Earthquakes

= USE THIS DOCUMENT TO ANSWER ALL QUESTIONS = = = =Chile's Buckling Part Of Earthquake Belt= by [|Richard Harris] [|www.NPR.org] February 28, 2010

Chile's 8.8 magnitude earthquake occurred on one of the most seismically active areas of the planet. In fact, the largest earthquake ever measured stuck not far away, back in 1960. It was a magnitude 9.5. It, too, spawned a tsunami that swept across the Pacific. Big earthquakes are usually caused by events that occur on a truly planetary scale — that is, giant chunks of the Earth's crust — tectonic plates — that are slowly moving around. Seismologist Kate Hutton at Caltech says the coast of Chile is the perfect example. "It's very seismically active because it's like California," she says. "It's on the boundary between two of the Earth's major tectonic plates. In this case, it's the Nazca plate and the South American plate." The Nazca plate is moving east at the rate of about 3 inches a year. Since South America is already sitting to its east, the plate has nowhere to go but down. It plows beneath the South American plate. Over geologic time, the result is obvious. "The deformation is causing an ocean trench — a very deep area offshore — and very high mountains, the Andes Mountains, with the volcanoes and everything on shore," Hutton says. In the short term, though, strain simply builds up in the rock. "When it finally does break, there's a lot of stored energy within the deformed crust that can be released as an earthquake," she says. In this case, not just an earthquake, but a tsunami — that sharp jolt underwater kicks up a wave. "It's sort of like sloshing in the bathtub," Hutton says. "If it's a big enough earthquake, then the waves that are caused on the ocean will travel all the way around the Pacific." This head-on collision of tectonic plates in Chile has been responsible for 13 quakes of magnitude 7.0 or higher since 1973, according to the U.S. Geological Survey. Still, this weekend's quake ranks among the strongest on record. "There were only three earthquakes in the 20th century that were bigger than this earthquake," says Brian Atwater, who works for the USGS out of the University of Washington. "Those three earthquakes were in Kamchatka in 1952, in Chile in 1960 and in Alaska in 1964." Of course, there was another giant quake in 2004, the Indonesian quake that triggered the devastating tsunami. These are very rare events. By comparison, the quake that devastated Haiti last month was a comparatively modest 7.0. The Chilean 8.8 released 500 times as much energy. So why was the Haiti quake so much more deadly? For one thing, the epicenter was much closer to a densely populated area. Atwater says it's also that the quake was surprisingly compact. "Nearly all the energy from the Haitian earthquake came from a patch just ... 5 or 10 miles long," he says. "So it's very, very concentrated." The Chilean quake, by comparison, caused a large area of ground to slip. Atwater says not to think of a dotlike epicenter in this case. Instead, think of a failure zone shaped like a giant sausage, 70 miles away from the nearest significant city. Also, in Haiti the buildings that collapsed were poorly constructed to begin with. Unlike Chile, people there didn't have experience building for earthquakes. Besides, Hutton says, Chilean buildings haven't just been built better, they've been put to the test. "Probably most of the really vulnerable buildings have already been destroyed in earthquakes," she says. Of course, a quake of this size still causes lethal damage, spread over a large area. But it made a big difference that Chile knew something like this was inevitable, and made an effort to be prepared.

