Seismic Waves The deepest places where people have seen the Earth’s interior are located in South Africa gold mines, some 3.5 kilometers below the surface. The deepest wells are about 10 km deep, which only scratch the surface of the Earth. Most of what we know of the Earth’s interior has come from the study of earthquakes. Earthquakes send out seismic waves in all directions similar to the way that throwing a stone into a pond sends out waves through water. Observing the behavior of these seismic waves as they travel through the earth gives us insight into the structure within the earth. HOW EARTHQUAKES FORM An earthquake occurs when rocks in a fault zone suddenly slip past each other releasing stress that has built up over time. The slippage releases seismic energy, which is dissipated through three kinds of waves: Primary (P)-waves, secondary (S)-waves, and surface (L)-waves. The distinction between these waves is similar to the wave properties we studied earlier in the semester: Primary (P)-waves are compressional or longitudinal waves that push and pull on the rocks through which they are traveling creating a back and forth movement on the Earth's surface. Primary waves travel the fastest and can move through solid and liquid rock. Secondary (S)-waves are transverse waves create an up-and-down or side-to-side motion in the underground rock through which they are traveling. S-waves travel only through solid rock and cannot travel through liquids. S-waves also travel slower than P-waves. Surface (L)-waves are produced by the motion of the primary and secondary waves striking the earth’s surface. Surface waves travel slightly slower than S-waves producing large displacements of the surface and are the most destructive of the three types of earthquake waves. epicenter The waves travel outward from the spot where rocks of the earth's crust snapped under the strain. This spot is called the hypocenter. The spot on fault line the earth's surface right above the hypocenter is called the epicenter. Earthquakes are felt the strongest at the epicenter. THE SEISMOGRAPH There are hundreds of earthquake recording stations around the hypocenter earth. Some have also been placed on the Moon and Mars to detect volcanic activity. A seismograph is an instrument that records the vertical and horizontal motion of the earth caused by an earthquake, recording the intensity of the vibration, as measured by the height of the line on the seismogram, and the vibration's frequency, which is measured by the distance between the peaks or valleys on the graph. Seismologists have developed a way of identifying the size, or magnitude of earthquakes using the Richter Scale, developed by Charles Richter in 1935. The Richter scale is a logarithmic scale so that as the magnitude increases by 1 unit the amplitude of the earthquake increases 10 times. In this laboratory you will investigate the process how the distance to an earthquake epicenter is located and how to estimate the magnitude of an earthquake. Seismic Waves 1 PART I: SIMULATING AN EARTHQUAKE During an earthquake, primary waves, or P-waves, move the fastest through the rock and thus are the first waves to be recorded by a seismograph. Secondary waves, or S-waves, are the second waves to be recorded since they travel slower than the P-waves. Knowing the difference in arrival times of these waves at a recording station allows us to determine the distance to the focus. If multiple recording stations are used it is possible to determine the location of the epicenter of the earthquake by triangulation. Launch the Gizmo from ExploreLearning The map indicates the locations of an earthquake and a recording station, and a seismogram displaying signal strength on the vertical axis and time on the horizontal axis. Click Play ( ). What does the purple circle represent? ______________________ What does the green circle represent? ______________________ Which wave (P wave or S wave) has the greater speed? _____________________ Which type of wave appears to be stronger as displayed on the seismogram? ___________________ Click Reset ( ) and then click Show Grid below the map and examine the geographical data in the legend in the lower left corner. What is the unit of distance represented by each square of the grid on the map? ______________ What is the distance from the station to the epicenter of the earthquake? ______________ Click Play ( ) to watch the animation. When the vibrations have mostly subsided, click Pause. When an earthquake occurs, it sends out fast-moving P waves and slower-moving S waves. P waves cause a fairly small vibration. S waves are much stronger and cause taller "spikes" in the seismogram. The amount of time between the arrival of the first P wave and the arrival of the first S wave is the lag time between waves, or T. Select Show time probe. Click and drag the left-hand (green) time probe to align with the onset of the first P-wave recorded on. Click and drag the right-hand (blue) time probe to align with the onset of the first S-wave recorded on the seismogram. How long did it take for the primary wave to reach the recording station? _______________ The secondary wave? _______________ Record the times for the P-wave and S-wave to reach the recording station, the difference in their times (Δ T) and the distance the recording station was from the earthquake in the table below. With the cursor grab the epicenter and move it to another location and click Play again. Record the P-wave, S-wave, Δ T, and the distance in the table. Repeat the process for three more earthquakes or a total of 5 simulations. Try to place the epicenter and recording station at distances apart that are convenient to plot, such as 400 km, 600 km, etc, keeping the distances below 1000 km. Plot the set of points for each earthquake simulation and draw a best-fit straight line for each set of data (P-wave, S-wave, Δ T). Label each of the lines. Seismic Waves 2 P-Wave Time S-Wave Time Δ T (seconds) Distance from (seconds) (seconds) Epicenter (km) S & P Wave Arrival Times 1000 900 800 700 600 500 400 300 Distance (kilometers) Distance 200 100 0 0 50 100 150 200 250 300 Time (seconds) Primary waves travel between 5.95 and 6.75 kilometers per second depending on the compressibility and density of the rock and are the first to arrive from a distant earthquake. Secondary waves move between 2.7 and 3.7 km/sec. What are the approximate slopes (speeds) for each wave? P-Wave _______________ km/sec S-Wave _______________ km/sec Δ T _______________ km/sec Seismic Waves 3 PART II: DETERMINING THE LOCATION OF AN EPICENTER To determine the location of an epicenter, data from multiple recording stations are triangulated. Launch the Gizmo from ExploreLearning Notice the three small graphs at the top left corner of the Gizmo. These are simulated seismograms from seismographs, which run constantly at many locations around the world. Click Play ( ) and watch the seismograms as an earthquake occurs somewhere in the world. When the vibrations have mostly subsided, click Pause. Drag one of the vertical probes and place it on the first seismogram where the first P wave arrived. Then place the second probe where the first S wave arrived. What is T for Recording station A? What is T for Recording Station B? For Recording station C? Station A Station B Station C T Distance T Distance T Distance (sec) (km) (sec) (km) (sec) (km) Using your graph from Part I, determine how far Recording station A was from the epicenter. (Make your best estimate, based on how this value for T fits in with your previous data.) In the Gizmo, select Show station A. Set the Radius slider to the distance from the epicenter. You will see a circle around Recording station A. A circle shows all the points that are the same distance from the center. In this case, the circle shows all the points that are the correct distance from Recording station A. According to your calculations, the epicenter lies somewhere on that circle. Repeat the process for Recording station B and Recording station C and record your distances in the table above. Once you have all three radii set, the three circles shown in the Gizmo should intersect at one point. That point was the epicenter of the earthquake. If you have found the correct epicenter you will see a red symbol like this: (A small red dot will appear if the three circles intersect at a point that is not the epicenter.) PART III: USING ACTUAL DATA TO DETERMINE THE EPICENTER In the real world, data from stations around the world recording the earthquake will be used to estimate the focus, epicenter, and magnitude. In this part you will use actual seismograph data to determine the epicenter. The data table on the next page shows the arrival times for the P-wave and S-waves from an earthquake that occurred in 1998. Determine the difference in arrival times (lag time) for the P and S waves. Using the distance vs time graph you obtained in Part I, estimate the distance of the epicenter to the recording station using the lag-time between the waves (ΔT). Using the scale provided on the map, spread the compass to the distance the first recording station is to the epicenter. Draw a circle centered on the recording station with a radius equal to the distance to the epicenter.