1 Did you feel it? That side to side, up and down, ground-shaking motion felt by many Californians every year, not to mention folks all around the world. The shaking motion is referred to as an earthquake, quake, or tremor. In any case, you are feeling a sudden release of energy or vibrations propagating through the earth as a result of shifting masses of the earth s crust. The created vibrations are in the form of seismic waves, similar to dropping a stone in a pond of calm water and watching the ripples emanate from a central point. Areas of the earth producing earthquakes are typically associated with stress (forces) building between shifting plates, specifically at plate boundaries. Once the buildup of stresses has exceeded the elastic limits of the particular rocks involved, potential energy converts to kinetic energy, and movement of the earth s crust takes place, or the crust snaps and releases seismic waves. Three types of seismic waves are released: the primary wave, secondary wave (body waves), and surface waves. The Primary Wave or P-Wave The primary wave, referred to as the P-wave, is the fastest moving wave and travels through both solid and liquid media. As this wave propagates through the earth s media, it compresses the material, similar to the action of a playing accordion. Therefore, this wave is also known as a compressional wave. The diagram below depicts the P-wave motion: P-wave compression propagating wave The primary wave or P-wave is propagating through the earth s surface. Note the circled areas being compressed. After compression, the media is expanded (dilation). The P-wave motion traverses through both liquids and solids. The Secondary Wave or S-Wave The secondary wave, referred to as the S-wave, travels approximately 2/3 the speed of the P-wave. As the S-wave propagates through the earth s media, it subjects the crust to sheering motions, where particles move up and down at right angles. This can be analogous to using a pair of shears or scissors when cutting. Shearing motions will move through solids but NOT liquids. For example, have you ever tried to use scissors and cut water? The diagram below depicts the S-wave motion: S-wave Shearing The S-wave is propagating through the earth s surface. Note the arrows showing how material is sheared and/or moving up and down at right angles. The S-wave motion travels through solids only, not liquids. propagating wave
2 The Surface Waves (L-Waves and Rayleigh Waves) Surface waves are the slowest waves propagating through earth and represent the most destructive type of waves since they primarily move across the surface. In other words, these waves are primarily felt by humans. Due to the motions of surface waves, they can be divided into two types of waves: the Love wave (L-wave) and Rayleigh wave (R-wave). Motions of the L-wave are characterized by the ground moving in a side-to-side direction, and the L-wave is the fastest of the two surface waves. Motions of the R-wave are characterized by rolling the surface up and down. This is analogous to sitting in a boat on the ocean going up and down from the body of water moving beneath you. Because the R-wave creates a rolling motion in the direction of wave propagation, most shaking on the earth s surface is primarily caused by the R-wave. The diagram below shows the motion of both the L and R waves. This is the motion of both the L-wave and R-wave. Note the side-to-side motion shown with the L-wave and the rolling motion depicted in the R-wave. Both waves are characterized by slow-moving waves and propagate across the surface. Mercalli Scale of Intensity Measuring the Energy Released by Earthquakes The Mercalli scale of measuring earthquake intensity was invented by Giuseppe Mercalli in This particular scale relies on damage observations stated by people who witnessed and experienced the earthquake. Ultimately, by observing various types of damage, people estimate the earthquake s intensity. The Mercalli scale is based on the Roman numeral system from I to XII. Roman numeral I indicates the least intensity while Roman numeral XII is used to show the highest intensity. How reliable or scientific is the Mercalli scale? Some observers of the earthquake may exaggerate the intensity, or in some cases, two witnesses may not agree with what happened to the same area with respect to the intensity. Additionally, different building codes applied to various communities all over the world may give the illusion of less or more damage with the same energy release. In other words, different parts of the world would experience various damage reports caused by the earthquake and not accurately record the real strength of the quake. Below is the Mercalli Intensity chart with damage reports and associated Roman numerals. In 1934, Charles Richter recognized the people factor when measuring the intensity of earthquakes and invented the Richter scale, in which one can calculate the magnitude of an earthquake eliminating the inconsistent observations of individuals. Here is the Mercalli Intensity scale showing Roman numerals assigned to various types of damage. Damage is reported by people and assigned a Roman numeral. Reports of damage are very subjective, and the Mercalli scale does not prove scientifically reliable for measuring the actual amount of energy release during an earthquake.
