1998-01-10_REVISION - M1988112

• Permit No: M1988112
• IBM Index Class Name: Revision
• Doc Date: 1/10/1998
• Doc Name: SAN LUIS PROJECT PERMIT AMENDMENT ADEQUACY RESPONSES M-88-112
• From: STEFFEN ROBERTSON & KIRSTEN
• To: MLRD
• Type & Sequence: AM1
• Media Type: D
• Archive: No

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BATTLE MOUNTAIN RESOURCES, SAN LUIS PROJECT
CMLRD ADEQUACY LETTER RESPONSES
Page 14 of 28
A review of seismic records of the past 110 years show that seismic
activity in the San Luis Valley has been virtually absent. A review
of technical literature pertaining to seismicity in the project
vicinity, however, revealed that Kirkham and Rogers (1981) in their
study of Colorado Earthquake potential, identified the Sangre de
Cristo Fault, which trends north-south at the base of the foothills
to the west of the project site. This fault is located relative to
the project on Figure D.1-1 in the April 1989 submission to the
Colorado MLRD. The interpretation of the fault was that it should
be classified as an active fault based on its apparent rupture having
occurred within the last 10,000 years (Holocene).
To augment the regional study, SRK commissioned Dr. Charles Glass
to further evaluate the seismic potential of this fault. Copies of
Dr. Glass' reports are contained in Appendix B of the April 1989
submission. Dr. Glass' findings indicated that the maximum credible
earthquake (MCE) for the portion of the Sangre de Cristo Fault in
the project vicinity is a Richter Magnitude of 6.8 to ~.0 and that
a very conservative and somewhat tentative estimate of the
probability of its occurrence in the next 100 yrs ranges from 0.6
to 12 percent. This equates to a probability of .06 to 1.3 percent
that the MCE will occur in the next 10 yrs or the operational life
of the facility. However, statistical models used to predict the
recurrence interval of earthquakes require a relatively complete
catalogue of historical seismicity. There is virtually no historical
seismicity for the San Luis Valley over the past 110 years. Dr.
Glass estimates that there is a 25 percent chance that the MCE along
this fault has a recurrence interval less than 10,000 years and
agrees with Kirkham and Rogers estimate that the Sangre de Cristo
fault last exhibited movement 1,940 to 4,715 years ago.
Current seismic theory suggests that the build-up of stresses along
' a large fault occurs prior to release of those stresses through a
major seismic event. Increase in stress is indicated by the
occurrence of minor seismic activity as the stress builds. The lack
' of seismic activity in the San Luis Valley indicates that stresses
across the Sangre de Cristo Fault have not been increasing and that
the occurrence of a major seismic event along this fault in the near
future is doubtful. Due to the indicated low probability of the
occurrence of the MCE event, SRK determined that the seismic design
criteria for the tailings facility should be based on the potential
for regional seismic activity.
' As indicated in Section D.1.2 of the approved permit application,
Algermissen et al (1982) suggest a peak ground acceleration for the
' project area of 0.139. This acceleration has an associated 90 percent
probability of not being exceeded in 250 years. To utiliae this data
fora pseudostatic analysis the peak ground acceleration is reduced
to produce an equivalent acceleration.