Surface Wave Propagation Around the Earth: A Problem Solving Exercise

In this activity, students examine a seismogram display called a seismic record section in which each trace is a seismogram recorded at a specific seismograph station. The seismograms are plotted according to the distance (in degrees, geocentric angle) from the earthquake location and time from the earthquake origin. The traces are of the vertical component of ground motion, and have been filtered to include only periods longer than 125 seconds. Students look for the prominent arrivals, called phases, that angle across the record section and are labeled and discover that they are called long-period Rayleigh waves. Students learn that these waves travel along the surface of the Earth and that surface waves penetrate (have particle motion) to depths of tens to hundreds of km but travel approximately parallel to the Earth's surface. Since the surface waves propagate in all directions from the source, the arrival times are approximately the same as if the stations were all located along a great circle path from the epicenter. From this, students are able to measure the distance traveled and calculate the velocity in kilometers per second and how long it takes for the wave to go around the Earth. Educational levels: High school