3 The Richter Scale From 1934 to the early 1970 s, the magnitude of earthquakes was measured using the Richter scale. Richter magnitudes are calculated based on the largest wave amplitude recorded on a seismogram. Amplitude lines are recorded on the seismogram by incoming P, S, and surface seismic waves. The diagram below shows a seismogram with amplitudes from both P and S- waves. Note the largest amplitude recorded on the S-wave. P-wave arrival Largest amplitude S-wave arrival The seismogram represents incoming P and S-waves. Note the recorded amplitudes on both P and S-waves. The largest amplitude is noted on the S-wave arrival. Richter magnitudes (M1, M2, M3,.) are based on a logarithmic scale using base 10. For example, for each Richter magnitude increase, the amplitude of the ground motion recorded by a seismograph increases by a factor of ten. Therefore, a magnitude 4 (M4) would result in ten times the level of ground shaking as an M3 earthquake and consequently release 32 times more energy. How much ground shaking and energy release would one expect between an M3 and M5? ANS: One would calculate that 2 Richter magnitudes have occurred (5M 3M = 2 Richter magnitudes). Therefore, since each Richter is equal to an increase in amplitude by a factor of ten, then 10 x 10 = 100. The amplitude increased by 100 times, or there is 100 times more ground shaking. The energy released between M3 and M5 can be calculated by multiplying 32 x 32 (each Richter releases 32 times the energy), so energy release is approximately 1000 more times. How much ground shaking and energy release would one expect between an M6 and M3? Use the above answer, and make your own calculations. The amount of energy released from different earthquake magnitudes can be analogous to the energy released by explosives. For example, an M1 release of energy is equivalent to the energy release from blowing up 6 ounces of TNT. However, an M8 would release as much energy as detonating 6 million tons of TNT. Note the logarithmic or exponential increase in energy between each Richter magnitude. Given the logarithmic nature of the Richter scale, it has no upper limit. Therefore, earthquakes can theoretically be measured up to M10 and beyond can t imagine the energy release from quakes greater than M10. The Richter scale assumes the focus of the earthquake at a single point and is best suited for energy release in small areas. Additionally, the intensity of the earthquake is based on amplitude measurements. To accurately measure the actual energy release from earthquakes, scientists have now made other magnitude scales, namely the Moment magnitude scale developed in the 1970 s and currently used by the USGS. The Moment magnitude scale is calibrated to incorporate Richter magnitudes and considers waves propagating through different rock properties.
4 Determining Earthquake Magnitude Interpreting the Seismogram / Determining the Earthquake Magnitude Determining the magnitude of an earthquake requires accurate interpretation of the seismogram. From the seismogram, a seismologist measures the wave amplitude height and S-P time interval. A typical seismogram will show a squiggly straight line, which is considered background noise during non-earthquake activity. When earthquake activity begins, the P-wave arrives and is interpreted as the first jog in wave activity (bigger than the background noise). The P-wave recordings will continue until the second jog on the recorder (larger than the recorded P-waves). The second large jog is interpreted as the first S-wave arrival. The S-P time interval represents the time difference between the first appearance of the P-wave (first jog ) and the first appearance of the S-wave (second jog ). In other words, the S-P interval is equal to the P-wave arrival time subtracted from the S-wave arrival time. The diagram below depicts a typical seismogram showing the amplitude height and S-P interval. The seismogram shows arrivals of P and S-waves, wave amplitude, and the S-P interval. To calculate the S-P interval, subtract the P-arrival time from the S-arrival time. The S-P interval for this seismogram is: 38 seconds P-arrival time = 0 seconds S-arrival time = 38 seconds S-arrival time P-arrival time: 38 sec 0 sec = 38 sec S-P interval is 38 seconds Typically, S-P intervals can be calculated directly from time intervals indicated on the seismogram as shown in the above diagram. However, in some cases, time intervals for P and S-wave arrivals are given, and one must calculate the S-P interval. For example, arrival times may be given using the military time clock. Below are examples of P and S wave arrival times using the military clock. P- wave arrival time S-wave arrival time S-P interval 21 hr, 32 min, 40 sec 21 hr, 33 min, 10 sec 30 seconds 15 hr, 23 min, 19 sec 15 hr, 24 min, 5 sec 46 seconds 3 hr, 16 min, 32 sec 3 hr, 17 min, 13 sec 19 hr, 43 min, 12 sec 19 hr, 43 min, 52 sec Once the S-P interval and amplitude are correctly interpreted from the seismogram, the magnitude and distance to the epicenter is determined for that particular seismograph. The values representing the S-P interval and amplitude wave height are plotted on the magnitude chart on the following page. Graphically plot the S-P interval. Graphically plot the amplitude. Construct a line, using a straight edge, from the plotted S-P interval to the plotted amplitude. The line will pass through the magnitude scale revealing the earthquake magnitude for the indicated S-P interval and amplitude height. Additionally, once the S-P interval is plotted along the S-P interval line, the epicenter distance is determined by observing the value adjacent to the plotted S-P interval.
