Learning target: I can describe how a 1980 eruption and a 1989 earthquake differed in the warning they gave, ask questions about why, and write a first explanation of how people could plan for an event no one can predict.
Read both case files and study the two figures. All the facts are real. Write what you notice before you write what you wonder. Your explanations in Part C are first drafts: it is fine to be unsure, but give a reason for every idea. Keep this sheet; you will return to it at the end of the chapter.
You will need: This worksheet, Pencil
At 5:04 p.m. on October 17, 1989, rock deep beneath the Santa Cruz Mountains suddenly slipped along a fault. The Loma Prieta earthquake, magnitude 6.9, shook the region hard for about 15 seconds. Roughly 100 km to the north, a section of the San Francisco-Oakland Bay Bridge's upper deck, about 15 m long, broke loose and dropped onto the deck below. In Oakland, a double-decker freeway collapsed. Sixty-three people died. No one had recognized any warning sign beforehand. Years later, engineers replaced the bridge's whole eastern half with a new span designed to reopen quickly after the strongest shaking expected in about 1,500 years.
Nine years earlier and about 1,000 km to the north, Mount St. Helens in Washington had behaved very differently. On March 20, 1980, a magnitude 4.2 earthquake began a swarm of small quakes beneath the volcano. A week later, steam explosions blasted a new crater in the summit. By late April, the volcano's north side was bulging outward by up to about 1.5 meters per day as magma pushed in from below, and officials had closed the area nearest the mountain. On May 18 an earthquake shook the bulge loose in a huge landslide, and the volcano erupted sideways. Fifty-seven people died. Scientists had forecast that an eruption was likely, but not its exact day or its sideways direction.
Mount St. Helens sits above a place where a small ocean plate is sinking beneath the North American Plate. The Loma Prieta earthquake happened in the San Andreas Fault system, where the Pacific Plate and the North American Plate slide past each other.
For q6: 'Before the eruption, something underground was ___, which caused ___. Before the earthquake, the rock was ___, but that did not show because ___.' For q7: 'Engineers would need to know how strong ___, how often ___, and how important ___. This matters because ___.' For q3, count 11 days left in March after the 20th, then add the days in April and the days in May up to the 18th.
The Bay Bridge and the Oakland freeway were about 100 km from where the rock slipped, yet nearer towns had less damage in some places. Look at a USGS page about the 1989 earthquake and find out what the ground was like under the collapsed freeway and in San Francisco's Marina District. Write a paragraph proposing why the type of ground could matter as much as distance, and what a hazard map would need to show because of it.