GLG 101
Week #3 White Paper: Earthquakes


Living in Southern California and near a major earthquake fault I have wondered why the Earth shakes and why the Earth only shakes in certain parts of the United States. In searching for the information to complete my paper, I was also able to find the answers to my own personal questions. This paper will answer these questions: how does the Earth move and when, what events cause the Earth to shake, how are earthquakes measured and what are the two types of body waves and earthquake creates.

Causes of Earthquakes
An earthquake is caused when there is an unexpected and sudden release of energy within the Earth??™s crust, creating seismic waves. A break in geological faults beneath the Earth??™s surface can also cause earthquakes. However, earthquakes can also be due to volcanic activity, nuclear experiments and landslides. A naturally occurring earthquake will happen within the Earth??™s surface where there is enough stored energy and heat which will cause plates to move. So why does the Earth move when these plates move
The Earth moves because when the two plates or faults move they sometimes get stuck together as they shift. The built up energy that moves the plates becomes stored and when that energy is finally released it causes the Earth to shake.
Earthquakes are measured with a seismometer, which is also called a seismograph. A seismometer measures and records the motions beneath the Earth??™s surface which includes seismic waves cause by earthquakes. A seismometer allows seismologists to create a map of the Earth??™s interior. Another instrument used to measure earthquakes is a Richter magnitude scale. This instrument gives a number to the amount of seismic energy released during an earthquake. The Richter magnitude scale was developed in 1935 by Charles Richter.
The intensity of an earthquake is measured using a Mercalli scale. The Mercalli scale determines the effects that the earthquake has on buildings, the Earth??™s surface and humans. To measure the effects earthquakes have on man-made buildings such as skyscrapers, seismologists use a scale of I through XII, with I meaning the earthquake was not felt and XII meaning the earthquake caused massive destruction. To determine how an earthquake affected humans, information is collected from people who felt the earthquake and each individual gives an intensity value using this scale to rate the intensity of the earthquake based on their location when the earthquake occurred.
During the moving and shaking of an earthquake the Earth moves in different waves. These waves are referred to as body waves. They are called body waves because they move through the body of an object. The names given to these body waves are p-waves and s-waves. There are also two other waves known as surface waves called Rayleigh waves and love waves. P-waves, also known as primary waves, are faster than s-waves and can travel through gases, liquids and solids. As p-waves travel across the Earth??™s surface they compress and stretch surrounding rock. S-waves, also known as secondary waves, come after p-waves because they are slower than p-waves and as they travel across the Earth??™s surface they shift the surrounding rock moves up and down and from side to side. Rayleigh waves, which are also known as ground rolls, move like the waves in the ocean over the Earth??™s surface in an up and down motion. Rayleigh waves are responsible for most of the shaking during an earthquake. Lastly, we have love waves which move quickly and cause the ground to move in a side to side motion.
An area that is prone to earthquakes is California. The earthquakes that can occur here are due to the major fault known as the San Andreas Fault. The San Andreas Fault, which is 20 miles west of where I live, measures 800 miles and runs from Southern through Northern California. In 1895, UC Berkeley geology professor Andew Lawson identified this major fault. This fault makes up the tectonic boundary located between the Pacific and North American plates. The San Andreas Fault was responsible for the 1906 earthquake that caused havoc in San Francisco. In the town of Parkfield, located in central California, seismologists conduct research to gain a better understanding of this powerful fault. Parkfield, which sits directly on the San Andreas Fault, is a popular spot for seismologists to capture and even record large data on earthquakes.
With all the new technology that we have today, we are more informed about earthquakes and are better prepared in case the ???big one??? hits. For years, scientists and geologists have tried to uncover the secret to predicting when an earthquake will hit and where, but I feel mother nature has secrets she doesn??™t want uncovered.
Powell, R.E., and Weldon, R.J., 1992, Evolution of the San Andreas Fault: Annual Reviews of Earth and Planetary Science, v. 20, p. 431-468.
Stoffer, Philip W. (2006). Wheres the San Andreas fault A guidebook to tracing the fault on public lands in the San Francisco Bay region. USGS. General Interest Publication 16.