Context: Earthquake magnitude affects how much damage is done in an earthquake. A larger earthquake damages more buildings and kills more people than a smaller earthquake. But thats not the only factor that determines earthquake damage. The location of an earthquake relative to a large city is important. More damage is done if the ground shakes for a long time. The amount of damage also depends on the geology of the region. Strong, solid bedrock shakes less than soft or wet soils. Wet soils liquefy during an earthquake and become like quicksand. Soil on a hillside that is shaken loose can become a landslide. Hazard maps help city planners choose the best locations for buildings (Figure 7.38). For example, when faced with two possible locations for a new hospital, planners must build on bedrock rather than silt and clay. The 1985 Mexico City earthquake measured magnitude 8.1. The earthquake killed at least 9,000 people, injured 30,000 more, and left 100,000 people homeless. It destroyed 416 buildings, and seriously damaged 3,000 other buildings. The intense destruction was due to the soft ground the city is built on. Silt and clay fill a basin made of solid rock. In an earthquake, seismic waves bounce back-and-forth off the sides and bottom of the rock basin. This amplifies the shaking. The wet clay converts to quicksand (Figure 7.39). Many buildings were not anchored to bedrock. They settled into the muck. This caused enormous damage. Water, sewer, and electrical systems were destroyed, resulting in fires. Acapulco was much closer to the epicenter, but since the city is built on bedrock it suffered little damage. The amount of damage depends on the amount of development in the region. The 1964 Great Alaska Earthquake, near Anchorage, was the largest earthquake ever recorded in North America. The gigantic quake had a magnitude of 9.2. The earthquake lasted for several minutes and the ground slipped up to 11.5 meters (38 feet). An area of 100,000 square miles (250,000 square km) was affected. The ground liquefied, causing landslides (Figure 7.40). The earthquake occurred at a subduction zone, and large tsunami up to 70 meters (20 feet) high were created. Despite the intensity of the earthquake, only 131 people died. Most deaths were due to the tsunami. Property damage was just over $300 million ($1.8 billion in 2007 U.S. dollars). The reason there was such a small amount of damage is that very few people lived in the area (Alaska had only been a state for five years!). A similar earthquake today would affect many more people. Buildings must be specially built to withstand earthquakes. Skyscrapers and other large structures built on soft ground must be anchored to bedrock. Sometimes that bedrock is hundreds of meters below the ground surface! Building materials need to be both strong and flexible. Small structures, like houses, should bend and sway. Wood and steel bend. Brick, stone, and adobe are brittle and will break. Larger buildings must sway, but not so much that they touch nearby buildings. Counterweights and diagonal steel beams can hold down sway. Buildings need strong, flexible connections where the walls meet the foundation. Earthquake-safe buildings are well connected (Figure Steel or wood can be added to older buildings to reinforce a buildings structure and its connections (Figure 7.42). Elevated freeways and bridges can also be reinforced so that they do not collapse. Important structures must be designed to survive intact. One of the biggest problems caused by earthquakes is fire. Fires start because earthquakes rupture gas and electrical lines. Water mains may break. This makes it difficult to fight the fires. The shapes of pipes can make a big difference. Straight pipes will break in a quake. Zigzag pipes bend and flex when the ground shakes. In San Francisco, water and gas pipelines are separated by valves. Areas can be isolated if one segment breaks. Strong, sturdy structures are expensive to build. Communities must decide how safe to make their buildings. They must weigh how great the hazard is, what different building strategies will cost, and how much risk they are 
 Question: The Great Alaska Earthquake occurred
Output: at a subduction zone.