Strings and Things for Locating Earthquakes

Abstract

When one hears that an earthquake has occurred, one of the first questions is, “Where was it?” For the general public, this question often determines the importance of another question, “How big was it?” Upon learning the location and size, one may wonder how this information was determined. For those interested in how earthquakes are located, we have developed an interactive, three-dimensional analog computer that uses a map, strings, and a time-distance scale to find the location of an earthquake (latitude, longitude, and depth) based on seismic wave arrival times. We also have developed a set of lesson plans to present the ideas used for locating earthquakes to grade-school students, college classes, and the general public. The device is suitable for both permanent mounting in a science museum or can be transported easily to schools or other educational and outreach venues. Our prototype analog earthquake locator is housed in the Public Earthquake Resource Center (PERC) at the University of Memphis and also is taken to local classrooms. The most popular introductory method for showing how to locate earthquakes is the circle method, which is based on drawing circles around seismic stations on a map or a globe (figure 1). The circles' radii are based on the length of time between the arrival of the P and S waves, Ts – Tp , at each station. This time difference is converted to a distance by using a table or multiplying by a given velocity. How the velocity or the values in the table are determined is usually not discussed. The distance determined from the Ts – Tp time is used as the radius of a circle that can be drawn on the map or globe by a student using a compass. Laboratory exercises and presentations about earthquakes …