PART 1:The Great Tohoku Earthquake of 11 March, 2011 – Rupture and Propagation The island country of Japan rides upon and is rooted to the eastward moving Okhotsk plate, an extension of the North American tectonic plate. Just 120km east of Japan, along the deep ocean Japan Trench, the Pacific plate moves westward at more than 9cm/yr and begins its descent down and under the Okhotsk plate. The motion is not smooth, and huge stresses build up and are stored in the rocks as the colliding plates lock up against each other. On 11 March, 2011, rocks along the boundary 100km east of northern Japan ruptured in a sudden and catastrophic release of years of built up stress, triggering a magnitude 9 earthquake – the largest ever recorded near Japan, and the fifth largest every recorded anywhere on Earth. In less than 2 minutes, literally hundreds of thousands of cubic miles of rock bent, stretched, and moved as much as 24m! Most of the adjustment occurred in the overriding Okhotsk plate, moving, shaking, and altering the size and shape of the Pacific seafloor and the nearby island of Japan. One of the most devastating effects of the sudden deformation of the seafloor was the generation of a massive tsunami that overran the eastern shore of northern Japan just minutes after the quake occurred. In this investigation, you will look at some of the data collected during the quake in an effort to understand and quantify some of the changes observed in the minutes and days following the quake. By the end of this investigation, you should be able to: • Interpret various forms of earthquake slip maps • Measure, calculate, and describe the various motions and adjustments of the bedrock involved in the Tohoku earthquake Open the "Tohoku Quake” KMZ file to begin this activity. Expand the “Tohoku Quake” folder and double click the “?” icon in the Places panel to fly to northern Japan. The red “+” off the coast of Japan marks the approximate epicenter of the quake. Clicking on it in the 3D display will open a balloon with links to the USGS web pages containing detailed scientific information about the quake. Without expanding the folder, turn on the “Tectonic Plate Boundaries” folder and notice the arrow and rate of movement that appear on the map just NE of the earthquake epicenter, and answer the following questions: 1. What term describes the seafloor feature that runs along the blue plate boundary at the arrow? 2. What type of plate boundary is the blue boundary here? 3. Describe the relative motions of the Pacific Plate (west of the boundary) and the Okhotsk Plate (east of the boundary). Turn off the red “+” marking the epicenter to unclutter the 3D display. Open/expand the “USGS Slip Distribution” folder. Turn on the “USGS Slip Distribution” layer. Double click the “USGS Slip Distribution” layer icon in the Places panel to zoom in on the chart.

Look carefully at the chart. It is a representation of the surface of the plate interface inside the subduction zone (see figure to the right or double-click the “Understanding the Slip Distribution Chart” placemark in the Places panel). The red star represents the focus of the quake, where the first rupture occurred (the epicenter of the earthquake is directly above the focus on the surface of the earth). The dashed isolines in the chart indicate the time at which the rocks in a particular place slipped (called “Rupture Front Contours”), and the colors indicate the amount of slip that occurred, measured in cm. Each colored block on the chart, then, represents the time, location, and magnitude of slip during the quake. Notice that even after 180 seconds, slip was still occurring (on the right and left sides of the chart), but that the magnitude of the slip was decreasing rapidly during that period of time. Answer the following questions: 4. What was the depth of the initial rupture (at the focus)? 5. At what time after the initial rupture at the focus did the maximum slip occur? 6. What was the magnitude of that maximum slip in cm?, and in m? 7. What was the depth of the area of maximum slip? Double click the “?” icon in the Places panel to re-center the view of the earthquake zone. Keeping in mind that the chart you are looking at is a representation of the surface of the plate boundary within the earth, answer the following question: 8. In what direction did the motion along the plate boundary propagate? (your description should have both vertical and compass direction components) 9. What effect might the movement of the rocks along the fault have had on the elevation of the seafloor immediately west of the trench? Turn off the USGS Slip Distribution overlay.

Open/expand the “Slip Models” folder which contains graphic data from Japan’s Geospatial Information Authority. Turn on the “japan-uplift-and-subsidence” layer. It is a model of the change in elevation of the seafloor as a result of the quake. The red area represents the area where the seafloor rose up. 10. Is that area of uplift surprising in light of your answers to questions 5 – 9 above? Explain why or why not.

