Transcript
LAB #24
LAB #1
Earthquakes
Recommended Textbook Reading Prior to Lab: • Chapter 14, Geohazards: Volcanoes and Earthquakes 14.3 Tectonic Hazards: Faults and Earthquakes 14.4 Unstable Crust: Seismic Waves Goals: After completing this lab, you will be able to: • Classify the types and characteristics of body and surface seismic waves. • Interpret a seismogram and determine the likely seismic wave characteristics. • Determine the arrival time difference between selected P waves and S waves, and use a travel time curve to determine distance to an earthquake’s epicenter. • Use a drawing compass to triangulate an earthquake’s epicenter. • Quantify earthquake magnitude with a magnitude nomogram after interpreting key variables on a seismogram. • Calculate energy-released comparisons, as well as ground-motion comparisons, for selected earthquakes of varying magnitude. • Identify how an earthquake intensity scale differs from an earthquake magnitude scale. • Carry out intensity interval sketching on a map following a hypothetical earthquake. • Judge the likely earthquake intensity of selected cities during a hypothetical earthquake. Key Terms and Concepts: • epicenter • magnitude • magnitude nomogram • modified Mercalli intensity (MMI) scale • P wave
• S wave • seismograph (or seismometer) • seismogram • travel time curve
Required Materials: • Drafting compass • Drawing compass • Ruler • Textbook: Living Physical Geography, by Bruce Gervais
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Problem-Solving Module #1: Earthquake Waves and the Travel Time Curve Earthquakes generate both P waves (primary) and S waves (secondary). Both P and S waves travel through Earth’s interior and are called body waves. Earthquakes also generate Rayleigh and Love waves. These waves travel on Earth’s surface and are called surface waves.
General Grouping Body waves
Surface waves
Travel Medium
Wave Name
Typical Speed (varies depending on medium)
Through Earth’s interior
P wave
~6 km/s
S wave
~3.5 km/s
Shearing (through Earth’s interior)
Love wave
~2.8 km/s
Shearing (across Earth’s surface)
Rayleigh wave
~2.7 km/s
Elliptical
On Earth’s surface
Motion Compression and dilation
TABLE 1–1 A seismograph (or seismometer) is an instrument that detects, measures, and records ground shaking. A seismometer’s record is called a seismogram. Seismograms are read like the pages of a book, from left to right, and they are always time-stamped using Greenwich Mean Time (GMT). Because P waves travel fastest, they appear first on any seismogram, followed by S waves, and then surface waves. Figure 1–1 is a hypothetical model of a seismogram.
FIGURE 1–1
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1. Use your textbook, along with the information in Table 1–1 and in Figure 1–1, to complete Table 1–2. Wave Characteristic
Seismic Wave Group(s) # (see Figure 1–1)
Includes Love waves Has a shearing motion through Earth’s interior Travels on Earth’s surface Arrives at a seismometer last Generally grouped as surface waves Travels fastest Includes Rayleigh waves Travels through Earth’s interior Classified as a surface wave Travels slowest Has the largest waves Travels at about 3.5 km/s Arrives at a seismometer first Has a compression and dilation motion Generally grouped as body waves Includes waves with an elliptical motion Produce the greatest shaking TABLE 1–2 Scientists use the difference between P-wave and S-wave arrival times to determine the distance to an earthquake’s epicenter using a travel time curve (Figure 1–2). A travel time curve is a graph that presents the arrival times of P waves and S waves as a function of distance from a seismic source. To use a travel time curve: 1. Subtract the P-wave arrival time from the S-wave arrival time to determine time difference. 2. Extend a drafting compass along the travel time curve’s y-axis, from 0 up to the time difference. 3. Keep the compass open at the determined time difference, and then move it to the right while keeping the compass point that was formerly on 0 along the P-wave curve. 4. Stop moving the compass when the other point lies on the S-wave curve, vertically above the P-wave curve. 5. Extend a vertical line to the graph’s x-axis to determine distance to the earthquake’s epicenter. 2. In Figure 1–1, how much time elapsed between the arrival of the P waves and the arrival of the S waves? __________________________________________________________________________________ 3. Using the arrival time difference determined in question 2, determine the distance to the earthquake’s epicenter using the travel time curve. How far away is the earthquake? __________________________________________________________________________________
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FIGURE 1–2
4. Complete Table 1–3 by calculating the arrival time difference of P waves and S waves and then the distance to seven separate earthquakes. P-Wave Arrival Time
S-Wave Arrival Time
19:12:15
19:18:45
07:34:00
07:37:30
02:28:30
02:33:30
12:16:45
12:20:15
22:58:15
23:06:15
09:01:30
09:11:30
03:15:15
03:19:30
Arrival Time Difference
Distance to Epicenter (km)
TABLE 1–3 Notice in Table 1–3 that only distance, not location, has been determined. Triangulating an epicenter’s location requires sketching a distance circle around each seismometer station. Where the three circles intersect is an epicenter’s location.
