Chapter 15 laboratory locating an earthquake epicenter answers

EXERCISE 11,4 Locating an Earthquakes Epicenter and Determining When It Occurred (continued) Name Course Section: Date: Arrival times and delays between seismic waves Boston Los Angeles Seattle Selmic wave arrival times Delays between seismic waves S-P Step 3: Estimating Distance from the Epicenter to Each Station Using the Travel-Time Diagram The next figure in this exercise shows how to use seismic wave delay data and the travel time diagram to measure the distance from each station to the epicenter. Start with one of the stations and then repeat the procedure for the others. Draw a horizontal arrow to represent the P-wave arrival anywhere near the bottom on a station worksheet (Appendix 11.1) or create your own scale using a separate sheet of paper. In this case, make sure to copy the scales in the worksheets exactly. Then indicate the arrival times of the S-, L-, and R-waves using the time delays you recorded in the table (continued)
Seismograms for Exercise 11.4 (continued). Seattle, Washington Boston, Massachusetts 5-30.00 wwwww Los Angeles, California
EXERCISE 11.4 Locating an Earthquakes Epicenter and Determining When It Occurred (continued) Name Course: Section Date: magram 14100 2.1800 Recogniring seismic wave arrivals and measuring time na 10500 106000700 14.00 Sudden chain amplitude and Frequency from previous vibration Indicates arrival of a new seismic wave type Using the figure above, determine the arrival times of each of the four different types of waves. Place a P next to the place where you believe the P-wave arrives, an S where the S-wave arrives, an L where the L-wave arrives, and estimate and place an R next to where the R-wave appears to arrive. Then determine the correct arrival time for each wave. Use the inset circles to determine precise times as necessary, Arrival of P-wave_1 21.00 Arrival of S-wave L: 24:00 Arrival of L-wave 12000 Estimated arrival of R-wave L:42:00 Looking at the amplitudes of the waves recorded on the seismogram, which wave types do you think might cause the most damage in an earthquake? Explain. 1 L WAS WALL cause the most damage the crise and dip the groundmore. Step 2: Measuring the Delay between Arrival of Different Waves Geologists use all four wave types to locate an earthquake, but to save time, your instructor may suggest using only the P and 5 wave data, so modify the following instructions as appropriate. The figure on the next page shows seismograms from three stations. Identify each of the wave types and record their arrival times in the table provided on page 286. Use the seismic reading overlay tool found in the back of the manual to get precise arrival times for each wave. (Place the overlay over each seismogram, aligning the vertical dashed lines, to see the 15-second intervals.) Then calculate the times between arrivals by subtracting the p wave arrival time from that of the S-wave, the S-wave from that of the L-wave, and the L-wave from that of the R-wave. (continued) 284 CHAPTER 11 EARTHQUAKES AND SEISMOLOGY
EXERCISE 11.4 ter and Determining When It Occurred PIREX mo 00 Locating an Earthquake's Epicenter and Determining When Jonathan Dolben Section: Date: ind let tackle an earthquake use the following seles stens to get all the informa identify the location of the earthquake and pinpoint when it occurred: Name: Course Park the cars, and let's tackle an earthquake Use of steps to get all the information you need to 1. Identify the four different seismic wave types on seismograms from three seismice m three seismic recording stations 2. Determine the arrival times and measure the delays between different wave types S. Use these data and the travel time diagram to estimate each station's distance from the epicenter 4. Use triangulation to locate the epicenter 5. Determine the time of faulting with the travel time diagram Step 1: Reading a Seismogram You will need to know what P-S-, L and R-waves look like on a seismogram in order to identify them correctly Each seismic wave causes the ground to shake differently (see Fig 112) and this produces a different appearance on seismogram. The differences are in wave amplitude Chripht) and frequency (the time between adjacent wave peaks These differences are summarized in the figure below and will help you interpret the seismograms in the next part of exercise This close-up of a seismogram shows the signals generated by different kinds of seismic waves. 2nd seismic 3rd seismic Normal background seismicity IP-wave Lowest amplitude Peaks 2-10 seconds apart S-wave ! Amplitude P-wave Reaks 2-10 seconds apart Amplitude > S-wave Peaks > 40 seconds apart R-wave Highest amplitude Peaks -20 seconds apart Now that you know what different types of waves look like on a seismogram, let's see how to measure their arrival times. The figure on the next page shows how waves look on a typical seismogram printout. The dashed vertical lines are time markers and are 1 minute apart. The waves reflect time moving forward from left to right on each row, and as a row ends, the time starts again on the left side of the next row below it (just like lines in a book). Seismic waves rarely arrive precisely on a minute marker, so you have to estimate the number of seconds before or after each minute [Note: To avoid confusion involving time zones and changes to and from Daylight Saving Time, seismogram times are recorded in Greenwich Mean Time (GMT)]. (continued) 11.3 LOCATING EARTHQUAKES 283