Name___________________________ Section________ Date__________ Geos251- Physical Geology Lab #8 EARTHQUAKES Objectives: Understand what causes an earthquake Explain with which geologic settings earthquakes are associated Explain the concentration of earthquakes in certain global regions Understand and define the following terms: o Hypocenter/focus o Epicenter o P-waves o S-waves o Surface waves (Rayleigh and Love) Work with: o Reading and interpreting topographic maps o Constructing contour maps o Constructing topographic profiles Recognize that different seismic waves travel differently through the Earth Triangulate an earthquake epicenter using a travel-time curve and seismograph readings Calculate the magnitude of an earthquake Estimate crustal shortening using a geologic cross-section Due Date: Monday, 11/12 at 12; Tuesday, 11/13 at 12:30; Wednesday, 11/14 at 12 No late labs will be accepted. Graded work: questions 1-18, all contour maps, cross-sections, and worksheets Note: This is the most quantitative lab of the semester. Please read the handout carefully as it contains the information that you need to complete this lab. Introduction If you watch the news or read the paper, then you are superficially familiar with earthquakes. This lab is designed to introduce you to some basic principles of seismology and how geologists use these principles to make calculations about earthquakes. You will also think about earthquake occurrence relative to geologic settings. Most earthquakes occur when stress from tectonic forces causes sudden movement of blocks in the Earth. As rocks continue to experience stress, energy builds slowly. Eventually, so much energy is stored that friction is overcome, allowing the two rock bodies to slip past each other along a fault plane. This release of energy generates seismic waves, which are responsible for earthquakes. Questions: 1. Name the three types of stress that we talked about in the structural geology lab and illustrate them with arrows. 2. Do you think that earthquakes are examples of brittle or elastic deformation in rocks? Briefly explain your answer. 3. Based on your answer to question 1, do earthquakes occur in the Earth’s: (circle one) (a) crust? (b) mantle? (c) core? EXERCISE 1: GLOBAL DISTRIBUTION OF EARTHQUAKES 4. The world map in the northeast corner of the room illustrates, among other things, the global distribution of over 20,000 earthquakes that occurred between 1960 and 1990. Study this map and answer the following questions: (a) Each data point represents an earthquake. In general, are the earthquakes distributed randomly, or does their occurrence follow a pattern? If applicable, describe the pattern. (b) Hypothesize the driving force behind these earthquakes. (c) Other than earthquakes, name another geologic phenomenon that correlates with this pattern of seismicity. EXERCISE 2: BENIOFF ZONES Among the world’s remaining frontiers are the deepest parts of the sea floor -- in the trenches associated with subduction zones. These trenches mark areas in which oceanic plates are being subducted or are beginning to descend into the mantle. As the oceanic plate goes further down into the mantle, stresses and friction along the subduction zone produce earthquakes. This surface of earthquakes is called the Benioff zone, and it defines the top of the plate that is being subducted. Geologists can trace the downgoing plate into the mantle if they have data on the depth and location of earthquakes in the Benioff zone. Figure A shows the relationship between the Benioff zone (denoted by stars), trench, and the subducting plate. . 5. In lecture, you have learned that subduction zones involve oceanic crust subducting beneath continental crust. Why is it this way and not the other way around? In this section of the lab, you will draw contour lines to produce a topographic profile and a profile of a Benioff zone. Please read the directions carefully as you go. Locate and read about the following worksheets: Benioff Zone Topographic/Bathymetric Contour Map Each data point on this map represents an elevation (in meters) above or below sea level. -Positive numbers represent the elevations of points on land. -Zero represents the continental shoreline (sea level.) -Negative numbers represent the depth to the ocean floor. -Elevation 4000 meters represents the location of a volcano produced by melting within the subduction zone. Benioff Zone Depth to Earthquake Contour Map Again, there are data points representing elevations above or below sea level. Elevation zero represents the continental shoreline. On this map, you also see a series of numbers in boxes. Each boxed number represents the depth to an earthquake…in other words, the depth to the Benioff zone. (For example, if you see “100” with a box around it, this means that an earthquake originated 100 km below Earth’s surface.) For both maps, notice the cross-section line running from the left of the map to the right of the map. Walking along the cross-section line, you would be walking on land, reach the shoreline, and then enter the ocean. Once in the ocean, you would drop down into a trench produced by the subduction zone. Topographic/Bathymetric Profile This worksheet is where you will draw your profiles. The top profile will represent the topography (what the Earth looks like on land) and the bathymetry (what the Earth looks like below the ocean floor). The bottom profile will represent the Benioff zone. Step 1: Contouring (USE PENCIL!!) On the Benioff Zone Topographic/Bathymetric Contour Map: Draw contour lines using a contour interval of 1000 m. Contouring the negative numbers will produce a