Copy of Lab 04 Earthquakes

GEOSCI/ENVIR ST-106: Environmental Geology Lab 4: Earthquake Detection & Seismic Hazards Earthquake: A sudden and violent shaking of the ground as a result of movements within the earth's crust or volcanic action Assignment Overview: The effects of plate tectonics can have major impacts on human life, in the form of volcanic activity and earthquakes. Geologists collect a variety of data about historical earthquakes and volcanoes in order to understand future risks. In this lab, you will use data from maps and seismic stations, along with simple computations, to explore these hazards. IMPORTANT NOTES: Download the .KML dataset associated with this lab from the Canvas course page. All instructions below reference commands used in Google Earth Pro, which is highly recommended for this lab. Submission: To submit the assignment on Canvas, use the following steps: 1. In Google Docs, generate a PDF: File → Download as → PDF Document 2. In Google Docs, use Share → Get Shareable Link, and copy the link address 3. In Canvas, upload your PDF to the assignment, and paste the link address to your Google Doc in the assignment comments. 1

Background: Seismic Data Analysis As we discussed, the energy release from an earthquake is transmitted through the earth as vibrations in several different ways. Body waves – much like sound waves through air – travel through the Earth itself, and are relatively fast. These include Pressure / Primary P-waves and Secondary / Shear S-waves. Surface waves – much like the ripples moving across a pond – travel primarily along the surface of the Earth, and are somewhat slower. Surface waves are especially important because they are the waves that are almost entirely responsible for damage to structures . The propagation and speed of these waves is shown below in Figure 1. Figure 1: Cartoon of energy movement away from an earthquake, including approximate propagation velocities for P-waves, S-waves, and surface waves An example of a record that might be recorded by a seismograph station is shown in Figure 2, along with the different wave arrivals highlighted on the record. Seismologists and, increasingly, computers, can detect and differentiate these different types of vibrations to determine the arrival time of each wave, i.e. when they have arrived at seismograph stations. Just like anything moving at a set speed, its arrival time at a location is determined by when it started, how fast it was moving, and how far it had to go. Said more mathematically: Arrival time = start time + distance / velocity For an earthquake, we are able to measure when each wave arrives ( arrival time ) and we can roughly know its velocity , based on how rocks respond to different wave types. We don’t know in advance how far away it was ( distance ), or the start time , meaning the time of the earthquake. In this lab, you will use information from seismic sensors to determine these important unknowns. Based on the equation above and some algebra (see the appendix at the end of this lab), we can use the following two equations to estimate an earthquake’s distance and arrival time: Calculated Distance = (S arrival time - P arrival time) / (1 / S velocity - 1 / P velocity) Calculated Start Time = S arrival time - Calculated Distance / S velocity 2

Assignment Questions and Responses Instructions: Fill out each red highlighted field (_________), according to the question instructions. As you read in the textbook, the difference in arrival time tells us something about how far away the earthquake event was. You have been hired by Amazon to assess the danger of a typical seismic event to their operational facilities in the Pacific Northwest. Amazon has chosen a particular historical seismic event as an “example case” to assess what its impact on operations would be. The data from this historical event is provided in terms of arrival times at seismograph stations in the table below. The locations of these seismograph stations is included in your Google Earth .KML file 1. Calculating Arrival Time Differences: (1 point) To start your analysis, fill in the table below by calculating the arrival time differences, which will be used to answer each of the questions below. Station P Arrival Time HH:MM:SS.SSS S Arrival Time HH:MM:SS.SSS Arrival Time Difference (s) S1 20:12:35.761 20:12:59.938 ____24.177____ S2 20:12:17.034 20:12:27.750 ____10.716_____ S3 20:12:29.761 20:12:49.625 ____19.864_____ 2. Estimating EQ location and timing: (4 points) Using the data above, and the seismic velocities given in Figure 1, determine when and how far away from each seismograph the actual earthquake occurred. First estimate the earthquake’s distance from each station and then its start time. (Note: To simplify times, you should give all answers as seconds after 20:12.) Station # 1 / Vs (s / km) 1 / Vp (s / km) 1/V s - 1/V P (s / km) Arrival time difference (s) Estimated distance to EQ (km) Estimated EQ start time (after 20:12) 1 0.3125 _0.1818_ _0.1307___ ___24.177_ __184.981_ _2.1314____ 2 ___10.716_ __81.989___ _2.1284___ 3 ___19.864_ __151.982__ __2.1306___ 4

3. Mapping the Event: (5 points) In Google Earth, draw a circle centered around each seismic station corresponding to the estimated distance to the EQ epicenter (each point on the circle is thus a location where the earthquake may have occurred). This can be accomplished by going to Tools → Ruler → Circle, generating a circle of approximately the right distance, and clicking “Save”. Based on these results, mark your guess at the actual location of the earthquake, and label it with its estimated start time, e.g., “EQ at xxxx seconds after 20:12”. Paste a clear screen capture of your results here. Briefly describe how you have color-coded / labeled the map, or add a legend. _______ EQ 2.13 seconds after 20:12 (time is averaged between the three estimated times I got) The red circles are the estimated distances from the stations to the epicenter of the earthquake. The small green dots represent these stations. The yellow pin marks the epicenter of the earthquake. 4. Data requirements: (2 points) What can you say about the earthquake location given the 3 seismic observations? What would you be able to say if you had only 1-2 seismic observations? 5

Appendix: From seismic data to EQ distance and time The following provides a brief description of how the equations presented earlier were developed. For each of the seismic waves, we can write the following equations: P arrival time = EQ start time + EQ distance / P velocity (1) S arrival time = EQ start time + EQ distance / S velocity (2) Here, note that our unknowns are the (italicized) EQ start time and the EQ distance. Thus, this is a set of 2 equations with 2 unknowns, so we should be able to determine both values. Since we know the S wave will arrive later and want to deal with positive numbers, Subtracting (1) from (2) gives: (S arrival time – P arrival time) = EQ distance x (1/S velocity – 1/P velocity) And solving for the EQ distance gives the first equation we used in lab: EQ distance = Arrival time difference / (1/S velocity – 1/P velocity) (3) Re-arranging equation (2), we arrive at the second equation used in lab: EQ start time = S arrival time – EQ distance / S velocity (4) Since EQ distance has already been estimated in equation 3, we can now solve for the EQ start time. 7