5. EARTHQUAKES AND EARTH S INTERIOR

Transcription

1 LAST NAME (ALL IN CAPS): FIRST NAME: 5. EARTHQUAKES AND EARTH S INTERIOR EARTHQUAKE An earthquake is ground shaking caused by sudden and rapid movement of one block of rock slipping past another along faults. Hypocenter (focus): point in the Earth s crust or upper mantle where an earthquake originates Epicenter: A point of the Earth s surface that lies directly above the hypocenter. Seismograph: Instrument that detect and record seismic waves Seismogram: The recordings made by seismographs GLOBAL DISTRIBUTION OF EARTHQUAKES About 95% of all earthquakes occur in the Circum-Pacific belt (coastal regions of Chile, Central America, Indonesia, Japan, and Alaska) and along the Alpine-Himalayan belt (Europe-Asia). THE EARTH S INTERIOR (FIGURE 6.8) The Earth s interior is divided into three compositionally distinct layers: the crust, mantle, and core. Crust: Earth s thin, rocky outer skin. There are two types of crust: continental crust, made of granitic rocks, and oceanic crust, composed of basaltic rocks. Mantle: A solid, rocky shell made of denser minerals; extends to a depth of 1800 miles. More than 82% of Earth s volume is contained in the Mantle. Lithosphere: Consists of the entire crust and uppermost mantle and forms Earth s relatively cool, rigid outer shell that averages about 60 miles in thickness. Asthenosphere: A soft (weak) layer that extends to a depth of about 200 miles beneath the stiff lithosphere. Outer core: A liquid layer 1410 miles thick that is composed mainly of an iron-nickel alloy. Inner core: A solid sphere with a radius of 754 miles, and like the outer core, it is composed of an iron-nickel alloy. SEISMIC WAVES PROPERTIES Seismic waves (P and S waves) generated by earthquakes are used to infer about the interior composition and rigidity of Earth. P waves can travel through all media, solid and liquid; S waves can travel through solid materials only. Seismic waves travel faster when rock is rigid (stiff) or less compressible. When rock is hotter, it becomes less stiff and waves travel more slowly. Waves also travel faster in mafic rocks (rich in Fe and Mg) compared to felsic rocks (rich in Al and Si). Page 1 of 7

2 1. DETERMINING THE DISTANCE TO THE EPICENTER Data required: S & P arrival times from a seismogram (Figure EQ-1); the difference in the arrival times between the P and S waves is then used to determine the distance to the earthquake epicenter by reading off from the chart in Figure EQ-2. FIGURE EQ-1 Seismogram of a New Guinea earthquake recorded at a location in Australia. FIGURE EQ-2. Graph to determine the distance to the epicenter when S-P time interval is known. Page 2 of 7

3 2. LOCATING THE EPICENTER OF AN EARTHQUAKE o Three seismographs needed to locate an epicenter o Each station registers the arrival of the first P wave and the first S wave for each earthquake o The difference in P and S wave arrival times is called the S-P interval o A travel time distance graph then determines each station s distance to the epicenter by reading from a graph (Figure EQ-2); the earthquake shown in Figure EQ-2 has a P-S interval of 5 minutes that corresponds to a distance to epicenter of 2100 miles (or 3400 km). o A circle with radius equal to distance to the epicenter (i.e miles) is drawn around the station o Following a similar procedure, draw circles for the other two stations. The point where all three circles intersect is the location or epicenter of the earthquake. 3. MEASURING THE SIZE OF EARTHQUAKES: MAGNITUDE (RICHTER SCALE) Procedures & required data to determine the Richter Magnitude: I) Determine the S-P interval from your seismogram (top box in figure EQ-3). The S-P interval is 24 seconds. On the left hand column (lower part of figure EQ-3) which is labeled Distance, km; S-P, sec., place a mark at 24 seconds. II) Using a millimeter scaled ruler, measure the amplitude (length) of the largest wave. In the example given on figure EQ-3, the amplitude is 23 mm. Mark this value on the column (right hand side of figure) marked Amplitude, mm. III) To determine the Richter magnitude, join the two points you marked in (a) and (b) above, using a straight line. That line crosses the middle vertical line, labeled Magnitude, M L. The value which the line crosses over is the Magnitude of the earthquake. In the example given in figure EQ-3, the Magnitude is 5. FIGURE EQ-3. Method to determine the Richter Magnitude of earthquakes. Page 3 of 7

