Program Name:
Utah Quaternary Fault Parameters Working Group
Administering Agency:
Utah Geological Survey
Contact:
William R. Lund, Senior Scientist
Address:
P.O. Box 146100, Salt Lake City, UT 84114-6100
Telephone:
801-537-3304
Fax:
801-537-3400
Email:
billlund@utah.gov
Program Summary
The Utah Quaternary Fault Parameters Working Group, a panel of experts
convened by the Utah Geological Survey (UGS) in 2003-04, has completed a
comprehensive evaluation of paleoseismic-trenching data available for Utah’s
Quaternary faults, and where the data permit have assigned consensus
preferred recurrence-interval (RI) and vertical slip-rate (VSR) estimates
for the faults/fault sections under review. Trenching data are available
for 33 (16%) of Utah’s 212 Quaternary faults/fault sections and related
structures. The available paleoseismic-trenching data are most abundant on
the six central, active segments of the Wasatch fault zone coincident with
the populous Wasatch Front, and typically are much less abundant for faults
elsewhere in Utah.
The general paucity of paleoseismic-trenching data, combined with large
uncertainties associated with some of the data, prevented using rigorous
statistical techniques to determine RI and VSR values. Consequently, the
Working Group relied on the broad experience and best professional judgment
of its members to assign preferred RI and VSR estimates to the faults/fault
sections under review. For some faults/fault sections, the trenching data
were insufficient for the Working Group to make RI and VSR estimates. The
Working Group also determined “best estimate” confidence limits for the RI
and VSR estimates that reflect both epistemic and aleatory uncertainties
associated with each fault/fault section. Until superseded by information
from new paleoseismic investigations, the Working Group’s preferred RI and
VSR estimates and associated confidence limits represent the best available
information regarding surface-faulting activity for the faults/fault
sections reviewed, and can be considered as approximating average RI and VSR
values and 2-sigma variability about those mean values.
The Working Group recommended additional paleoseismic study of
20 faults/fault sections to characterize
Utah’s earthquake hazard to a minimally acceptable level. The Working Group
considered NEHRP minimum slip-rate criteria and specific fault priorities
for urban areas in Utah when evaluating which faults to recommend for
additional study. However, the Working Group selected some faults precisely
because so little is known about their recurrence or slip
history. Others, while not located adjacent to urban areas, are near major
transportation, utility, and pipeline corridors critical to Utah’s citizens
and economy.
Consensus RI and VSR estimates are necessary to (1) update the National
Seismic-Hazard Maps, (2) characterize seismic sources, (3) perform
probabilistic seismic-hazard analyses, and (4) provide data for research
into other earthquake topics. The Working Group’s consensus RI and VSR
estimates are currently the best available data in Utah to meet those needs.
Program Operational
Since January 2003
Major Purposes
The purposes of the Utah Quaternary Fault Parameters Working Group review
were to (1) critically evaluate the accuracy and completeness of Utah’s
paleoseismic-trenching data, particularly regarding earthquake timing and
displacement, (2) where the data permitted, assign consensus, preferred
recurrence-interval (RI) and vertical slip-rate (VSR) estimates with
appropriate confidence limits to the faults/fault sections under review, and
(3) identify critical gaps in the paleoseismic data and recommend where and
what kinds of additional paleoseismic studies should be performed to ensure
that Utah’s earthquake hazard is adequately documented and understood.
Although used
extensively by researchers and geologic and engineering practitioners, prior
to this review, Utah’s
Quaternary fault paleoseismic-trenching data had not been critically
evaluated to establish consensus fault parameter values and confidence
limits. Consequently, users unfamiliar with the database and unaware of
important caveats often did not recognize variations in the quality and
completeness of the data. Consensus RI and VSR estimates are a critical
component in four areas directly related to reducing losses from earthquakes
in Utah: (1) updating the National Seismic-Hazard Maps, (2) characterizing
seismic sources, (3) performing probabilistic seismic-hazard analyses, and
(4) providing consensus paleoseismic data for research into other earthquake
topics. With a widely distributed consensus dataset, all users can have
access to expert-reviewed paleoseismic-trenching data that are qualified
with appropriate caveats, and from which they can make informed judgments
regarding their own research and projects.
