The day a magnitude-6 earthquake strikes the Denver area, it could claim 50 to 130 lives.
More than half of the hospitals would be so damaged they couldn’t take in the sick and injured.
The quake would hurl 80,000 truckloads of debris onto roads and sidewalks.
In seconds, the earthquake would trigger about $10 billion in damage, a Federal Emergency Management computer simulation estimates.
A likely event? Absolutely not.
Then again, few scientists imagined a magnitude-9 earthquake could shake the Indian Ocean floor, triggering a tidal wave and killing more than 283,000 people in Indonesia and 10 other countries – a scenario that came true in December.
“The Sumatra tsunami showed us that extreme events do happen,” said Susan Hough, a seismologist with the U.S. Geological Survey in Pasadena, Calif. The disaster left scientists questioning whether emergency planners should take even very unlikely scenarios more seriously.
Vincent Matthews, director of the Colorado Geological Survey and the man who ran the FEMA computer simulations, said he is worried about the state.
Mysterious earthquakes from magnitude 5 to 6.5 have rattled the state several times since 1880, often rupturing through geological faults that still haven’t been identified.
Some quakes were triggered by wastewater injected deep underground in the 1960s but may involve faults capable of moving on their own.
“The whole state needs to be aware of the magnitude of what can happen,” Matthews said. “I want people to ask the tough questions. What do we do if the sewage system is damaged? How will we treat people if medical facilities are damaged?”
Federal hazard maps highlight, in bright colors, the well-known hot spots for shaking – Alaska, Washington state and California – and less well-known hot zones, such as Charleston, S.C., and the New Madrid region, which stretches across parts of Missouri, Illinois, Kentucky, Tennessee and Arkansas.
On the maps, Denver is shaded blue, a sign it isn’t likely to experience violent earth shaking in the next 50 years. Denver shares a similar level of risk with Boston, Topeka, Kan., and Santa Fe.
Probabilities, however, are wily things when applied to earthquakes, said Jim Harris, a structural engineer and earthquake expert with the Denver engineering firm JR Harris & Co.
“Say a truly damaging earthquake will occur in the Denver basin every 10,000 years,” Harris said. “It could happen tomorrow or 10,000 years from now. Of course, an asteroid can strike you, too.”
State geologist Matthews takes local earthquake risk more seriously, while conceding there are many places in the United States where the danger of dying in a quake is much higher.
Matthews says federal earthquake hazard maps, created by the U.S. Geological Survey’s Denver office, may underestimate Colorado’s shake risk.
Building designers and insurers use those maps as a guide to building codes and setting premiums.
The map makers set high standards, requiring studies of known faults or extensive analysis of small earthquakes, which can help researchers calculate the risk of big ones.
Since Colorado has long been considered seismically uninteresting, Matthews said, no one has done much research here.
Serious earthquakes, however, have rocked Colorado, the largest one in 1882, possibly magnitude 6.6. It started from an unknown epicenter west of Fort Collins, according to the U.S. Geological Survey.
Damage to a power plant cut Denver’s electricity, buildings shook violently, and plaster and windows cracked as far away as Laramie.
“A modern recurrence of an earthquake like that of 1882 could cause significant damage,” the Geological Survey wrote.
Matthews modeled a more moderate quake with the FEMA program – a magnitude-6.0 event centered 15 miles northeast of the state Capitol, at the Rocky Mountain Arsenal.
At the arsenal during the 1960s, the Army injected wastewater deep underground, triggering hundreds of small earthquakes and a few big ones, including a magnitude 5.5 on Aug. 9, 1967. That quake struck just before 7:30 a.m., tossing chairs around Denver kitchens and knocking Denver radio station KIMN off the air for several minutes. Windows rattled in Fort Collins.
The FEMA computer model of a present-day Denver earthquake predicted:
33,000 buildings with at least moderate damage; 1,332 destroyed.
Nine highway bridges with moderate damage.
Dozens of breaks in drinking-water pipelines.
While other geologists agree that the state’s quake geology needs to be better understood, some doubt there is a great risk.
University of Colorado geologist Anne Sheehan, who studies the state’s seismic structures, said federal maps might even overestimate the risk in the state.
“I know an earthquake in Colorado could be devastating,” said Marilyn Gally, state hazard-mitigation officer, who has worked closely with geologist Matthews analyzing earthquake risks in Colorado.
Still, floods, snowstorms, wildfires, landslides and tornadoes happen in the state every year, Gally said.
“Earthquakes are just one of the competing interests, which makes it tough,” she said.
Federal geologist Hough put it this way: “Outside of the known earthquake zones in the U.S., cities are kind of like targets on a dart board: The odds that a dart will hit any one area are low.
“But the odds that a dart will hit somewhere over the next 30, 50, 100 years are a lot less low.”
Staff writer Katy Human can be reached at 303-820-1910 or khuman@denverpost.com.