5 The chart below shows how the magnitude of an earthquake is determined: Using the chart, the magnitude of an earthquake is determined as well as the distance to the epicenter. In this case the S-P interval is 24 seconds, and amplitude is 23 mm. The line connecting both the S-P interval and amplitude passes through the magnitude scale resulting in an M5 earthquake. Using the S-P interval time, the distance to the epicenter is determined to be approximately slightly greater than 200 km. The Triangulation Method for Locating the Epicenter The use of triangulation provides a method for locating the epicenter of an earthquake. Triangulation involves the use of three or more seismic stations recording the same seismic waves during an earthquake at different distances from the epicenter. Distances to the epicenter can be determined by the S-P time interval from each seismic station. Using the seismic station as the center point, circles can be drawn with the epicenter distances represented by the radius of each circle. After constructing at least 3 circles, the intersection of all three circles determines the point or location of the epicenter. The diagram below shows the triangulation method and location of an epicenter. SS-A Distance radius SS-B Triangulation is used to determine the location of epicenters. Note that 3 seismic stations (SS-A,B,C) are used along with circle radii representing the distance from each seismic station to the epicenter. Epicenter distances are determined from S-P intervals. epicenter SS-C The intersection of all three circles represent the epicenter location.
6 LAB EXERCISES PART A: Earthquake Definitions Provide the definition for each vocabulary term listed below: earthquake seismic waves potential energy kinetic energy primary wave secondary wave surface waves Mercalli Scale amplitude Richter Scale S-P time interval triangulation
7 PART B: P, S, and Surface Waves and the Earth s Interior 1. Draw a colored diagram that represents the propagated motion of the P-wave, and list three distinct characteristics. 2. Draw a colored diagram that represents the propagated motion of the S-wave, and list three distinct characteristics. 3. Draw a colored diagram that represents the propagated motions of both the Love wave (Lwave) and Raleigh wave (R-wave). Identify at least 2 characteristics each for both waves. 4. Given the diagram below (the layered earth) and your descriptive wave characteristics in questions 1, 2, and 3, how do scientists understand the layered nature of our earth? Label the interior parts of the earth, and show how the P and S-waves move through the interior acting as predictors regarding the earth s internal layers. The Earth s Interior
8 PART C: Measuring the Magnitude of Earthquakes Reading a seismogram 1. Find the S-P time intervals. S-P time interval S-P time interval 2. Given the P-wave arrival of 21h 45m 34s, S-wave arrival of 21h 46m 09s, what is the S-P time interval? 3. Given the P-wave arrival of 19h 16m 17s, S-wave arrival of 19h 16m 52s, what is the S-P time interval? 4. Given the P-wave arrival of 22h 32m 42s, S-wave arrival of 22h 33m 13s, what is the S-P time interval? 5. An earthquake is recorded at three different seismic stations (A, B and C). (Station A) P-wave arrival 20h 12m 56s, S-wave arrival 20h 13m 22s (Station B) P-wave arrival 20h 12m 40s, S-wave arrival 20h 12m 49s (Station C) P-wave arrival 20h 13m 12s, S-wave arrival 20h 13m 44s Indicate the proximity of each seismic station A, B, and C to the epicenter. Which stations would report the greatest and least Mercalli intensity values?
