EARTHQUAKES Vicki M. Harder

Introduction An earthquake is the sudden movement of portions of the Earth's crust along a fault. Tectonic forces within the Earth produce a build-up of stress that is released abruptly. The device used to record the energy released from an earthquake is a seismograph and the recording of the earthquake a seismogram.

Seismic Waves Two types of waves are generated during an earthquake, body waves and surface waves. Body waves travel through the Earth, emanating from the earthquake focus, or subsurface source. Body waves are useful in determining the location, or epicenter of an earthquake and for determining the amount of energy released, or magnitude. Body waves can be further subdivided into p-waves and s-waves. P-waves are compressional waves and have the greatest velocity of all seismic waves--6 km/s being a typical value--and are the first waves to be recorded by a seismograph (primary waves) after an earthquake.

S- or shear waves consist of an alternating series of sidewise movements perpendicular to the direction of wave travel. A typical velocity of an s-wave is 3.5 km/s--and therefore are recorded at a later time than the p-wave. As the distance from an earthquake increases, the arrival times between the p-wave and the s-waves also increases. This feature allows the earthquake location (epicenter) to be determined when the travel-time curve is used. Surface waves, as their name implies, travel along or near the Earth's surface. Because they travel along the surface, surface waves are studied for their effect on the population such as structure collapse, fire, and landslides.

Measuring Earthquakes Earthquakes can be measured in several ways, either by the amount of energy released (Richter magnitude scale) or by the intensity of the vibrations (Mercalli intensity scale). The Richter scale, which provides an estimate of the energy released by an earthquake, is calculated from the maximum amplitude of the body waves on a seismogram and is corrected for distance. No matter the distance from the earthquake, the Richter scale will be the same at all recording stations. The Mercalli scale is based on the amount of vibration people feel during an earthquake and the amount of damage to structures. There is no correction for distance from the epicenter so a single earthquake will have a number of Mercalli intensities. Mercalli intensities are particularly useful in city planning and the development of building codes.

World Distribution of Earthquakes Although no one place is entirely safe from the possibility of an earthquake, most occur within well-defined seismic belts. These belts are in turn related to plate tectonic boundaries. The circum-Pacific belt is responsible for approximately 80% of all recorded earthquakes. This belt follows the active margins of the Pacific plate. Lesser seismic belts follow the mid-oceanic ridges.

Southern Arizona Seismological Observatory

The SASO site allows you to create your own seismogram. Follow the instructions and create a seismogram. To see what a seismic record should look like check in your textbook or have your instructor show you one. You will most likely have to make several tries to get the record to look like an actual seismogram. Once created, save it and print it out. (To get as much of your seismogram onto a sheet of paper, be sure to set your printer to legal paper size and oriented in landscape.) On your print-out, label the first arrival of the p-wave, the s-wave and the surface waves. Determine, as closely as possible, the difference in arrival time between the s-wave and p-wave. Using the travel-time curve determine the distance from the seismic station to the earthquake epicenter. View the "Map of Recent Seismic Activity" within the SASO site. Determine the total number of earthquakes that have occurred recently worldwide. What percentage of the earthquakes occurred within the circum-Pacific belt?

Active Tectonics Web Site

Under "Images" you can view photographs of the 1994 Northridge, California earthquake damage. Using the Mercalli intensity scale, assign a Mercalli number to each photograph. Which picture was taken closest to the epicenter? Which picture was taken farthest from the epicenter?

International Data Center Home Page Science News (vol. 149, pg. 298, May 11, 1996) Policing the Peace: How nations will monitor a nuclear test ban

An international seismic monitoring system designed to check on violations of the Comprehensive Test Ban Treaty is now operational. The completed seismic network will detect quakes and blasts of magnitude 4.0 or greater with a high degree of confidence. Determine how many earthquakes this represents on a per year basis by going to Recent Seismic Events by clicking the link at the left. From reading the article, illustrate the difference between a seismogram generated by an earthquake versus one generated by a nuclear test. (You may also want to access the information from the International Data Center's home page for more information.)