4 QUESTIONS Instructions: Refer to Exercise 6 in your Lab Manual on pages to answer the questions in this work sheet. Your work will be graded on the basis of its accuracy, completion, clarity, neatness, legibility, and correct spelling of scientific terms. When you are done with your lab work, clean your desk and leave all materials you worked with in the same way you found them! EARTHQUAKES Answer Q1-4 by referring to Figure 6.2. Q1. How many minutes elapsed between the arrival of the first P wave and the arrival of the first S wave? Minutes elapsed: Q2. How many minutes elapsed between the arrival of the first P wave and the first surface wave? Minutes elapsed: Q3. How much time elapsed between the arrival of the first S wave and the first surface wave? Minutes: Q4. Is the maximum amplitude (wave height) of the surface waves (less than or greater than) the maximum amplitude of the P waves? The maximum amplitude of the surface waves is than that of the P waves Q5. Using the seismograms given in Figure 6.5, enter the S-P interval, in minutes. S-P interval New York, NY Nome, Alaska Guadalajara, Mexico Q6. Using the S-P time intervals (Q5, above) and Figure EQ-2 (page 1, this handout), determine the distance to epicenter for each recording station. New York, NY Kilometers Nome, Alaska Guadalajara, Mexico Kilometers Kilometers Page 4 of 7

5 Q7. I) Use Figure 6.6 (see below) and a drafting compass to draw a circle around each of the three recording stations. The radius of each circle, in miles, is equal to the distance from epicenter determined for the station in Q6. Note: Use the distance scale provided on the map (Figure 6.6). II) What are the approximate latitude and longitude of the epicenter of the earthquake that was recorded by the three stations? Lat. and Long. must be written in formal notations, with the appropriate suffixes (N, E, W, or S). Latitude: Longitude: Q8. What is the name of a major fault that occurs near the epicenter? FIGURE 6.6. Map for locating the earthquake s epicenter Q9. Refer to Figure 6.7 in your Lab manual. With what plate boundary type is each of the following earthquake belt associated? I) Western and south-western Pacific Ocean basin: II) Western South America: III) Mid-Atlantic Ocean basin: Page 5 of 7

6 Use the information in Table 8.2 in your Lab Manual to answer Q Q10. Plot the temperature values from Table 6.2 on the graph in figure 6.10 (given below). Using a pencil draw a line to connect the points. Label the line geothermal gradient. Your plot must be neat. Points may be deducted for plots drawn in pen or are not neat enough. Figure Graph for plotting temperature curve. Q11. Referring to the graph in Figure 6.10, the Earth s internal temperature increase at a rate with increasing depth. Choose one. A) Constant B) Changing Q12. The rate of temperature increase from the surface to 100 km is than the rate of increase below 100 km? Choose one. A) Greater B) Less Q13. The temperature at the base of the lithosphere (~100 km below the surface) is approx.. Choose one. A) 600 o C B) 1400 o C C) 1800 o C Page 6 of 7

7 EARTH S INTERIOR Figure 6.9 shows the average velocities of P and S waves at various depths. Use this figure to complete Q For answers involving numbers, you must estimate as accurately as possible. Points will be deducted if answers are off by more than 0.5 from the correct answer. Q14. The velocity of P waves and S waves with increased depth in the lithosphere? A) Increase B) Decrease Q15. What are the approximate velocities of P and S waves at the very bottom of the lithosphere? P waves velocity: km/sec S waves velocity: km/sec Q16. The velocity of P waves and S waves immediately below the lithosphere? A) Increase B) Decrease Q17. The change in velocity of seismic waves as they enter the asthenosphere indicates that the asthenosphere is rigid than the lithosphere. A) More B) Less Q18. How does the velocity of seismic waves change with increasing depth in the lower mantle? Q19. The change in velocity of seismic waves with increasing depth in the lower mantle indicates that the rocks in the mantle becomes rigid with depth. A) More B) Less Q20. What happens to S waves when they reach the outer core, and what does this indicate about this layer? Q21. In the outer core, P waves in velocity with depth. A) Increase B) Decrease Q22. What are the approximate velocities of P and S waves at the bottom of the mantle? P waves velocity: km/sec S waves velocity: km/sec Q23. What are the approximate velocities of P and S waves at the bottom of the inner core? P waves velocity: km/sec S waves velocity: km/sec Q24. Based on your answers to Q22 and Q23, compare the rigidity of the material in Earth s inner core to that of the lowermost mantle. Page 7 of 7