Lab 9 Locating Earthquake

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Geol 116, Fall 2023 Name: Anya Donders Lab 9: Locating Earthquakes OVERVIEW Millions of earthquakes occur every year around the world and are recorded by a network of seismometers. Seismometers are calibrated to sense even the smallest motion of seismic waves from distant earthquakes. By understanding how seismic waves travel, the ground-motion records, called seismograms, can be interpreted to locate an earthquake’s source. In this activity, you will use 3-component seismograms (motion recordings on the N/S, E/W, and up/down axes) to locate earthquakes. Traditionally, exercises like this have you identify P and S waves on the seismograms, measure the time between arrival of the P and S wave, then use the time difference to estimate the distance from the station the earthquake by using a travel-time curve; Fig. 1 illustrates this. Figure 1: Determining P- and S-wave arrival times on seismogram. This example was derived from IRIS’s online app “Triangulation” But that’s not how it’s actually done! Rather, an iterative process is used, called Residual Minimization. This approach to locating earthquakes uses only P wave arrivals from many seismic stations. Using a mathematical model, P wave arrivals times are predicted from trial event locations, which are refined through comparisons to observed arrivals. That initial trial location is based on a small number of stations, using p-wave timing and direction to “guess” where the earthquake might have originated. Then more stations are analyzed, identifying the p-wave arrival time and direction. The basic idea is to minimize the differences among locations predicted until a best fit is obtained. This is done automatically by computer, but (especially for large earthquakes) will be checked by a seismologist and refined if needed.

You will use seismograms recorded at different locations to estimate the location of the epicenter. We’ll start with some recordings of an earthquake that occurred off the Mexican coast to get you familiar with what the p and s waves look like on a seismogram, then we’ll shift to an online tool to use just p- wave arrivals to try to “nail down” the location of several recent large earthquakes using an iterative procedure (basically a brute-force way of doing what computers do in near-real time). Modern seismic stations produce a 3-component seismogram simultaneously. One trace of the seismogram shows vertical motion, one shows east to west motion, and another shows north to south motion (Figure 2). • • • • Figure 2: The different behavior of P, S, and surface waves explain how the 3 seismometers in the seismograph station have different seismograms. 1) The vertical component shows the compressive P wave bumping up from beneath; it has very little horizontal movement. 2) The S wave is moving side to side in the direction of travel, so it has less effect bumping up from beneath the station than it does in its side to side motion. 3) Surface waves have a huge effect on all components for shallow earthquakes, particularly when recorded at local and regional distances. Because body waves come up from below the surface, P wave motion is usually greatest on a vertical component seismogram, while the S wave usually registers a stronger signal on the horizontal component seismograms (Figure 3). But the horizontal components of the p wave are important in estimating the direction of the seismic waves. P wave S wave Surface waves start

FIGURE 3 –Since the motion of a P wave is in the direction the wave travels, for a wave coming up from below the seismograph station, the motion of the ground will be mostly vertical (up and down), with a smaller amount of motion on the horizontal components (North and East). By contrast, since the S wave motion is perpendicular to the direction of wave travel, an S wave coming up under the station will produce larger motion in a horizontal direction which will be some combination of the North and East component directions, and smaller motion on the vertical component . Materials You’ll Need •3 three-component seismograms for the same earthquake (attachments) • Computer for access to the IRIS earthquake locator tool • Map with global and/or regional seismicity illustrated ( IRIS earthquake browser ) Part 1. Reading the seismograms and locating the “old-fashioned” way. 1.Identify P and S waves on each seismogram on the accompanying pages 2.Measure the time between arrivals of the P wave and the S wave on each seismogram. Think about which trace(s) (vertical, E-W, N-S) should show the P wave best, which the S wave. 3.Use this time to mark on the Y axis of the travel-time curve the distance the epicenter is away from the station. 4. Go to the IRIS Triangulation website and follow the instructions to input your data from the three stations: Locate each station using its latitude and longitude. Mark the distance you have computed in step 2. Repeat for each station.

