Homework 6 Earthquake Location EPSc 1021, Spring 2007 Due: Monday, March 26 Name: ________________________ Because P waves travel faster than S waves through rock, the difference in their arrival times can be used to estimate the distance to an earthquake’s epicenter. However, with only one seismograph, the location of the earthquake cannot be determined (in other words, a seismograph can’t determine which direction the seismic waves came from). Multiple seismographs are therefore used to triangulate earthquake locations. In this exercise, P-S travel time differences are used to locate an earthquake in the New Madrid Seismic Zone. This 5.0 magnitude earthquake occurred in June 2002 and could be felt in St. Louis. Use the following figures to find the earthquake’s epicenter. We assume a P wave velocity of 6.5 km per second and an S wave velocity of 3.5 km per second through the crust. Figure 1. Seismograms at St. Louis, Nashville, and Memphis. The P and S wave arrivals are denoted by vertical red lines. The PS interval is the time between P and S arrivals. 60 55 ave s 50 Sw 45 time (s) av e s 40 w al rv P 35 PS 30 e int 25 20 15 10 5 0 0 50 100 150 200 250 distance (km) 300 350 400 Figure 2. Travel-time plot for P and S waves. The line marked “PS interval” is the time difference between P and S wave arrivals. This interval gives the distance from the epicenter. Figure 3. Map of the New Madrid Seismic Region. Each segment of the scale bar represents 100 km. Use the seismogram and travel-time plots on the previous page to triangulate the location of the June 2002 earthquake. Indicate the epicenter on the map. Draw a line indicating the position of a plate boundary, and arrows indicating plate motions, had the mid-continent rift fully developed. If a powerful earthquake occurred in the seismic zone above, how/why would the Mercalli intensity rating for that earthquake be different in St. Louis than, say, Memphis?