9 PART C: Measuring the Magnitude of Earthquakes Using the magnitude chart below, answer questions 1, 2, and Given the P-wave arrival of 10h 26m 41s and S-wave arrival of 10h 27m 13s and with an amplitude of 8mm, fill in the blanks below: S-P time interval Magnitude Epicenter distance 2. Given the P-wave arrival of 14h 19m 22s and S-wave arrival of 14h 19m 40s and amplitudes of 0.3mm and 4mm, fill in the blanks below: S-P time interval Magnitude Epicenter distance 3. Given the S-P interval of 10 seconds and wave amplitude of 0.4mm, fill in the following blanks: a. What is the Richter magnitude? b. If the amplitude were 40mm, what is the Richter magnitude? c. Given answer b, the amplitude increased by a factor of d. Given answer c, the Richter magnitude increased by a factor of e. If the amplitude were 4mm, the Richter magnitude = f. If the amplitude were 400mm, the Richter magnitude = g. If the amplitude were 0.04mm, the Richter magnitude = 4. Given the seismogram below, label the P-wave and S-wave arrivals and fill in the blanks below. The wave amplitude is.5 mm. S-P interval EQ magnitude Epicenter distance
10 PART D: Locating Epicenters Using the Triangulation Method 1. Seismic activity is recorded at 3 separate seismic stations from the same earthquake. Given the world map (handout), as a seismologist, determine the location of the epicenter using the triangulation method. Accuracy when determining S-P intervals, epicenter distances, plotting longitude/latitude locations, and compass settings is extremely crucial to proper triangulation. 1. Obtain the handout that includes the world map, seismograms, and the time difference vs. distance to epicenter chart. 2. Fill in the table below using data provided from question 1. Seismic station P-wave arrival time S-wave arrival time S-P time Interval (min) Distance to epicenter (km) Honolulu Sitka Charlotte 3. Plot the longitude and latitude locations for each city (Honolulu, Sitka, and Charlotte) on the world map. 4. Using your epicenter distances, construct circles from each city location. Use the provided compasses, and match the distance scale on the world map with proper compass opening. 5. Once your circles have been constructed and your S-P intervals accurately determined (epicenter distances and compass techniques are extremely accurate), the circles should show an intersection point. 6. From question 5, the epicenter of this earthquake is, which is located at longitude and latitude. 7. Measure the highest amplitude using the seismogram generated from Sitka, Alaska. Determine the magnitude of the earthquake. The earthquake magnitude is. 2. Seismic activity is recorded at 3 separate seismic stations from the same earthquake. Given the California map (handout), as a seismologist, determine the location of the epicenter using the triangulation method. 1. Obtain the handout that includes the California map, seismograms, and the time difference vs. distance to epicenter chart. Below are latitude/longitude readings for the California seismograph stations. Bakersfield: San Francisco San Diego 35.4 O N. Latitude, O W. Longitude 37.8 O N. Latitude, O W. Longitude 32.7 O N. Latitude, O W. Longitude
11 2. Follow the directions outlined in the previous question, and determine the location of the California epicenter. Seismic station P-wave arrival S-wave arrival S-P time interval Distance to epicenter (mi) Bakersfield San Francisco San Diego 3. Using a map of California, determine the epicenter of this earthquake. The epicenter is located at. 4. Given the location of the epicenter, what plate boundary does the epicenter represent? Critical Thinking Questions: 1. An M7 magnitude earthquake takes place and three seismic stations at different locations record the incoming waves. What is the magnitude of the earthquake at each station? Explain your answer. 2. Given your answer in question 1, describe how the S-P time intervals would differ at each seismic station. Draw a picture that would represent the S-P time interval from the closest, middle and farthest seismic station. 3. Given your answer in question 2, how does the Mercalli scale relate to the proximity of each seismic station from the epicenter and reported damage?