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LOCATING AN EARTHQUAKE WITH SEISMIC DATA 1. Identify P and S waves on your seismograms. Station TUC What is the P arrival time? 3 hr 21 min 40 secs What is the S arrival time? 3 hr 23 min Station COR What is the P arrival time? 3 hr 22m 20 secs What is the S arrival time? 3 hr 24 mins 20 secs Station CCM What is the P arrival time? 3 hrs 25mins What is the S arrival time? 3 hrs 29 mins 2.Determine the amount of time between arrival of the P and S wave (S minus P time) Station TUC: 1.3 minutes Station COR: 2 minutes Station CCM 4 minutes 3.Determine the distance the epicenter is away from each station using the travel time curve. Station TUC 800 km Station COR 1200 km Station CCM 2600 km Do this by taking your S-P time (e.g. 4 minutes) and locate a position between the S and P lines on the graph that is 4 minutes apart. 4. Enter the three station sites (latitude, longitude, distance) into the IRIS Triangulation interactive map. It will provide a location for you based on your interpretations.

5. What was the latitude and longitude of the location to which you have triangulated? Take a screen shot of your result and provide it to your TA. Latitude: 37.17 Longitude: -115.22 6. The actual location was very near Ridgecrest, CA. How close were you? Why might you have missed the location? Mine included Ridgecrest CA in the epicenter. The area might vary depending on when someone determines the S wave and P wave hit and therefore the amount of time in between the two which determines the Km. Part 2: Iterative earthquake location Even from the simple exercise you just did, you can begin to appreciate how challenging it can be to identify P and S first arrivals. That’s true even for a seasoned veteran. So seismologists developed a technique that relies just on first arrivals, recorded at multiple stations. The more stations, the better the location (both lat-long and focal depth). IRIS has developed an interactive tool, IRIS EQLocate, that simulates the way that earthquake locations are now done. 1. Navigate to the IRIS earthquake locator tool (linked above). 2. Choose the Residual Minimization method (it actually does work) 3. Choose an earthquake from the Global List. This will open up a map that shows seismic stations near where the earthquake occurred, with the closest ones marked with a dot in the center. 4. Select up to 12 stations (remember, the more the merrier). The program will help with this, telling you if you’ve got good geographic coverage. 5. Click on Next (upper right-hand corner of the screen) to get to the seismograms. Now you’re getting only one trace—the vertical component—from each station. Pick the initiation of the p wave (first arrival) by sliding the read bar. You may need to zoom out (- sign next to Default) in

order to see the first arrival from the more distant stations you chose. Once you’ve picked your first arrivals, click Next, and it will take you back to the map. 6. Now for the iterative part. You’re trying to solve for the best 3D (latitude, longitude, and focal depth) location for the earthquake that minimizes the misfit (residuals) between the location and the arrival of the p waves at your stations. You control this in 2 ways: adjust the depth to obtain the smallest residual, and adjust the lat-long location, again to minimize the residuals even further. But you need to provide a first guess: some location close to the stations that are marked as nearby. 7. Search for an optimal solution (lowest RMS) by moving the depth scale and the location of the epicenter until you’re satisfied you’ve got a good solution. The green arrow on your location map will show you what direction to move your epicenter guess; the best depth will show up as a green value on the depth scale. Click on Show Result. Your goal: to get four stars. Take a screen shot of your best result and submit it to your TA by email. 8. Repeat this for one of the Special (large) earthquakes.

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Seismic Station TUC (Tucson, AZ) Latitude 32.31 degrees, Longitude -110.78 degrees

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COR (Corvallis, OR) TIME, in hour/min Seismic Station Latitude 44.59 degrees, Longitude -123.30 degrees Latitude 38.06, Longitude -91.24

Seismic Station CCM (Cathedral Caves, MO)

Lab 9 Locating Earthquake

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