and boundaries because of the differential movement along the plates. The movement of the plates against each other is not smooth. Stress builds up along the boundary resulting in strain or deformation of the plates. Finally the strain builds to a point that cannot be continued and the rocks slip past each other, the stored strain energy is released, resulting in the propagation of seismic waves from the point of the slip (Bolt, 2003).

When the Earth begins to quake, there are several things happening at once. Seismic waves begin propagating out from the center or focal point of the earthquake. Some of the waves only travel along or near the surface of the Earth while others travel through the body of the Earth. The waves slow down or speed up depending on the material that they are passing through. Some waves are reflected and refracted and others will not pass through liquid (ie, molten) material such as the outer core of the Earth.

Two types of the waves that pass through the body of the Earth are called P-waves and S-waves. The P-waves are primary or pressure or push-pull waves. The best way to envision how this pressure wave travels through the Earth is to visualize a Slinky stretched out on a table. Taking one end of the Slinky, push it toward the other end. The compressions through the coils travel the length of the toys until they reach the end and then they are reflected back again. These waves are the first of the body waves to be detected by seismometers following earthquakes.

The second type of body wave is the S-wave that stands for secondary, shear, or shake. Taking one end of the imaginary Slinky again, whip it from side to side or up and down. This represents the motion of the S-wave. The motion of the wave is perpendicular (or 90 degrees) from the direction that the wave is traveling.

Earthquakes are measured by the amplitude of the seismic waves generated by the quake. C.F. Richter developed a magnitude scale where the jump from one whole number to the next represented a 10x increase in the strength of the quake. Prior to the development of the Richter scale, earthquakes were given a rating based on how intensely it was felt and how much damage was done. This method meant that an earthquake would have a different rating depending on how far away a location was from the epicenter, whereas the Richter scale gives a measurement of intensity at the source of the quake.

The study of earthquakes is still evolving as geologists continue to make new discoveries about the science of how the earth moves. In the future, it is hoped that this knowledge will help in the predictions of deadly quakes.

References

Bolt, B.A. 2003. Earthquakes, Fifth Edition. W.H. Freeman.

Fowler, C.M.R. 1990. The Solid Earth, An Introduction to Global Geophysics.

Cambridge University Press.

Louie, J. October 9, 1996. Earthquake Effects in Nevada Seismology Laboratory.

Retrieved October 5, 2008, from http://www.seismo.unr.edu/ftp/pub/louie/class/100/effects-kobe.html