= = = = =Sizing Up The Tsunami: Why It Wasn't So Big= by [|Christopher Joyce] [|www.NPR.org] March 1, 2010 The earthquake in Chile on Saturday not only brought down buildings and killed hundreds of people — it also created a tsunami. The tsunami set off alarms around the Pacific basin. Eventually, the waves turned out to be pretty small, at least beyond Chile. To understand why, let's start with how a tsunami is created. You can make one in your bathtub. Put your hand, palm open, under the water, parallel to the surface. Then quickly push down. You'll create an underwater wave that will push water to the end of the tub and up the side. That's what a thrust earthquake on the sea floor did in Chile. A big tectonic plate, the Nazca plate, thrust itself underneath an adjoining plate, the South American plate, which is what Chile sits on. That jacked up a stretch of sea floor about 400 miles long. Geophysicist Brian Shiro from the Pacific Tsunami Warning Center describes how that creates a wave: "A tsunami is basically the ocean, all the way to the sea floor, kind of being picked up and dropped. That's what the earthquake does — it lifts the sea floor a little bit and then drops it back down." When you lift the sea floor, you lift the water column above it. Then gravity smacks the water back down and creates the underwater wave. The wave front stretches vertically from the ocean surface all the way to the bottom — with a lot of water following behind. The thrust creates two wave fronts moving in opposite directions away from the fault. Now, the Chilean quake was the fifth-biggest in the past century. But the biggest displacement was deep //under// the sea-floor, about 22 miles down. It's the sea floor movement that creates the tsunami. Geophysicist Harley Benz, with the U.S. Geological Survey, says there's some evidence that the floor didn't move that much. "We're not seeing a lot of coastal uplift from this earthquake," says Benz, "so it appears that there wasn't a large amount of displacement of the sea floor to cause a tsunami." There was //some// lift though — enough to create a tsunami. But one thing that may have kept it small was that the quake hit fairly near shore, in relatively shallow water. "It was not in deep enough water to have enough water to actually excite and move," says Shiro, "to create the initial pulse of the tsunami. There's less mass to start the process off." Less water above the fault means less comes crashing down after the quake. Nonetheless, a special device lying on the sea floor near Chile — called a DART buoy (deep-ocean assessment and reporting of tsunamis) — did detect an underwater wave. "This thing is sitting down there measuring the weight of the water over it," says Shiro, "and it's very sensitive, so as the wave comes over it, it measures it as a pressure increase." That's when computer models at the tsunami warning center in Hawaii started to calculate how powerful the tsunami waves were and where they were headed. Since the original fault line lay northeast to southwest, one wave would propagate to the northwest, toward Hawaii and Japan. After the first DART buoy picked up the signal, the Pacific alert went out. Then more DART buoys picked up the waves as they traveled across the Pacific; scientists have added more than 30 of the devices since the big quake and tsunami in the Indian Ocean in 2004. The buoys showed that the waves weren't that big. But the final measure of a tsunami can't be taken until it hits shallow water near a shoreline. Say's Harley Benz of the USGS: "You have this big pulse traveling at 500 miles an hour, and then when it gets to the other end, it starts piling up because it's slowing down." The shallow water allows the back end of the wave to catch up and pile onto the front end. Since the shape of every coastline is different, it's hard to predict how even a small tsunami will behave near shore. In this case, the piling up amounted to only about 3 feet in Hawaii. The tsunami reached Japan as well, but did little damage. Benz says that with each tsunami, the computer models for tracking them get better. They'll need to; he says the Chilean fault that buckled over the weekend

