Supplementary Materials for

Transcription

1 Supplementary Materials for Engaging Students in Earthquakes via Real-Time Data and Decisions Anne E. Egger *To whom correspondence should be addressed. This PDF file includes Materials and Methods Published 29 June 2012, Science 336, 1654 (2012) DOI: /science

2 Context of the Seismicity and Relative Risk Module Anne E. Egger April 29, 2012 I have used this module (or one like it) in several different types of courses, but most commonly in an introductory geoscience course. A description of that course, a syllabus, and the main activities included in that course are all available here: This module comes near the beginning of the course, but after a thorough introduction to and exploration of the data that support plate tectonics. A description of the activity and supporting resources for how I cover plate tectonics prior to the use of this module is available here: This activity provides them with general background about the distribution of earthquakes and their relationship to plate boundaries, which is the starting point for this module. Prior to starting the seismicity module, they have also read the following online readings: Plates, plate boundaries, and driving forces Earth Structure The slides provided as supplementary materials here are meant to show an example of how to facilitate this activity, not as a comprehensive introduction to earthquakes. I do not go into detail about the Richter scale of magnitude or about how earthquakes are located. One of the things I enjoy about using this activity, however, is that often students generate these kinds of questions on their own, and I will provide some additional impromptu lectures on specific topics. Through our discussion of the difference between magnitude and intensity, we generate a working definition of the difference between hazards and risks that helps them frame their follow-up writing assignment.

3 Seismicity and types of faults In this activity, you will work in pairs to explore real time data available on the web at the U.S. Geological Survey Earthquake Hazards Program web site ( Your exploration is interspersed with short lectures to clarify or introduce new concepts. Learning goals Content By the end of this activity, you will be able to: 1. Describe and interpret the distribution of earthquakes around the world 2. Describe the types of data collected by earthquake monitoring programs 3. Describe the difference between intensity and magnitude and how they are measured 4. Describe how earthquake focal mechanisms relate to plate motion 5. Describe the frequency of earthquake occurrence Skills By the end of this activity, you will be able to: 1. Navigate the U.S. Geological Survey Earthquake Hazards Program web site to find and collect real time data about earthquakes 2. Read a focal mechanism indicator and ShakeMap Higher order thinking By the end of this activity, you will be able to: 1. Predict the focal mechanism and effects of an earthquake based on its location 2 Analyze the relative risks associated with a living in a variety of earthquake prone regions You can type your answers into this document and save it as you go. You will not turn it in, but consider this as guided note taking. Make sure you and your partner both end up with copies. PART 1: Exploring recent seismic activity We ll start by looking at real time earthquake data at 1. Click on the Real time Earthquake Map. You should see a map showing earthquakes that have occurred around the world in the last 7 days. In the control panel on the left hand side of the page, adjust the magnitude slider down to 1. Describe the distribution of these earthquakes: how many are there on the map, where are they located, where are the largest earthquakes, where are the densest concentrations. Don't go overboard here just make a quick survey. 2. Above the map, click on the link to jump to the US. Again, make sure to adjust the magnitude slider down to 1. Same thing here describe the distribution of quakes in the US in the past week. 3. As we discussed in class, most earthquakes fall along or near the plate boundaries. Does this generally hold true for earthquakes in the last 7 days in

4 the US? Are there any "unexpected" areas with earthquakes? If so, what are your thoughts about why they are there? Don't worry about being right or wrong, we'll discuss this in a bit. Feel free to speculate. PAUSE we ll come together as a group and discuss at this point. PART II: Data collected about earthquakes Let s now explore a recent large earthquake. On the page with the world map, look at the list of earthquakes below the map. Click on the M (magnitude) in the leftmost column in order to sort the table by magnitude. (You might need to click it again so that it sorts from largest to smallest.) Look through the largest magnitudes and select one that is in a relatively populated place. If there aren t any greater than magnitude 6, you can adjust the Earthquake Age slider in the control panel on the left to show more earthquakes. When you click on an earthquake in the list, a new window should open with a zoomed in map of just the area around that earthquake and a summary. The following questions are about the type of data collected for each earthquake and the kinds of processing that data undergoes. 4. Which earthquake are you looking at? What is the magnitude, depth, and location? (This information is all on the Summary page.) 5. Click on Historical Seismicity in the left navigation. Initially, this will show seismicity just in the last year. You can look at additional earthquakes since 1990 and 1900 by clicking on those links in the left navigation. How does this earthquake fit into the historical record for this area? Does it seem to be similar to past earthquakes, or is it different? 6. Now take a look at the ShakeMap link above the map. What is the ShakeMap showing you, and how does it differ from the magnitude? PAUSE we ll come together as a group and discuss at this point. PART III: What earthquakes can tell us about fault types Now that you ve had a chance to look at a single earthquake in some detail, we ll look at how the kinds of earthquakes that occur are related to the kinds of faults they occur along and the type of motion that occurs. 7. Click on Technical in the left navigation for your earthquake and look at the moment tensor on the right. Describe the "beach ball" that you see, and relate it

5 to the type of fault motion that occurred. (You might get a hint from the Summary tab as well.) 8. Finally, how does this earthquake fit into the tectonic setting of the region? And how does that tectonic setting translate to seismic hazards?

