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SALT LAKE CITY — Had that seven magnitude earthquake really hit the Salt Lake Valley Tuesday morning, what would we have experienced?
Keith Koper, who directs the University of Utah Seismograph Stations, said the energy released from a magnitude 7.0 shaker along the Wasatch Fault is analogous to a 20 kiloton atomic bomb - similar to the one dropped over Hiroshima in World War II. But instead of the energy from a single bomb, imagine several.
Koper said that "a magnitude seven quake would be about a factor of 25 times larger than that. It would be equivalent to 25 bomb detonations. It would be enormous."
In our case the weapon is not a bomb but the infamous Wasatch Fault. It's 240 miles long, extending from Malad City, Idaho south to Fayette, Utah.
The Brigham City, Weber, Salt Lake, Provo and Nephi segments of that fault run through the most populated areas of our state.
At least 23 magnitude seven quakes have hit these segments over the past 6500 years - an average of a big shakeup every 300 years.
"I don't want to say we're overdue, but based on this paleo-seismic evidence and the geologic evidence from trenching the fault, it would not be unexpected for one to happen soon," Koper said.
Tomorrow, five, fifty, a hundred years from now? The experts don't know but say it will happen.
It might not be just one segment of the fault. Researchers now know a big earthquake on one segment might trigger movement on another.
"It could actually increase the stress of adjacent segments and could lead not really to an aftershock but a big damaging earthquake on one of these adjacent segments," Koper said.
So exactly what is under us?
Let's take the 25-mile-long stretch of the Salt Lake segment of the Wasatch Fault along our east bench. It dips at about 50 degrees, angling to a depth under the center of the valley floor of up to 9 miles. In a big earthquake, the fault plane ruptures. The land mass drops away from the east bench.
If the rupture is deep enough, portions of the valley floor could tilt. That could alter the shoreline of the Great Salt Lake, allowing surges of water to flow eastward, perhaps flooding portions of the Salt Lake International Airport, maybe at about knee length.
Because the city is located on an old prehistoric lake bottom, unconsolidated silts and sands will almost surely amplify the shock waves.
It will be a very terrifying experience. There's no other way to put it.
–Keith Koper, director of the the University of Utah Seismograph Stations
The first wave is called the "P" wave. As Koper described, "people might hear it. It might be a sharp jolt. You might think something hit the building."
Next comes the "S" waves or shear waves and the enhanced surface or long period waves.
"When the "S" waves and surface waves arrive, everybody's world is going to be turned upside down," Koper said.
With these shock waves, expect severe destruction, landslides in the canyons and something called liquefaction.
In the valley where the water table is high, shaking can mix the water and surface soil together liquefying it until the consistency is almost like mush. Heavy buildings on the surface can sink. Sometimes they even tip over.
Scientists know the Great Basin is slowly stretching westward. In fact, all kinds of land masses around us are moving like pieces of taffy.
Is the Wasatch Fault feeling the stress. If so, when will it break loose?
The quake will happen, Koper said, and with a seven magnitude "it will be a very terrifying experience. There's no other way to put it."
