Lab3_Assignment (1)

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Dec 6, 2023

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LAB 3: SEISMIC HAZARDS 100 points Due at the beginning of lab next week in physical or digital forms. LEARNING OUTCOMES At the end of this lab, you will be able to 1. Understand hazards associated with earthquakes 2. Identify seismic wave signals in a seismograph 3. Use a seismogram to identify an earthquake epicenter 4. Explain how uncertainty builds over multiple calculations INTRODUCTION Earthquakes are the spontaneous release of energy within the lithosphere. Stress applied by tectonic forces becomes potential energy stored in the ground. Most earthquakes are released along faults within the first 25 miles of the Earth’s surface. Sudden movements along faults cause shock waves to radiate away from the focus of movement. Earthquakes are measured with several scales (e.g., Moment magnitude, Richter)—both exponential scales used to quantify the magnitude of an earthquake. Seismic waves transfer energy from one place to another through rock and sediment. P- waves are the first to arrive at a recorder, called a seismometer. S-waves are the second. P- waves travel through solids and liquids, while S-waves only travel through solids. P- and S-waved are used to determine the magnitude of an earthquake. You can use a Richter Nomogram to draw a line connecting the values of distance and magnitude, which require the identification of when the waves reach the seismometer, recorded on a seismogram (Figure 1). The line will cross the magnitude plot showing the value of the earthquake. A seismogram records earthquake intensity. Because P-waves and S-waves travel at different speeds, the lag time (i.e., S-P interval) can be used to calculate how far away the earthquake happened if we know the wave velocities. Figure 1 is a simplified seismogram showing P-wave and S-wave traces. Seismologists use three or more recording stations to locate the epicenter of an earthquake, or the spot on a map below which the earthquake focus is located. The epicenter’s distance from the recording station can be found from a plot of time versus distance. Circles drawn around the recording stations at the proper distance will intersect at the epicenter, where three circles will intersect at only one point, the epicenter. 1

Figure 1. Example seismogram recorded by a seismometer with the wave arrival times and signal amplitudes labeled. The x-axis is Time in seconds with time increasing to the right. The y-axis is the signal amplitude measured in millimeters – this is how much shaking there was at the seismometer. For another example of a seismogram with surface waves, arriving later and with larger amplitude than is shown above, click this link . There are many different hazards associated with earthquakes. Landslides, rock fall, and cracking of the ground are some of the hazards associated with earthquakes. Surface waves (not S-waves) are the most damaging of the seismic waves, especially to buildings not made to withstand earthquake hazards. One of the more hazardous effects of earthquakes is called liquefaction, where sediment behaves like a liquid when shaken. Only certain soils are prone to liquefaction. When these soils are dry, they are stable, although once saturated and a shock is applied, they may lose their internal cohesion. This may cause catastrophic damage to any structures on the surface. 2

LAB 3: EARTHQUAKES AND SEISMIC HAZARDS [100 pts] Name: Alexis Rojas__________________________ Section: ____8:35am_________________________ In this lab, you will locate the epicenter of an earthquake that occurred in the San Francisco area and calculate its magnitude using seismograms from 3 recording stations, included after the lab questions. The x-axes of the seismograms are in seconds, like in Figure 1. 1a) Identify the arrival times for P- and S-waves in the seismograms (x-axis). Calculate P-wave/S- wave intervals by subtracting the arrival times and enter the intervals into the table below. [15 pts] 1b) Enter the maximum seismic wave amplitude of each station into the table below (y-axis). [15 pts] 1c) Using the graph of time versus distance, identify the distance from each station to the epicenter, and enter them into the table below. [15 pts] Recording station Latitude/Longitude S-P Interval (seconds) Amplitude (mm) Distance from epicenter (km) Eureka, CA 40.79 Latitude -124.14 Longitude 50 seconds 270mm 480km Las Vegas, NV 36.17 Latitude -115.13 Longitude 65 seconds 100mm 640km Elko, NV 40.84 Latitude -115.76 Longitude 70 seconds 55mm 680km. 1d) Go to www.iris.edu/app/triangulation . Use the “+Station” function to input the latitude, longitude, and distance from epicenter of each of the recording stations above. Take a screenshot and insert it here. [15 pts] 3

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1e) Where is the location of the epicenter? [10 pts] The Epicenter is in San Jose, California 2) Draw a straight line (using a ruler or the draw tool) on the nomogram from distance to amplitude for each recording station. They should all intersect on the “Magnitude” axis. [10 pts] 4

3) What is the magnitude (Richter scale) of this earthquake? [5 pts] The Magnitude of the earthquake is 7.2 4) Would you rather have been visiting San Francisco or Los Angeles during this earthquake? Why? [5 pts] I would rather be visiting Los Angeles due to the reason that that further from the epicenter. Around the location of Location of the epicenter San Francisco is the closet to that location. This is my reason why I would rather be at Los Angeles than San Francisco. 5) Would another seismogram (i.e., recording station) help identify the epicenter location better? Why or why not? [10 pts] No, just due to the sear factor that these three locations all circle one specific area such as San Jose. Another Seismogram wouldn’t be any help to locate an area. So another Seismogram wouldn’t be any help trying to identify the location of the epicenter. 5

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Figure 1: P-wave/S-wave interval and amplitude of earthquakes in Eureka, CA, Las Vegas, NV, and Elko, NV LAB 3: EARTHQUAKES AND SEISMIC HAZARDS Name: _____________________________ Section: _____________________________ Figure 2. Nomogram for calculating the Richter Magnitude of an earthquake (lef t), and a graph of the relationship between the S-P interval time and the distance represented by the time interval (right) , based on the difference in velocity between P- and S- waves. The nomogram example (dashed line) shows that an earthquake 100 km away from a station that recorded a maximum amplitude of 1 mm would have measured a magnitude 3 earthquake on the Richter Scale. 7

Figure 3. Map of the western USA to use for drawing circles representing equal seismic wave travel distances. The intersection point of three circles will identify the epicenter of the earthquake recorded in the seismograms. 8

Lab3_Assignment (1)

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