6 Earthquakes What causes them How we measure them What they tell us about faults Sunday, April 1, 2012

7 Part I What causes them? Sunday, April 1, 2012

8 We are already well aware that earthquakes are distributed unevenly around the world, and that the locations of these earthquakes are very strongly related to the locations of the plate boundaries. The depths of the earthquakes vary according to the type of plate boundary as well. In general, they occur because the plates are moving different directions, meeting up at the boundaries.

9 How do we reconcile discrete earthquakes with being able to measure rates of plate motion using GPS data?

10 We need to reconcile the idea that plates are constantly moving with the discrete earthquake events. In fact, plates ARE constantly moving, but they are stuck to each other along the faults due to friction. So they move in a stick-slip fashion--once enough strain builds up in the crust,

11 The plates slide past each other and an earthquake results. The rocks on either side of the fault also rebound to their original shape. This is called elastic rebound, and the foundation for our understanding of this process was laid in the wake of the 1906 San Francisco earthquake, about which you will hear much more in section and GES 1X. This diagram represents a terrain more like what we have along the San Andreas Fault, by the way - not the Tonga trench. But the concept of stick-slip is the same. We can actually measure this around real faults with strainmeters.

12 We can actually measure the strain as it builds up, usually by measuring deformation in a borehole (which starts out round, becomes progressively deformed over time, flattening perpendicular to the greatest stress). When the stress is lessened or released suddenly in an earthquake, the borehole may return to its original shape. Such a sudden change is recorded here in this tensor strain data. This is an earthquake of particular interest which we will look at in detail a little bit later.

13 Your job now: Complete part 1 of the assignment Sunday, April 1, 2012

14 Why do earthquakes occur away from plate boundaries? Sunday, April 1, 2012 Undoubtedly, you have now found that many earthquakes do NOT occur along plate boundaries. Stick-slip doesn t account for all of them, nor does tectonic plate motion. What are our other options?

15 Other earthquakes aren t associated with movement of plates, but with the movement of something else, like magma. This picture is from April It s Mt. St. Helens, which began erupting September 04, just before classes started. We ll talk a lot more about volcanoes and volcanic eruptions in a few weeks, but let s just focus on the earthquakes that are associated with this particular volcano right now.

16 Earthquakes in the last year Sunday, April 1, 2012 LAST DECADE: Red dots in the last month, green in the last year. LAST YEAR: Red dots in the last day, green in the last month. Here s a map showing epicenters. Not surprisingly, there have been a lot of recent earthquakes (small ones) right underneath Mt. St. Helens. But it is not rocks moving against each other that cause these earthquakes, because that isn t happening.

17 Earthquakes in last decade

18 Here s a cross-section showing the depth of these earthquakes, and we can get a better idea of what s going on. The most recent earthquakes are nearest the surface, which causes geologists to think that magma is moving up towards the surface as a harbinger of an imminent eruption. It almost looks like there are two DEEPER conduits leading to the same main vent.

19 When you look at the same data over a longer span of time, you can also see much deeper events that may indicate migration of fluids up from deeper levels. However, notice that the ENTIRE depth scale here fits into the shallow designation on our other maps: less than 33 km. So although this is near a plate boundary, the earthquakes associated with plate motion here are much, much deeper, while these occur relatively shallowly in the crust. This is not stick-slip motion.

20 When you look at the same data over a longer span of time, you can also see much deeper events that may indicate migration of fluids up from deeper levels. However, notice that the ENTIRE depth scale here fits into the shallow designation on our other maps: less than 33 km. So although this is near a plate boundary, the earthquakes associated with plate motion here are much, much deeper, while these occur relatively shallowly in the crust. This is not stick-slip motion.

21 When you look at the same data over a longer span of time, you can also see much deeper events that may indicate migration of fluids up from deeper levels. However, notice that the ENTIRE depth scale here fits into the shallow designation on our other maps: less than 33 km. So although this is near a plate boundary, the earthquakes associated with plate motion here are much, much deeper, while these occur relatively shallowly in the crust. This is not stick-slip motion.

22 Why do earthquakes occur away from plate boundaries and away from volcanoes? Sunday, April 1, 2012 Still, there are LOTS of other earthquakes that aren t anywhere near plate boundaries OR volcanoes, and these appear to have multiple causes.

23 New Madrid Fault Zone is probably the most famous, where a series of magnitude 8 earthquakes struck in the winter of What the heck? There are several things going on here: isostatic rebound of the crust following the removal of the ice sheets is one thing. More importantly, however, is that this is the scene of a failed rift--the Atlantic Ocean tried to open up here, leaving behind a deep weakness in the crust. Whenever anything acts to stress the crust, that stress is accommodated along this pre-existing weakness.

