1 Name: Block: Earthquake Lab The goal of this portion of the lab is to learn how recording of earthquakes seismograms are used to locate earthquakes, determine their magnitudes, and to understand the sense of fault motion related to the earthquake. Typical seismogram of a small earthquake recorded fairly near the source. The P-waves arrive first and are later swamped by the arrival of the S-waves. The S-P interval (referred to as lag time) is the time between the first arrivals of the P and S waves; it is used to determine the distance from the epicenter. This interaction produces additional seismic waves (phrases) which will be detected by seismographs. Refer to Chapter 10, pages in your textbook for an explanation on how seismographs work. A. Locate the Epicenter 1. Measure the S-P interval from the 3 seismograms shown on the following page, and enter the results in the table below. Then, use the S-P travel time curve (follows the seismogram) to determine the distance between the station and the epicenter and enter the date into the table. 2. Next, take a compass and set its radius to the epicenter distances for each station and draw circles or arcs around each station. Ideally, the three circles/arcs should intersect at a single point, which is the epicenter. If they do not, you should go back and check your work. Clearly mark the location of the epicenter. Station S-P(sec) Distance(km) Elko Eureka Las Vegas
2 Las Vegas, Nev Elko, Nev Eureka, CA
3 P-waves travel between 6 and 13 km/s. S- waves are slower and travel between 3.5 and 7.5 km/s. In most regions, study of numerous earthquakes with well-known epicenter locations results in an empirical S-P curve, such as the one shown to the right. B. Magnitude of the Earthquake The magnitude of an earthquake provides a convenient measure of its size scaled to a small number usually less than 9. It is a unitless number, derived from the amplitude of ground motion associated with S-waves, adjusted for the distance between the epicenter and the seismograph station (the amplitude generally drops off with increased distance from the epicenter). The table below helps to put earthquake magnitudes into perspective.
4 1. Measure the maximum amplitude of the S-waves from the 3 seismograms, and enter the results in the table below: add the distances from epicenter you obtained in the previous problem. 2. Then, use the graphical device to the right (called a nomogram) to determine the Richter magnitude of the earthquake as indicated by each of the three seismograms. Ideally, they should all converge on a single answer. If they do not, you should go back and check your work. Station Elko Distance from Epicenter (km) Max S-wave Amplitude (mm) Magnitude Eureka Las Vegas Analysis Questions: (Answer all questions in complete sentences) 1. Where is the epicenter for the earthquake? (Mention a direction and distance from the closest major city). 2. Is there a known fault in the vicinity of the epicenter that is likely to have slipped to create the earthquake? If so, name the fault. 3. Why did you need to draw three circles in order to locate the epicenter of this earthquake?
5 4. Which type of seismic wave reaches the seismic station first? Second? 5. How is the Lag Time between the arrival of the P and S waves used to calculate the location of the earthquake? 6. What is the relationship between the length of the Lag Time and the distance to the epicenter? Use the diagram below to answer questions 7 and 8 7. Which station is the closest to the epicenter and why?
6 8. List the seismograph stations from closest to furthest from the epicenter. 9. Determine the Richter magnitude of an earthquake that was located 500 kilometers from this seismic station.
7 Extension Activity The graph below shows travel time in minutes and distance traveled for primary and secondary waves. Primary and secondary waves start at the same time but do not travel at the same speed. Study the graph. Use the graph to help answer the questions that follow. 1. How long does it take for a primary wave to travel 2000 kilometers? 2. How long does it take for a secondary wave to travel 2000 kilometers? 3. How far does a secondary wave travel in 10 minutes? 4. How far does a primary wave travel in 10 minutes? 5. What happens to the time difference between primary and secondary waves as the distance traveled gets longer? 6. Suppose a primary and secondary wave both traveled a distance of 5000 kilometers before they are picked up by a seismograph. Which wave will arrive first? 7. How much time lag at 5000 km will there be between these two waves? 8. Suppose both a primary and secondary wave start together and travel for 5 minutes. Which wave will travel farther?