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5. Use seismogram data in Table 1–4 and your drawing compass to triangulate the location of an earthquake on Figure 1–3. Identify the earthquake’s epicenter with a star where the three distance circles overlap. Be sure to use the scale provided on Figure 1–3 to adjust the width of your drawing compass.
Station
Distance to Epicenter (km)
Station #1
800
Station #2
700
Station #3
500
TABLE 1–4
Data source: National Atlas
FIGURE 1–3
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Problem-Solving Module #2: Magnitude versus Intensity An earthquake’s magnitude is a quantitative measure of its size. Of the several methods that exist to determine magnitude, “Local Magnitude” (ML) is good for quantifying nearby earthquakes. Determining ML requires knowing 1) the arrival time difference between P waves and S waves, and 2) the amplitude of the largest S wave. Amplitude is an absolute value and is determined by measuring the largest S-wave peak.
FIGURE 2–1 From Figure 2–1 we determine that an earthquake’s arrival time difference is 30 seconds and its amplitude is 23 mm. These two variables can now be plotted on a nomograph—a graphical calculating tool that yields a desired value after two variables are aligned with a straight line. Figure 2–2 is an ML nomograph.
1. Using data from Figure 2–1, plot the arrival time difference on the nomograph’s left vertical axis (notice that this automatically yields the distance to the earthquake’s epicenter). Plot the amplitude on the nomograph’s right vertical axis. Connect both points with a straight line. Your line intersects the middle vertical axis at the earthquake’s ML. What is the ML? _________________________________________________________________________________
2. Use Figures 1–2 and 2–2 to complete the data for each earthquake listed in Table 2–1.
Arrival Time Difference
Distance to Epicenter (km)
P-Wave Arrival
S-Wave Arrival
14:01:04
14:01:14
15
03:12:09
03:12:15
75
01:30:01
01:30:05
5
17:14:10
17:14:55
35
23:14:32
23:14:57
0.7
08:56:17
08:56:57
100 TABLE 2–1
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Amplitude (mm)
Earthquake ML (Magnitude)
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FIGURE 2–2 Every whole number increase in earthquake magnitude yields 10 times more ground shaking and 32 times more energy released. For example, a magnitude 6 earthquake moves the ground 10 times more and releases 32 times more energy than a magnitude 5 earthquake. Here is an example of how to determine how much more (or less) ground shaking occurred, and how much more (or less) energy was released, when comparing two different earthquakes. Assume a magnitude 6.7 earthquake occurred, and compare it to a magnitude 5.3 earthquake.