PART 2: The Great Tohoku Earthquake of 11 March, 2011 – Effects on Earth’s Surface Turn on the “japan-uplift-and-subsidence” layer. It is a model of the change in elevation of the seafloor as a result of the quake. The red area represents the area where the seafloor rose up. 1. Is that area of uplift surprising in light of your understanding of plate motion at convergent boundaries? Explain why or why not. 2. What was the maximum magnitude of the uplift represented on this chart? 3. There was also an area of subsidence just west of the uplifted area. What was the magnitude of the subsidence along the coastline of northern Japan there? 4. Explain how the sudden upward bulging of the seafloor near the trench could generate a tsunami. 5. Explain how the subsidence of the coastline could have increased the destructiveness of the tsunami as it came ashore. Turn off the “japan-uplift-and-subsidence” overlay, Turn on the “japan-mainshock-slip” overlay. Notice the dashed isolines on the map…they represent the depth to the plate boundary below. The green arrows represent the amount of slip at various places along the plate boundary. The white star represents the earthquake focus. Answer the following questions: 6. What is the magnitude of the maximum slip indicated here? 7. What is the approximate depth at which the maximum slip occurred? 8. In what general direction does this chart indicate that slip occurred? (your description should have both vertical and compass direction components) Compare your answers to 8, 9, and 10 in "The Great Tohoku Earthquake of 11 March, 2011 – Rupture and Propagation" with your answers to questions 6, 7, and 8 above. The movements indicated in both charts are generally consistent with each other, but exact amounts vary. These variations in the reported motions are expected as different models were used to calculate them. As more data is collected and analyzed, there is a good possibility that the values calculated will be readjusted, and become more similar. Turn off the “japan-mainshock-slip” overlay, Turn on the “japan-slip” overlay and examine it. It represents the displacement of the land on the northern part of the island of Japan that occurred during the quake. Notice that the displacement of the land was in the general direction of the epicenter. Notice too that the displacement of the land was greatest on the coastline near the epicenter, and less on the west coast of the island. What this means is that the eastern part of the island moved east farther than the western part! Turn on the “japan-mainshock-slip” overlay once again and notice that both the island and the seafloor moved as a single unit (the island is rooted to the larger seafloor below it), and the magnitudes of motion

increase to the east. The island and the seafloor east of it actually stretched out during the quake – the distance from the west coast of the island to the trench is now greater than it was before the 'quake. Turn off the “japan-mainshock-slip” overlay once again, look at the “japan-slip” map again. 9. What, approximately, was the maximum displacement measured on this map? 10. What, approximately, was the displacement of the west coast of the island west of the area of maximum displacement? 11. Use your answers to 9 and 10 above to calculate the net expansion of the island between those points. The expansion, or stretch you calculated above took place over a distance of about 125km. Assuming that the stretch was distributed evenly over that distance (it most likely was not), calculate the amount of expansion accommodated by each kilometer of the width of the island (report your answer in cm/km). Turn off the “japan- slip” overlay Turn on the Verrazano Narrows Bridge placemark Double click the icon for the bridge to fly to it. In the Layers panel, turn on 3D Buildings 12. Use the ruler tool to determine the distance (in m) between the bridge towers. 13. Calculate how much the roadway between the bridge towers would have to stretch to accommodate bedrock deformation similar to that in Japan if it were to occur in the NY metropolitan area (a highly unlikely scenario) Turn on the “japan-mainshock-slip” overlay and double click its icon to fly to it. In the Layers panel, open the Gallery folder and turn on the Volcanoes layer. 14 Describe the relationship between the location of volcanoes on the island of Japan and the depth to the subducted plate below Japan. 15. What is a possible explanation for the fact the there are no volcanoes over areas where the subducted plate is less than 100km deep?

KML for this exercise is at: http://stevekluge.com/geoscience/tohakuquake.kmz Contact [email protected] for more information on this exercise and on the lab manual "Encounter Earth"