Big Earthquake Rattled Utah 500 Years Ago

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MIKE STARK,Associated Press Writer

SALT LAKE CITY (AP) — Geologists say a major quake could strike at any time in Utah, with new research showing the last large temblor along the Wasatch Front was more recent than previously thought.

The Utah Geological Survey and U.S. Geological Survey said in a study released Thursday that an earthquake about 500 years ago tore a deep gash along a 35-mile (56-kilometer) section of the fault known as the Weber segment.

The quake was likely a magnitude 6.5 or 7 — large enough to cause major damage if it occurred today, according to Christopher DuRoss, a geologist with the Utah Geological Survey.

Previous research indicated the last major quake in the area was 800 to 1,500 years ago.

But scientists excavating a 100-foot (30.5-meter) trench near North Ogden found clear signs of the quake 500 years ago along with evidence of large quakes about every 1,500 years going back 10,000 years.

"This really tells us one thing: that the Wasatch Fault is very active," DuRoss said.

The North Ogden trench indicates the quake 500 years ago caused the ground to slip six to 12 feet (1.8 to 3.6 meters), DuRoss said.

Back then, the region was lightly populated, nothing like today's sprawling metropolitan landscape full of subdivisions, shopping centers and vast networks of roads and rail.

A similar quake today could cause significant damage, particularly to unreinforced structures built prior to the 1960s and 1970s, DuRoss said.

"Damage would be felt all along the fault," DuRoss said.

About 700 earthquakes hit Utah each year, but only about 2 percent are ever felt.

The USGS last year published new hazard maps for the region showing that the worst-case earthquake could be a magnitude 7.4 earthquake along the 240-mile (386-kilometer) Wasatch Fault that runs from Malad, Idaho, through Utah's population centers to Gunnison.

A large quake along the central portion of the Wasatch Fault could cause $4.5 billion in damage to buildings in several counties, according to the University of Utah.

Although the geological record along the Weber segment indicates large quakes about every 1,500 years, there's no reason to think the fault will stick to a schedule.

"It's still a significant hazard. We just don't know enough about the fault to predict earthquakes," DuRoss said.


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