July 9, 1999
"That's off the charts from what we see around the country for the effects at
those distances for an earthquake of that size."
Off the charts?
Yes, that's how geologists describe it, after discovering last week's small
earthquake near Park City probably triggered an unusual landslide.
Science Specialist Ed Yeates reports from North Salt Lake, which is quite a
ways from the epicenter of the earthquake.
We're more than 20 miles away from where that earthquake occurred.
Geologists would have never expected to find a landslide here - this far away -
from a small 3.6 magnitude quake.
Within 30 minutes after the tremor hit near Park City, more than 70,000
cubic yards of ground began slowly slipping away from a terraced hillside in
North Salt Lake.
It usually takes a magnitude 4.5 earthquake to cause a landslide like
this.
But in the 3.5 range and this far away? The Utah Geological Survey says,
Unheard of!
Gary Christensen, an earthquake geologist at the U.G.S., says, "But here we
have one - and over 20 miles away - that caused a fairly significant landslide
from a magnitude three and a half, which generally won't even cause a landslide
near the epicenter."
If the quake in fact triggered this slide, and it appears that way -
geologists told the Utah Seismic Safety Council it's one more example of how
little we know about the effects of earthquakes along the Wasatch Front.
What makes this even more unsettling is that our infamous Wasatch Fault
may have acted like a conduit, actually transferring energy from the Park City
tremor.
"There does seem to be stronger ground motions sometimes along existing faults
within very close to a few hundred feet either side of the fault."
The landslide in this gravel pit is just above the Wasatch Fault.
So is the home in Salt Lake which was damaged last week. The owner
inside said it felt like a Mack truck hit the corner of her house.
Unexpected events from such a small tremor? That's what worries
geologists.
Lee Allison, director of the U.G.S. explains, "One of our big concerns is that
when we have a moderate or large earthquake in Utah, we're going to have a much
different response than we're prepared for or expecting."
Allison says an extreme example of how a distant earthquake's shock waves can
reach so far, happened in 1985 in Mexico City -- lots of damage from a quake
400 miles away.
Last week's quake also triggered a strong ground motion detector at the
Jordanelle Dam near Park City.
But the ground motion there was not significant.
