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Shake, Rattle, and Roll 

One of the most active seismic areas on the planet Earth is California. Located on two tectonic plates, the Pacific and North American Plates, tremors in this region are constantly being recorded at seismic stations around the world.

To give you an idea of how active this area is, read the eyewitness accounts of earthquakes in the Santa Barbara area of Southern California. Written accounts have been recorded as far back as 1812. Once you have read the eye witness accounts of almost 200 years of quakes, you will learn how to determine the epicenter of one.

The above image (USGS) is a copy of the seismogram recorded in Gottingen, Germany. Click on the seismogram to learn more.

As you have read, earthquakes are serious business, and California is struck with many of them througout the year. Now that you have read the personal accounts from people who have experienced them on a personal level, you will learn how to read a simulated earthquake seismogram, calculate an earthquake's epicenter, and determine its magnitude.

Although an actual seismogram will contain several types of waves, the ones you will be working with are the P (Primary) waves, which are very similar to sound waves, and S (Secondary) waves, which are a type of shear wave. P waves can travel through both liquids and solids, whereas S waves travel only through solids. P waves travel between 6 and 13 km/sec and S waves travel between 3.5 and 7.5 km/sec. As these waves travel through the earth, they are reflected by its various layers and boundaries. It is these waves and their interactions which produce the seismic waves which are detected by seismograms around the world.

The only way to determine the epicenter (location) of an earthquake is to analyze the seismograms which have been recorded at three different seismic stations. Three stations are required in order to triangulate the epicenter, since the direction from which the waves arrive at a station is unknown. For each station the following tasks will need to be accomplished:

• Determine which wave signature is the P - Wave and its arrival time at the station.
• Determine which wave signature is the S - Wave and its arrival time at the station.
• Calculate the difference in the arrival times between the P - Wave and S - Wave.
• Translate the time difference into distance of the earthquake's epicenter from that particular seismic station.
• Using a drawing compass and the distance legend on the seismic map of the Western United States, scale the compass to the correct distance of that particular station and draw a circle around the station. It is important that the distance represents the radius from the station; the station is the center point.

The above procedure is very time consuming when accomplishing the task manually. However the University of California at Los Angeles has designed a viirtual earthquake lab to practice these skill. Click on the Virtual Earthquake Lab link. When you are successful in locating the epicenter of an earthquake, you will be rewarded with a certificate.