ES 104 Laboratory # 5 EARTHQUAKES: Epicenter Determination, Seismic Waves, and Hazards Introduction Earthquakes are vibrations of Earth caused by large releases of energy that accompany volcanic eruptions, explosions, and movements of Earth's crust along fault lines. The earthquake vibrations are waves of energy that radiate through Earth away from the focus. These waves of energy can be recorded on a seismograph, which produces a recording called a seismogram. Seismographs record the two types of body waves: Primary waves (P-waves) and Secondary waves (S-waves). They also detect Surface waves called Love waves (L-waves) and Rayleigh waves (R-waves). Travel-time curves are graphs that indicate how long it takes each type of seismic wave to travel a distance measured on Earth's surface. The difference between the S-wave arrival time and the P-wave arrival time corresponds to the distance of the seismograph station from the earthquake focus. This time difference can be converted easily into distance using the travel-time curves (Figure 2). Goals and Objectives - Learn to locate an earthquake epicenter using p-wave and s-wave arrival time differences and travel time curves. - Know the essential components of a seismometer and how seismometers record earthquakes. - Describe the relation between earthquakes, volcanoes, and plate boundaries. - Understand earthquake-induced liquefaction and landslide hazards and how they relate to site geology. Useful Websites - http://quake.wr.usgs.gov/info/1906/got_seismogram_lp.html - http://www.jclahr.com/science/earth_science/tabletop/earthshaking - http://www.sciencecourseware.org/VirtualEarthquake/ VQuakeExecute.html - http://staff.imsa.edu/science/geophysics/geosphere/tectonics/ seismogram.htmlName______KEY________________________ Lab day ______Lab Time_____________________ Pre-lab Questions – Complete these questions before coming to lab. Briefly define the following key words. 1. Earthquake Break or rupture of rock, emanating from a focus, sending seismic waves through Earth materials 2. Primary Wave Compressional wave from earthquake, moving fastest, and through all types of Earth materials. Material is alternately compressed and diliatated, parallel to the direction of wave propagation, 3. Secondary Wave Shear wave from earthquake, moving slower than primary waves, through solid material only. Material is sheared side to side, perpendicular to the direction of wave propagation. 4. Epicenter Location on Earth’s surface directly above the focus of the earthquake 5. Richter scale Method of earthquake measurement based on the amplitude of seismic waves recorded at the seismometer. Must be corrected for distance, since the waves are smaller for earthquakes further from the station. 6. Tsunami Sea wave initiated by undersea displacement of material, including landslides, volcanic eruptions and earthquakes Question for Thought 7. How do earthquakes relate to plate tectonics? Plate movement builds up strain that is released during earthquakesPart A – Epicenter Determination The epicenter of an earthquake is the point on Earth's surface at or above the earthquake's focus. In this exercise, you will determine the location of the epicenter of an earthquake that was recorded on seismograms at three different locations (Figure 1).Figure 2: Travel-time curves for P-waves, S-waves, and L-waves.Figure 3: Travel-time graph to determine the distance to the epicenter. S wave arrival-- P wave arrival012345670 500 1000 1500 2000 2500 3000Distance (miles)Time difference: S-P wave (minutes)1. Estimate to the nearest tenth of a minute (NOT seconds), the times of the first arrival of the P-waves and S-waves at each station in Figure 1. Times show it arrived after 8 AM. Record this in Table 1, below Subtract P-wave arrival time from the S-wave arrival time to determine the difference in travel time of P-wave and S-wave in minutes and tenths of minutes. Table 1: Arrival times at seismic stations Location of seismic station First P-wave Arrival (time as hour: minute.tenths) First S-wave Arrival (time as hour: minute.tenths) Difference in travel time between S & P Sitka, AK 8:07.4 8:11.5 4.1 Charlotte, NC 8:08.5 8:13.5 5.0 Honolulu, HI 8:09.3 8:15.2 5.9 2. Using the S-minus-P times and the travel-time curve (Figure 3), estimate the distances from the focus that correspond to these values. Record these in Table 2, below. Table 2: Distance of focus to seismic station Location Distance (miles) Sitka, AK 1450 Charlotte, NC 2100 Honolulu, HI 2650 3. Find the earthquake's epicenter using the distances you just obtained. a. Locate and mark the three seismic stations on the world map, Figure 3 (page 5-7): Sitka, AK: 57° N latitude, 135° W longitude Charlotte, NC: 35° N latitude, 81° W longitude Honolulu, HI: 21° N latitude, 158° W longitudeFigure 4: Map of Earth, for use in plotting data and locating the earthquake's epicenter.b. Use a drafting compass to draw a circle around each seismic station. Make the radius of the circle equal to the distance between the station and the epicenter that you determined above. Use the scale for the world map to set this radius on the drafting compass. The circles you draw should intersect at one point, which is the epicenter. (If the three circles do not intersect at a unique point, choose a point equidistant between the three circles.) The location of the epicenter is: Latitude between 30oN and 40oN Longitude between 110oW and 13oW Need to indicate N and W, and report correct one for each, not have them switched. 4. What is the origin time of the earthquake? That is, at what time did the earthquake occur? Using data from a single station, and Figure 2 or 3 to find out how long it takes to arrive, and the distance determined from epicenter to quake. Note the station, and show your calculations. Using P waves Sitka 8:07.4 minus 4.4 minutes = 8:03.0 AM Charlotte 8:08.5—6.4 minutes= 8:02.1 AM Honolulu 8:09.3—7.7 minutes=8:01.6 AM The stations don’t give the same origin time. Times in the 8:00+ ballpark are correct. Times between 2 AM and 4 AM are not, nor are times after arrival of P wave from table 2…you subtract minutes from minutes. 5. What time would you estimate did the L-waves from this earthquake begin to arrive at the Sitka station? (Use Figure 2) Use your origin time, and add the length of time for L waves to arrive. Depending upon
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