• The magnitude 6.7 earthquake shook the ground 25 times more because 6.7 – 5.3 = 1.4 magnitude difference 101.4 = 25.11 (rounded to 25)
• The magnitude 6.7 earthquake released 128 times more energy because 6.7 – 5.3 = 1.4 magnitude difference 321.4 = 128 3. How much more ground shaking occurs during a magnitude 6.3 earthquake compared to a magnitude 5.3? (Be sure to show your work.) __________________________________________________________________________________ 4. How much more ground shaking occurs during a magnitude 6.3 earthquake compared to a magnitude 4.7? (Be sure to show your work.) __________________________________________________________________________________ 5. How much more ground shaking occurs during a magnitude 6.3 earthquake compared to a magnitude 2.8? (Be sure to show your work.) __________________________________________________________________________________ 6. How much more energy is released during a magnitude 7.8 earthquake compared to a magnitude 6.8? (Be sure to show your work.) __________________________________________________________________________________ © 2014 W. H. Freeman and Company
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7. How much more energy is released during a magnitude 7.8 earthquake compared to a magnitude 5.3? (Be sure to show your work.) __________________________________________________________________________________ 8. How much more energy is released during a magnitude 7.8 earthquake compared to a magnitude 3.1? (Be sure to show your work.) __________________________________________________________________________________ An earthquake’s intensity is a qualitative measure of the ground shaking at a particular site. It is based on the effects felt by people and on the observed damage to building structures. The modified Mercalli intensity (MMI) scale is the current U.S. standard and has 12 levels of increasing intensity. While it is an arbitrary ranking based on the observed effects of an earthquake, often it has more meaning than an earthquake’s magnitude for people because it classifies the actual experiences of people and buildings. Table 2–2 is the MMI scale. 9. What levels of the MMI scale refer specifically to effects on people? __________________________________________________________________________________ 10. What levels of the MMI scale refer specifically to effects on buildings? __________________________________________________________________________________ Modified Mercalli Intensity Value
Intensity Description
I
Not felt except by a very few under especially favorable conditions.
II
Felt only by a few persons at rest, especially on upper floors of buildings.
III
Felt quite noticeably by persons indoors, especially on upper floors of buildings. Standing cars may rock slightly. Vibrations similar to the passing of a truck.
IV
Felt indoors by many. At night, some awakened. Dishes and doors disturbed; walls make cracking sound. Sensation like heavy truck striking building.
V
Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.
VI
Felt by all. Some heavy furniture moved; a few instances of fallen plaster.
VII
Slight to moderate damage in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.
VIII
Considerable damage in ordinary buildings with partial collapse. Damage great in poorly built structures. Chimneys and factory stacks fall.
IX
Well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.
X
Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.
XI
Few masonry structures remain standing. Bridges destroyed. Rails bent greatly.
XII
Damage total. Lines of sight and level are distorted. Objects thrown into the air. Credit/courtesy of USGS (adapted)
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11. If a magnitude 5 earthquake occurs in an uninhabited region with no buildings, what is its MMI? Explain your reasoning. __________________________________________________________________________________ __________________________________________________________________________________ __________________________________________________________________________________ __________________________________________________________________________________ 12. In your own words, explain the difference between a quantitative value and a qualitative value. __________________________________________________________________________________ __________________________________________________________________________________ __________________________________________________________________________________ __________________________________________________________________________________ 13. Figure 2–3 is a map of western Washington with hypothetical MMI values following an M 6 earthquake. Finish the map by sketching interval lines connecting equal MMI point values.
FIGURE 2–3
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14. Complete Table 2–3 by identifying the city on Figure 2–3 that experienced the described intensity. Each city is used only once.. Described Intensity
City
“After the earthquake my well-built home had some cracks and my chimney lost some bricks.” “It felt like someone hit my house with their car. I woke up, but my neighbor slept right through it.” “Two collectable plates fell and broke, and the pendulum stopped swinging on my clock.” “My home moved 8 inches off its foundation, and none of my doors close properly anymore.” “I didn’t feel a thing, but people on the 8th floor said they noticed some small vibrations.” TABLE 2–3
Summary of Key Terms and Concepts: • An epicenter is the location on the ground’s surface immediately above an earthquake’s focus where ground shaking intensity is the greatest. • The magnitude scale is a quantitative earthquake ranking system based on the amount of produced ground movement. • A magnitude nomogram is a graphical calculating tool that yields an earthquake’s magnitude after the P-wave and S-wave travel time difference and amplitude are both plotted. • The modified Mercalli intensity (MMI) scale is a qualitative measure of ground shaking and is based on the effects felt by people and on the observed damage to buildings. • A P wave moves through Earth with a compression and dilation motion and is the fastest seismic wave. • An S wave moves through Earth perpendicular to its direction of travel and arrives at a seismometer after a P wave. • A seismograph (or seismometer) is an instrument used to detect, measure, and record ground shaking. • A seismogram is the record of ground shaking produced by a seismograph (or seismometer). • A travel time curve is a graph that presents the arrival times of P waves and S waves as a function of distance from a seismic source.
© 2014 W. H. Freeman and Company