=Chile, Haiti Quakes Explained= March 1, 2010 from All Things Considered [|www.NPR.org] Significant aftershocks continue to rock Chile two days after a magnitude 8.8 earthquake brought down buildings and bridges, and triggered a tsunami. And yet it's already clear the devastation won't reach the levels seen in Haiti. Walter Mooney, a seismologist with the U.S. Geological Survey, explains the differences between the two quakes. Significant aftershocks continue to rock Chile two days after a magnitude 8.8 earthquake brought down buildings and bridges and triggered a tsunami. The death toll stands at more than 700. Some areas have yet to be reached. And yet, it's already clear that the devastation in Chile will not reach the levels that we saw in Haiti after the 7.0 earthquake there in January. For an understanding of the differences between the two quakes, we've turned to Walter Mooney. He's a seismologist with the U.S. Geological Survey in Menlo Park, California. Thanks so much for being with us. Dr. WALTER MOONEY (Seismologist, U.S. Geological Survey): It's my pleasure, Michele. NORRIS: Many people right now point to the fact that the Haiti quake was smaller in magnitude. But when it comes to earthquakes, you're not just looking at magnitude, it seems like you're also looking at the frequency of shaking. Could you explain that concept to us, what shaking means? Dr. MOONEY: Sure, Michele. Every earthquake of course causes some ground shaking. But, you know, the number of shakes per second is different for a very big earthquake. There you have relatively fewer shakes per second because it's a big earthquake like a cello. And you have an earthquake like Haiti, which I would compare to a violin with a higher pitch. And so the number of shakes per second is going to make a difference in how buildings and bridges and hospitals and schools respond. Schools and hospitals are usually one story and two story, and they don't do very well with these high frequency shakes. In contrast, a tall building, like the 12 and 14-story apartment buildings in Chile, they don't do very well with these long period, few shakes-per-second kind of waves. So, with this frequent shaking near to the source, near to the epicenter, that kind of circumstance like in Haiti, people don't have a chance. They can't evacuate and the buildings come down very, very quickly. In contrast, in Chile, the earthquake would've started a little bit more with a rumble and people would have some chance to evacuate. NORRIS: Does this have something to do with depth also? Dr. MOONEY: Oh, boy. Depth off an earthquake is a really important parameter. And we can recall the earthquake that occurred only about a year ago in L'Aquila, Italy. Here, the earthquake was only a 6.3, but it ruptured right up to the surface with devastating results, the reason being that the city was sitting basically right on the fault. So, any structure that had not been designed with earthquake engineering in mind, it never had a chance and it came down right away. NORRIS: It's very unsettling to see these back-to-back earthquakes. Where else in the world are seismologists predicting major quakes right now? Dr. MOONEY: Boy, we have a lot of places that we're very, very concerned about. Tehran is one city in the Middle East where we have more than 10 million people living right on a fault. You know, the population pressure is so great for people to move to the capital cities and to occupy that area that's - it's an economic pressure. Other cities that we worry about a lot is Caracas, Venezuela, Quito, Ecuador, Lima, Peru and, well, you know, the list goes on. Northern India is a highly populated area that hasn't seen a really big earthquake for about 100 years. And we worry about that region as well. So, I would have to say that, you know, we don't have a list of the top five. Unfortunately, our list of the top concerns would probably be 25 mega cities, all of which are in peril from the great earthquake. NORRIS: Walter Mooney, thank you very much. Dr. MOONEY: You're welcome. NORRIS: Walter Mooney is a seismologist with the U.S. Geological Survey in Menlo Park, California. Copyright © 2010 National Public Radio®.

Earthquake USGS WebQuest (courtesy of Mr.Brown, Dickerson Middle School, Marietta GA)
Go to [] Each answer should be a minimum of 3-4 complete sentences. Please put into your own words. Include all important vocabulary. 1. What is an earthquake? 2. What causes earthquakes and where do they happen? 3. Why does the earth shake when there is an earthquake 4. How are earthquakes recorded? 5. How do scientists measure the size of earthquakes? 6. Describe P waves. 7. Describe S waves. 8. How can scientists tell where the earthquake happened? 9. Can scientists predict earthquakes?
 * foreshock:
 * mainshock
 * aftershock:

Navigate to: []
 * __Part 2__**

**__ Part 3 __** [] 1. Using the world map, where do you see the most earthquakes? 2. Where and when was the most recent earthquake? What was its magnitude? 3. Now click on the US map. where do you see the most earthquakes? 4. Where and when was the most recent earthquake? What was its magnitude?
 * What happens when two plates meet?
 * Describe folding:
 * Describe faulting:
 * What are dip slips?
 * What is a strike slip?
 * What causes earthquakes?
 * What can earthquakes do to change the landscape?
 * Why can we feel earthquakes hundreds of miles away?
 * Define epicenter
 * Define focus
 * Define seismic waves
 * Now take the quiz on earthquakes.