| Large Recent Fault Scarp in Southern Colorado
Seismological recordings of earthquakes have only been made for
a little over a hundred years. Our record of earthquakes before
that time can be extended into the geological past by field observations
of seismic effects, such as fault scarps (offsets of the Earth's
surface created by faulting in large earthquakes) or sand
boils. Shown is a six meter-high fault scarp in the Villa Grove
Fault Zone on the eastern side of the San Luis Valley, Colorado.
The San Luis Valley is the northern extension of the Rio Grande
Rift, which extends south from Colorado into New Mexico. A series
of fault scarps in the Villa Grove Fault Zone have been mapped and
dated by McCalpin, who concludes that they were created by magnitude
7+ earthquakes having a repeat time of 10,000 to 40,000 years. The
most recent event appears to have occurred approximately 8,000 years
ago. In 2000, engineering students at the Colorado School of Mines
installed a leveling line across this feature and performed a baseline
leveling survey. Since that time, students in the Department of
Geophysics have monitored this feature for evidence of slow deformation,
with results still pending!
[Photo courtesy of Tom Boyd, Colorado School of Mines.]
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