24 Your job now: Part II: What do we measure? Sunday, April 1, 2012

25 Here s some of the information we get - how do we get it?

26 Location of seismometers monitored by Berkeley Seismological lab

27 Location of seismometers monitored by Berkeley Seismological lab

28 Intensity - different from magnitude. Intensity is how we feel it on the surface. Naturally, intensity does not change abruptly at county lines, but this is how it has been mapped in this instance.

29 One way to measure that is through how people feel it.

30 One way to measure that is through how people feel it.

31 1906 San Francisco and 1989 Loma Prieta Earthquakes USGS Shaking Intensity Animations Sunday, April 1,

32 animation

33 How do earthquakes help us understand faults? Sunday, April 1, 2012 The focal mechanism relates us back to not only different types of plate boundaries, but on a smaller scale, different types of faults. Not all faults are the same!

34 Three types of faults Normal Reverse/thrust Strike-slip/transform Sunday, April 1, 2012 There are three basic types of faults: normal, reverse, and strike-slip. They form according to the regional tectonic stresses.

35 Three types of faults reflect three types of stress Normal à Tension Reverse/thrust à Compression Strike-slip/transform à Shear Sunday, April 1, 2012 There are three basic types of faults: normal, reverse, and strike-slip. They form according to the regional tectonic stresses.

36 Thrust (reverse) faults Sunday, April 1, 2012

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38 Normal faults Sunday, April 1, 2012 Normal faults

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40 Strike-slip faults

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42 This brings us to

43 Your job now: Part III: Find the moment tensor solutions Sunday, April 1, 2012

44 We also measure the moment tensor, which tells us more about the directionality of the earthquake. White means tension, dark color means compression. These vary according to the different types of earthquakes, and are one of the ways we can tell that the deepest earthquakes are not quite the same as the ones nearer the surface. And it adds more data to our plate boundary observations--depth and moment tensors vary with the type of plate boundary,

45 We also measure the moment tensor, which tells us more about the directionality of the earthquake. White means tension, dark color means compression. These vary according to the different types of earthquakes, and are one of the ways we can tell that the deepest earthquakes are not quite the same as the ones nearer the surface. And it adds more data to our plate boundary observations--depth and moment tensors vary with the type of plate boundary,

46 Most earthquakes, therefore, appear to be caused by plate motion, as the rocks that make up the plates rub against each other. Earthquakes in the Tonga trench, for example, go very, very deep. Remember how thick the lithosphere is? And below that is the aesthenosphere, which is too weak to generate earthquakes. So what is causing the deepest earthquakes? Probably NOT plates grinding against each other. Instead, it s more likely the breaking apart of the slab as it goes deeper and deeper. We ll come back to this later when we look at focal mechanisms, which help us determine the directionality of the earthquake motion.

47 What does this mean for us, here?

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50 Writing Assignment: Relative Risk Content goal Writing goal Apply knowledge learned in class about earthquakes to understand the risks associated with living in different earthquake-prone areas. Compare and contrast data; develop an argument based on scientific data. You have had a chance to take a look at the San Andreas Fault in some detail, and you ve explored the USGS earthquake website. Now it s time to use some of that information. Suppose you re nearing graduation (as some of you are) and you ve decided to avoid real life further by going to graduate school. You re choosing between schools in three cities: San Francisco, CA; Seattle, WA; and Salt Lake City, UT. All three of these are sizable cities prone to earthquakes. You are curious what is the relative risk due to earthquakes you face in each of these cities? You need to start off by collecting data: the spatial distribution, frequency, depth distribution, focal mechanisms, magnitude and shaking intensity of potentially destructive earthquakes in and around each location and the types of faults they occur along (please cite all sources that you use). After a short introduction, you should summarize all of this data in a table to compare the different locations. Follow that up with an analysis of the relative risk faced by residents in each of those locations. This should be based on three things: what is the probability of a potentially destructive earthquake, how destructive is it likely to be, and how prepared are the cities to deal with major earthquakes? Finally, make an argument for which city you would choose to live in based on your assessment of the relative risk. There is no correct answer really. I m looking for a little analysis of the available data. Here s how you will be graded (20 points total): Points Content (15 points 5 per location) Composition (5 points) 5 Complete descriptions of earthquakes in each area covering all data Hazards and risks evaluated well Overall structure easy to follow. Individual paragraphs well constructed. No grammar or spelling mistakes. 4 Description lacks one important component OR Hazards & risks only minimally addressed 3 Description lacks one or more important component Hazards & risks minimally addressed 2 Descriptions seriously lacking detail No hazards or risks included 1 Very little detail whatsoever Little structure 0 No effort No effort Overall structure relatively easy to follow One or two paragraphs lack structure A few grammar or spelling mistakes. Overall structure wanders A few paragraphs lack structure Several grammar or spelling mistakes Overall structure difficult to follow Most paragraphs lack structure Several grammar or spelling mistakes