June 19, 2000
Salt Lakers can sleep a little easier now. It appears
the risk of an earthquake along the Wasatch Fault is
much lower than projected.
The new evidence comes from a careful analysis of
samples pulled from a trench near Little Cottonwood
Canyon. Science Specialist Ed Yeates has the latest.
On the high end of the scale, experts originally had
projected a 57% risk of a major earthquake
over the next 100 years along the Salt Lake segment
of the Wasatch Fault.
That's roughly a 25-mile stretch
of the Fault which extends along the east bench
through the heart of the city.
When geologists disected the fault last year with a
trench, they saw a frequent pattern of earthquakes.
But they needed a careful analysis of the evidence to
come up with an accurate probability of what to
expect, say, over the next 100 years.
It's sort of like forecasting the weather. But instead of
rain, it's earthquakes.
The data went to a geological
consulting firm called GEOHAZ. After months of
analysis now, it's new forecast may surprise
residents.
Instead of a low of only 7% or a high
of 57%, which is a sure thing, GEOHAZ says a
probability of a major earthquake along the Salt Lake
segment of the Wasatch Fault over the next 100 years
is more realistically 16% to 17%.
GARY CHRISTENSEN, UTAH GEOLOGICAL
SURVEY: "THE PURPOSE OF THIS STUDY
WAS TO TRY TO LOOK AT A LONG TERM
RECORD OF EARTHQUAKES. WE'VE ONLY
BEEN ABLE TO GET BACK ABOUT
6,000 YEARS WITH OUR PREVIOUS
TRENCHES."
Salt Lake's new earthquake forecast is based partly
on the weight of water from prehistoric Lake
Bonneville, then the lightening of that load when it all
disappeared, and how both events affected the
frequency of earthquakes.
"THE ONLY THING LEFT OF LAKE
BONNEVILLE NOW IS THIS DEAD SEA
CALLED THE GREAT SALT LAKE - AND
THE WATER WEIGHT HERE IS A FAR CRY
FROM WHERE IT WAS WHEN THE WHOLE
VALLLEY WAS FILLED WITH WATER."
All the weight of old Lake Bonnevile pushed down on
the Wasatch Fault, increasing stress and the
frequency of large earthquakes. But as the Lake
disappeared, the load lifted and we didn't have as
many quakes.
Now, the Fault is probably returning to a normal
pattern of producing earthquakes, as it did in a time
before Lake Bonneville.
One caution about resting too easy with these new
numbers. Gary Christensen says major earthquakes
along other numerous faults and other segments of the
Wasatch Fault not included in this study could happen
at any time and could produce devastating damage
here in the valley.
