Sunday, October 20, 2013

Star Valley Earthquake Potential

With the occurrence of the two weak earthquakes in the Star Valley area so far this year , it is a reminder that we live on, or very close, to an active fault.

While the chances of a major, damaging earthquake are low in any given year, nevertheless, it is just a matter of time before one does occur.

Following is a link to  a paper discussing the location of fault lines in Lincoln County WY and the potential of a major quake.

The following was taken from this paper as it describes the fault systems of Star Valley in particular.

The Star Valley fault system is the third active fault system in Lincoln County. This fault system,
which has been subdivided into north and south segments, bounds the eastern edge of the Star
Valley. Investigations of the Star Valley fault system determined that Holocene and late-
Pleistocene offsets exist along the south fault segment (Piety et al., 1986; McCalpin et al., 1990;
McCalpin, 1990). Several maximum magnitude earthquakes have been suggested for the Star
Valley fault system. Piety and others (1986) proposed that the Star Valley fault system is capable
of generating a maxiumum credible earthquake of magnitude 7.5 with a recurrence interval of
5,000 to 7,000 years. Based upon a surface rupture length of 27 miles, McCalpin and others
(1990) determined that the Star Valley fault system could produce a maximum magnitude 7.2
earthquake. When McCalpin (1990) trenched a portion of the Star Valley fault near Afton, he
determined that a magnitude 7.3 earthquake with a recurrence interval of 2550-6000 years is
possible on this system. Approximately 5,500 years (radiocarbon age) has elapsed since the latest
event on the fault system at the Afton locality. Based upon this evidence, the Star Valley fault
system is near the maximum limit for the recurrence interval assigned to the system. Because of
the extensive seismic activity associated with the area surrounding the Star Valley fault, and
because of the close proximity of towns to this fault system, a maximum magnitude of 7.5 will be
used for this analysis. It should also be noted that it has been approximately 5500 years since the
last confirmed event on the Star Valley fault at Afton. This fault system is therefore nearing its
recurrence interval limit. A magnitude 7.5 earthquake could generate peak horizontal
accelerations of greater than 80%g at Afton, approximately 79%g at Alpine, approximately
6.6%g at Cokeville, approximately 3.2%g at Kemmerer and Diamondville, approximately 5.2%g
at La Barge, approximately 2.9%g at Opal, and approximately 47%g at Thayne (Campbell, 1987).
These accelerations are roughly equivalent to intensity IX earthquakes at Afton and Alpine, an
intensity VIII earthquake at Thayne, intensity V earthquakes at Cokeville and La Barge, and
intensity IV earthquakes at Kemmerer, Diamondville, and Opal. Afton and Alpine could sustain
heavy damage. Moderate to heavy damage could occur at Thayne. Light damage could occur at
Cokeville and La Barge, but Kemmerer, Diamondville, and Opal should sustain no damage.

The following map shows where the fault lines in this portion of Wyoming have been located.

Fault Line Locations 

Based on the the known threat for a major earthquake in the Star Valley area, it seems prudent to factor this into any future construction.  Additionally earthquake insurance might be considered.

Here is another link to a discussion of Wyoming Earthquake Hazards  including Star Valley

Wyoming Earthquakes

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