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Earthquakes and Subduction Zones Name __________________________ Date _____________ Earthquakes occur when the brittle rock of the lithosphere moves in response to stress. The worldwide distribution and depths of earthquakes are important evidence for plate tectonics. They help geologist delineate active plate boundaries and infer locations of ancient plate boundaries. Most earthquakes have shallow focus depths (0 to 70 km below the surface), which makes sense since earthquakes do not occur in the hot, plastic rock of the asthenosphere or deeper in Earth’s interior. Intermediate (70-350 km) and deep (350-700 km) focus earthquakes do occur, but only near subduction zones. This indicates that solid, rigid material is being forced into the mantle in these areas. Plots of earthquake focus depths along subduction zones provide geologists with profiles of subducted plates and enhance our understanding of what happens at subduction zones. The attached map shows an area in the western Pacific Ocean where several deep ocean trenches mark the locations of subduction zones. Different symbols on the map represent different earthquake focus depths. The table below summarizes the focus depths of 24 earthquakes near the Izu Trench in the western Pacific and the distance from the focus of each earthquake to a seismic station on Bonin Island. Plot a graph of earthquake focus depths on the graph paper on the back of the map. The graph paper is already labeled. Select colors to represent shallow, intermediate and deep focus earthquakes and make a key on the graph. Give the graph a title. Earthquake Number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Focus Depth (kilometers) 390 32 295 190 540 54 90 215 32 630 230 150 13 680 335 460 70 10 400 530 110 55 58 605 Use the graph and map to answer the questions on the back. Relative Distance (km X 100) 400 120 350 290 530 200 220 340 150 520 310 240 80 570 390 460 190 50 440 480 260 165 190 520 Questions 1. What is the approximate angle of descent of the subducted plate at the Izu Trench? _______________ 2. Identify the plates that are involved in this subduction zone. _______________________________________________________________________ Which plate is being subducted? _________________________ What is the direction of subduction? ______________________________ There is oceanic crust on both sides of this plate boundary. What determines which of the two plates is subducted? _______________________________________________________ _______________________________________________________________________ 3. Compare the earthquake distribution at the Izu Trench (A-A- on the map) with the earthquake distributions at the Mariana (B-B’) and Japan (C-C’) Trenches. The steepness of the angle of plate descent can be inferred from the spacing of the earthquake locations. The closer together they are, the steeper the angle of descent and vice versa. Using these relationships, analyze the distribution of earthquakes along B-B- and C-C’ compared to A-A’ (the ones you just graphed). At which of the three trenches is the angle of plate descent steepest? _________________ Where is the angle of descent shallowest? _________________________ 4. Why don’t earthquakes occur at depths greater than approximately 700 km? What happens to the subducted plate at greater depths? _________________________________________ _______________________________________________________________________ _______________________________________________________________________ 5. Examine the map of tectonic plates. Identify three other places around the world where you would expect to find the same distribution of earthquakes as you see near Japan. _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________ 6. Places on the east coast of the United States occasionally experience earthquakes along faults that are left over from ancient plate activity. Do you think that these are likely to be shallow or intermediate and deep focus earthquakes? EXPLAIN your reasoning. _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________