Earthquakes and Volcanoes Week 4 Reading

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Geology

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Dec 6, 2023

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CHAPTER 4 Plate Tectonics and Earthquakes 1. Explain why earthquakes at subduction zones are many times more powerful than spreading- center earthquakes. They are more powerful because at subduction zones, 70-100km thick plates are being pushed back into the mantle, involving an immense amount of energy. 2. Explain the seismic gap method of forecasting earthquakes. Researchers have concluded that if segments of one fault have moved recently, then it seems more reasonable to expect that the unmoved portions will move next and thus fill the gaps. 3. Which tectonic plate edges fail most commonly in shear? In tension? In compression? In shear the slide-past motion occurs as rigid plates move around the curved earth. In tension the divergent or pull- apart motion at spreading center causes rocks to fall in tension. In compression the convergent or push-together motions at subduction zones and in continent collisions cause rocks to fall in compression. 4. Why are fires in cities so commonly associated with major earthquakes? Earthquakes are associated with fires because they break gas pipes and bring down electrical lines. 5. If present seafloor spreading trends continue, what will happen to Baja California, San Diego, Las Angeles, and Santa Cruz? These cities will continue to move northwestward along the Baja California microplate in a very short amount of time. 6. Which part of the United States sits in an opening ocean basin? Evaluate the earthquake threat there. The San Francisco section of the San Andreas fault is positioned near an opening ocean basin and the other plates “locking” the fault in pose a higher risk of a massive earthquake occurring there at some point in the future. 7. How long were the surface ruptures in the 1906 San Francisco and 1857 Fort Tejon earthquakes? What was the maximum offset of the ground surface during each earthquake? The 1857 one had a rupture of 360km and maximum offset in the Carrizo Plain at 9.5 m. The 1906 one had surface displacement of up to 28 feet and a rupture distance of nearly 500 km.

8. Evaluate the earthquake hazards in locked versus creeping segments of the San Andreas fault. Are the biggest cities in locked or creeping segments? The biggest cities are in the locked part of the fault which poses the greater long-term risk for a potential damaging earthquake. 9. What factors contributed to cause the resonance in Mexico City that was so deadly after the 1985 earthquake? How far was the city from the epicenter? There were multiple earthquakes with magnitudes ranging from 5-7 and Mexico City rests on an ancient lakebed, so many buildings and structures fell. The city was 320 km away from the epicenter. 10. What are the four stages of formation of an ocean basin? 1. Rifting of continents by mantle diapirism. 2. Continental drift, seafloor spreading and formation of ocean basins. 3. Progressive closure of ocean basins by subduction of ocean lithosphere. 4. Continental collision and final closure of ocean basin. 11. In the 2011 Japan earthquake, how large an area of plate moved? What was the maximum slip? What was the earthquake magnitude? When did the last earthquake of this size occur in the same area? The magnitude was around 9Mw. The rupture length was nearly 500 km. The maximum slip was 40 m. The last earthquake of this size to occur in the same area happened in 2015. 12. What is the largest earthquake ever measured? In 1960, a 9.5Mw earthquake was measured in southern Chile. 13. Why do so many mega-killer earthquakes occur in the China-India-Pakistan region? These countries sit on the convergence zone between the South Asian and Eurasian plates, which are very mobile plates.

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