bathymetric map of the trench and the seafloor. Contouring the positive numbers will produce a topographic map of the land and volcanic arc. The shoreline (zero elevation) and several other contours are shown as examples. On the Benioff Zone Depth to Earthquake Contour Map: Draw contour lines using a contour interval of 100 km. Contour the depth to the earthquakes using the numbers in boxes. This will illustrate the depth of the upper surface of the oceanic plate subducting beneath the continental plate. The 100 km contour line is shown as an example. Step 2: Profiling Draw a topographic/bathymetric profile following the cross-section line A - A’ on the Benioff Zone Topographic/Bathymetric Contour Map. Note that the vertical scale of this profile is exaggerated, so slopes appear steeper than they actually are. Then draw a profile that shows the Benioff zone and the down-going plate following the B - B’ profile line found on the Benioff Zone Depth to Earthquake Contour Map. In this section, the vertical scale equals the horizontal scale, so there is no exaggeration. 6. Using a protractor, determine the average dip of the Benioff zone. 7. What is the approximate depth to the top of the downgoing plate directly beneath the chain of volcano (in kilometers)? 8. Most volcanic arcs in the world are located where the Benioff zone (which represents the top of the downgoing plate!) is at approximately the depth that you determined in question 6. Why do you think this is so? EXERCISE 3: DETERMINATION OF EARTHQUAKE EPICENTER Earthquakes and Seismic Waves Earthquakes are caused by sudden movements of large rock masses. When rocks suddenly slip past each other along a fault, seismic waves are produced. The location within the Earth where the seismic waves are produced is called the earthquake’s focus, or hypocenter. The location on the surface of the Earth directly above the focus is called the epicenter. Two major categories of seismic waves, body waves and surface waves, are produced at the foci of earthquakes. Body waves travel directly through the earth in all directions from the focus. Two types of body waves, P-waves (Primary waves) and S-waves (Secondary waves), are produced during earthquakes. This table compares some of the characteristics of P-waves, S-waves, and surface waves. Name Type Motion Can travel Relative velocity Arrival at through: seismic stations P-wave Body Compressional: Solids, liquids, Fastest seismic First (primary) Alternate and gases waves. ~300 m/s in “push-pull” between (Earth’s air; 300-1000 m/s compressing interior) in soil; over 5000 and expanding m/s in solid rock at media through the surface which they travel in the same direction that the waves move S-wave Body Transverse Solids only More slowly than P Second (secondary) (shear): side- (Earth’s waves; faster than “Side to side” to-side and interior) surface waves up/down vibration of rock Surface waves Surface Travels on Slower than either Last (Love and Earth’s surface body wave, but Rayleigh) only, in all greater amplitude directions from (wave height). the epicenter More destructive than body waves. Earthquake detection: Seismographs are devices that record ground motion (seismic waves) at a site. They often consist of a pen moving across a piece of paper, which is attached to a rotating drum. The pen is electronically linked to a seismometer, which detects ground motion caused by body and surface waves. The pen is moved back and forth by impulses from the seismometer, recording the amount and direction of ground motion. The record of ground motion produced by a seismograph is a seismogram. Modern seismometers record data digitally, making interpretation of seismic data much easier than it used to be. Your seismogram worksheet contains three seismograms from three locations on the map: Site X on the volcanic island, Site Y in the river delta flats, and Site Z in the alluvial plains. The seismograms all record ground motion (seismic waves) from the same earthquake, but the time, location, and magnitude of the earthquake are unknown. Notice that the first arrivals of the different types of seismic waves are marked on the seismograms. An important (and very difficult) part of interpreting seismograms is determining the first arrival times of the different seismic waves. P-waves are fairly easy to identify because they are the first seismic waves to appear on a seismogram. Notice that on the seismograms in the figure below, the P-waves quickly build up to maximum amplitude (wave height), then slowly decrease in size until interrupted by the next arrivals, the S-waves. The first arrival of S-waves can be identified on a seismogram as a sudden increase in wave amplitude among the waning P-waves. Surface waves are the last (and longest lasting) seismic waves to appear on a seismogram. The first surface-wave arrival can be identified as a marked increase in seismic wave amplitude, much greater than P- or S-waves. Determining the epicenter of an earthquake Remember: each type of seismic wave travels at a different velocity through the Earth. The relationships between velocities of P-, S-, and surface waves are fairly well known and can be used to determine distance to the focus or epicenter of an earthquake. A seismograph acts as a stopwatch, recording the time when P-, S-, and surface waves pass. By knowing this and the velocities of each type of seismic wave, the distance to the focus of the earthquake can be calculated. Alternatively, a travel-time curve can be used to obtain an estimate of the distance to the earthquake’s focus. This estimate is made by correlating the first arrivals of each wave type with lines on a