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When Disaster Strikes —

|| Rockfall || Earthquake ||

When a natural disaster threatens or occurs, scientists are among the first people to respond. While the first priority is to save human life, the next is to understand as much as possible about what happened in order to prevent or minimize the harm from future natural hazards.



A color photograph of a mountian with a rockfall in Happy Falls Park, Calif., photo: Pacific Aerial Surveys

Tens of thousands of tons of rock plunged from this cliff face 2,000 feet down to the valley floor.

Rockfall
Yosemite National Park July 10, 1996


Trail worker Ernie Milan was out for a jog in Yosemite National Park, Calif., when he heard a loud boom. He thought a jet airplane was about to crash. Then dust started swirling around him like a tornado. Milan took cover by hugging a cliff wall. Day turned to night.

The sound and dust were caused by two massive rockfalls that occurred just seconds apart at a site in the park called Happy Isles. A giant slab of granite weighing nearly 70,000 tons fell from a cliff face more than 2,000 feet to the valley floor. As the slab hit the ground, it created a massive air blast and a dust cloud full of sharp rock particles. The air blast and billowing dirt knocked over trees, killing one person and seriously injuring several others.

Gerald Wieczorek of the U.S. Geological Survey (USGS) estimated that the slab of rock was traveling more than 250 miles per hour when it hit the ground. Wieczorek is a geological engineer. He was on the team of scientists who went to the park to document what happened, before wind and weather could erase the evidence. Most geologists look at how rocks formed, what minerals they contain, and where particular rocks exist. In contrast, a geological engineer is an expert in the mechanics of rocks. Wieczorek explains, "In this case, what was it about the rocks that made them slide? What qualities [of the rocks] contributed to the dynamics of the situation?" Other members of the investigating team included scientists who study aspects of volcanic eruptions, such as dust clouds and air blasts.

Jim Snyder, now the park historian for Yosemite, contributed another expertise. Snyder worked for many years in the park's back country and often encountered the debris and damage from rockfalls on park trails and roads. As a result, he became interested in rockfalls and researched both historic and prehistoric slides at Yosemite. His study revealed that rockfalls are common in Yosemite, but a slide accompanied by a massive air blast had happened only once before, in 1872.

As Snyder wrote in Yosemite magazine, rockfalls are a natural part of Yosemite. There is no way to prevent rockfalls in Yosemite's mountainous terrain or to forecast precisely when they might occur. But understanding the history and mechanics of the rockfalls at Happy Isles can help identify where a rockfall might happen elsewhere in the park.

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Earthquake
Northridge, California January 17, 1994

At 4:31 a.m. on January 17, 1994, some 10 million people were shaken awake by an earthquake. The Northridge earthquake collapsed buildings, ruptured gas lines, and toppled freeways. The 6.7 magnitude quake killed 57 people, injured more than 9,000, and displaced more than 20,000 people from their homes.

Within minutes of the main shock, earthquake scientists were analyzing data and dispatching information about the quake to emergency response agencies. "When I felt the earthquake, I jumped out of bed, threw on clothes, and came in. When you feel an earthquake, you run in," recalls Lucy Jones, a seismologist at the U.S. Geological Survey (USGS). Jones is Chief of the USGS field office in Pasadena, Calif., the office that operates the network of seismic instruments that monitors earthquakes in southern California. Seismic instruments detect, measure, and record the vibrations that travel through the Earth and over its surface when an earthquake happens.

"I spent the whole day in what we call the operations center dealing with a variety of tasks," Jones explains. "One was talking to the media. And one was tracking aftershocks." Smaller earthquakes called aftershocks, which also can cause damage, especially to buildings and other structures weakened by the initial shock, usually follow a major earthquake. Jones, an expert in earthquake statistics, estimated the probability of damaging aftershocks and where they would most likely occur.

This information was of vital importance to emergency service agencies that were coordinating and dispatching response teams.

Many other scientists quickly began collecting and analyzing data to learn how to reduce hazards from future earthquakes. It is unfortunately true that the best laboratory for studying earthquakes is an actual earthquake.

Daniel Ponti is a USGS geologist who studies the surface effects of earthquakes. On the day after the earthquake, Ponti recalls, "I spent most of the day in a helicopter doing reconnaissance work from the air." He was looking for where, or if, the fault — the fracture in the Earth's crust that caused the earthquake — had ruptured the surface. When a large fault ruptures and produces an earthquake, it usually causes ground cracks that displace the foundations of buildings, fracture swimming pools, and rupture water and gas pipelines. Knowing where the greatest damage was concentrated could help Ponti and his team of geologists locate the fault.

Ultimately, Ponti and other scientists determined that the earthquake was caused by a hidden fault, called a blind thrust fault, which did not rupture the surface. The minor ground cracks were caused by shaking produced by the earthquake. Ponti and his team also dug trenches in the areas where the greatest damage occurred and looked for evidence of past earthquakes. Their investigation uncovered cracks left by two earthquakes that occurred during the past 1,300 years. Because these cracks were remarkably similar to the surface cracks caused by the Northridge earthquake, the geologists concluded that the two older earthquakes most likely occurred on blind faults as well. This information is helping geologists locate other blind faults as they study the region's earthquake history, which will improve their ability to forecast where future earthquakes might strike.

A color photograph showing a collapsed freeway bridge in a mountainous area.

Collapsed freeways symbolized the Northridge earthquake more than any other type of damage.



A photograph of a young woman with with earthquake measuring equipment.

Seismologist Lucy Jones fields questions from the press about an earthquake that struck Landers, Calif. in 1992.



A small US earthquake map, most of the earthquake regions are on the west coasts.

As this map shows, California is not the only place at risk from earthquake damage. Architects, engineers, and land use planners throughout the country use these maps to assess earthquake risks when planning buildings and other structures.

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