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Roger Weller, geology instructor

by Ian Leavelle
Physical Geology
Spring 2010



            Earthquakes are one of the most feared natural disasters. One of the reasons is because they cause a lot of damage, depending on the size, and can’t be stopped.  These natural disasters occur when there are ruptures in geological faults, and the earth’s crust is moving, creating seismic waves. Earthquakes are recorded with a seismograph, which tell how big or small the earthquake is. The earthquakes place of initial rupture is called the “hypocenter”, and the term “epicenter”  refers to the spot at ground level where the earthquake occurred.

Above: Focus and epicenter, which shows how vibration waves move through the ground.

Fault Types

            Depending on the area, or region in the world, earthquakes may be more or less destructive. All of this depends on the type of fault where the earthquake is created. There are three type of faults, normal, reverse, and strike slip. The San Andreas Fault is known for having all three of these types of faults.The normal and reverse faults are simply a vertical or up or down in movement. Strike-slip faults are where two sides of the fault move past each other horizontally. Some of the movements by these fault are also enough pressure to create volcanic eruptions.

Above: Fault types, horizontal movement, shock waves shown below.


Size and Frequencies:

            There are about 500,000 earthquakes that occur every year. However, only about 100,000 of these can be felt. The size and frequency of earthquakes is measured using a seismograph. This tool uses vibrations in order to determine how violent the earthquake is. Places such as California, and Alaska are known hotspots for earthquakes in the U.S. However, there are also many known places around the world in which they occur. Guam, Peru, New Zealand and Japan are to name a few. According to Gutenberg-Richter’s law , certain earthquakes only occur after a certain number of years. Earthquakes of 3.7-4.6 on the Richter scale, occur just about every year. Some earthquakes of 4.7-5.5 every ten years, and some of 5.6 or larger every century. In fact, most of the worlds earthquakes happen in the Pacific seismic belt, also known as the “Ring of Fire”.

Above: Seismograph


Earthquakes and other Natural Disasters:

            Earthquakes may seem like they are only responsible for moving the ground, but they are also responsible for causing other known disasters. Earthquakes can start fires, because when an earthquake occurs, it can rupture underground gas lines, causing a fire. The San Fransisco fire in 1906, is an example of how earhtquakes can also cause other naturl disasters. Earthquakes can also create landslides which are also very dangerous. Floods are another result of earthquakes and plate movement. As the plates move, above or below sea-level, it causes the water to rise or fall, which can cause flooding. An earthquake of 7.4 on the Richter scale caused a tsunami in the Indian Ocean and causing mass destruction in India in 2004.

Above: San Francisco fire-1906


            There is no way to be able to stop an earthquake. However there are some ways that scientists are using to figure out the likelihood of seismic activity, in order to save lives, and prevent more damage during earthquakes. Scientist are paying closer attention to the most active faults, and determining when the fault needs to move, break, ect. This way they will be prepared to help out others in the present and future, of the science of earthquakes.

Above: Earthquake preparation steps.



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