travel-time curve. The graph gives the distance to the focus, not the epicenter. (But if the focus is shallow, the distance to the epicenter is roughly the same as the distance to the focus.) The following seismogram worksheet contains three seismograms from the different locations of sites X, Y, and Z, all on the same time scale. (Time is not shown in detail because we are not concerned with any time calculations in this exercise.) Directions for determining the epicenter: (refer to the figure on the previous page) -place each seismogram (for stations X, Y, and Z) over the large travel-time graph -move the seismogram along the curves until the first P, S, and surface waves marked on the seismogram intersect the corresponding lines on the travel time graph. Be as precise as possible. -remember to keep the axes of both figures parallel during this process. -read the distance to the epicenter off the horizontal scale of the large travel-time graph 9. How far is each seismic station from the earthquake’s epicenter? Use the seismograms and travel-time curve to determine the distance. (Units in kilometers.) Site X ___________________________________ Site Y ___________________________________ Site Z ___________________________________ 10. Use a compass to draw a circle around each of the above Section VI sites, representing the distance from that site to the epicenter (see the above for an example). If the entire circle won’t fit on the map, just draw what you can. The circles should all intersect or nearly intersect at one point -- the earthquake’s epicenter! (Tips: Don’t forget to convert the distance units of kilometers to centimeters using the map scale. Use the converted value to set the compass radius.) 11. Draw a star where all three circles intersect...the star represents the earthquake epicenter. 12. Which geologic structure is closest to the epicenter? Which type(s) of force(s) generate this type of structure? EXERCISE 4: DETERMINATION OF AN EARTHQUAKE’S MAGNITUDE Seismograms also provide data about an earthquake’s magnitude. Magnitude is a measure of the energy released by the earthquake at its focus and is related to the height of the waves recorded on seismographs. Generally, strong earthquakes produce bigger seismic waves than weak earthquakes. The vertical axis on a seismogram measures the amplitude (distance between middle and top) of seismic waves. For the seismograms in this exercise, amplitude is measured in micrometers ( 1,000,000 micrometers = 1 meter). Magnitude can be measured in several ways that yield similar, but not identical, results. Magnitude can be calculated using the amplitude of body waves or surface waves. Because the seismograms in this exercise are long-period, surface wave magnitude is the easiest to calculate. Formula: Ms = log10(a) + 1.66[log10(s)] + 2.0 Where: Ms = magnitude based on surface waves a = amplitude of surface waves, measured in micrometers s = distance to the epicenter, measured in km (all logs base 10) 13. Using the surface-wave magnitude formula given in the exercise, calculate the magnitude of the earthquake shown on the Site X, Site Y, and Site Z seismograms. Show your work. Site X Site Y Site Z 14. Because of geologic variations along the paths of the seismic waves, the calculated magnitudes are slightly different at each location. The average magnitude is a better estimate of the magnitude of the earthquake. What is the average of your calculated magnitudes? EXERCISE 4: CRUSTAL SHORTENING (review from structural geology lab) Thrust faults result in the overall shortening of continental crust. This occurs due to compressive forces that cause Earth’s crust to essentially crumple. Terranes can be displaced up to hundreds of kilometers along a thrust fault. In fact, thrust faults are responsible for much of the uplift of the Rocky Mountains in the western U.S. and Canada. Generally, the greater the amount of shortening, the greater the number of thrust faults are needed to accommodate the total amount of shortening. Note that the cross-section on the following page has many thrust faults, indicating that the crust was shortened significantly. We do not know how far the “marker bed” (shown in black) traveled until it came to rest on top of the fold-belt rocks. However, you do have the means of making an estimate. The following steps will allow you estimate the amount of crustal shortening that the thrust faults have produced. 15. First, mark the faults using a marker or highlighter. Then label each hanging wall (HW) and footwall (FW) for each fault. 16. Have the hanging walls moved up or down relative to the adjacent footwalls? Which type of force (that you learned about last week) causes these thrust faults? 17. Determine the amount of crustal shortening in kilometers and as a percentage as follows: a) Calculate and record the present length of the entire cross-section in kilometers. b) Find the total length of the marker bed by measuring each individual section. Note that folds in the right, upper portion of the cross-section need to be “unfolded” because the layers were originally horizontal. Record your answer. c) Calculate and record the amount of shortening in kilometers or in percent as follows: kilometers: [original length - present length] percent: [(100) x (original length - present length)/(original length)] 18. What surface process has been occurring continuously since the rock layers were deposited? Based on this knowledge, do you think that the percentage of crustal shortening that you just calculated is a minimum value (ie, this is the minimum amount that the crust has shortened), or a maximum value? Explain your answer.