Specific Activities
and Operations
Various seismic-hazard-evaluation initiatives in California have
successfully employed the concept of working groups composed of technical
experts in a field of interest to critically evaluate various datasets and
arrive at consensus decisions regarding data values and reliability. The
UGS employed a similar strategy and convened the Utah Quaternary Fault
Parameters Working Group composed of 18 technical experts in the fields of
paleoseismology and seismology in 2003-04. The paleoseismologists on the
Working Group collectively represented many decades of experience in
conducting paleoseismic investigations in Utah as well as throughout the
United States and around the world. Likewise, the seismologists on the
Working Group were familiar with Utah tectonics, and have worked directly
with Utah’s paleoseismic data. The Working Group included two categories of
experts, all serving in a volunteer capacity. The first category consists
of paleoseismologists having direct knowledge of Utah’s Quaternary fault
dataset. The second category consists of knowledgeable experts capable of
providing critical analysis of the paleoseismic data, but who have not
conducted paleoseismic studies in Utah and therefore has no vested interest
in the Utah data; this group included both paleoseismologists and
seismologists. The experts were drawn from state and federal government,
academia, and the private sector.
Although the Utah paleoseismic-trenching database is small compared to
California’s, where similar evaluations of paleoseismic data have been
conducted, it was neither reasonable nor practical to expect Working Group
members serving in a volunteer capacity to review each of the more than 60
paleoseismic source documents available for Utah’s Quaternary faults. To
expedite the process, a UGS Working Group Coordinator summarized the
available paleoseismic-trenching data and forwarded the summary information
to Working Group members for their review. The Working Group convened three
times to evaluate the data, and to come to consensus decisions regarding
preferred RI and VSR estimates for the faults under review. The Working
Group Coordinator then summarized the paleoseismic data and the results of
the Working Group’s deliberations in a final report on a CD.
Results of the Working Group review were presented at local, regional, and
national professional society meetings (2004 Utah Earthquake Conference,
2/26/04; Seismological Society of America 2004 Annual Meeting, 4/17/04;
Western States Seismic Policy Council Basin and Range Province Seismic
Hazards Summit II, 5/17/04). The UGS used the Working Group’s results to
revise the Quaternary Fault and Fold Database and Map of Utah, and a
UGS Bulletin presenting the Working Group results is currently in press.
The Bulletin will make the Working Group results widely available to users
who require expert-reviewed paleoseismic-trenching data.
New Approaches and
Methods
This program successfully adapted seismic-hazard-evaluation techniques first
pioneered
in California for faults chiefly in a strike-slip tectonic environment to
normal-slip faults typical of the Basin and Range’s extensional tectonic
regime. The program serves as a prototype for other Basin and Range states
to use as they compile their Quaternary fault databases and need to develop
consensus RI and SR values.
Program Funding
Start-up Costs:
$98,492
Source: USGS
NEHRP ($69,915), State of Utah ($28,577)
Annual Budget:
approximately $5,000
Source:
State of Utah
Program Staffing
1.25 FTE
The UGS devoted 1 FTE to the Working Group process in 2003-2004. In
addition 17 other technical experts from federal and state government,
academia, and the private sector donated numerous hours of their time to
this project. Ongoing Working Group activities require approximately 0.25
FTE from the UGS.
Program Origin
The Working
Group concept originated in California, we know of no similar programs
within the Basin-and-Range states.
Achievement of Stated
Purposes
(1) The
Working Group consensus data have been used to update the Utah Quaternary
Fault and Fold Database and Map, (2) the consensus data for the Wasatch
fault zone (WFZ) are being used in an on-going evaluation of possible
multiple or partial segment rupture scenarios for the six central WFZ
segments with evidence of Holocene surface faulting, (3) geologic and
engineering practitioners have requested the new consensus data for use in
Probabilistic Seismic Hazard Analyses, and (4) the consensus data will be
incorporated in the next update of the National Seismic Hazard Maps.
Program Modifications
The Utah
Quaternary Fault Parameters Working Group was convened by the UGS to perform
a specific task – evaluate the paleoseismic-trenching data available for
Utah’s Quaternary faults and assign consensus preferred recurrence-interval
(RI) and vertical slip-rate (VSR) estimates for the faults/fault sections
under review. Upon successful completion of that task, the Working Group
morphed into a permanent Utah Earthquake Working Group that also acts as
part of the Geoscience Standing Committee of the Utah Seismic Safety
Commission (USSC). As a part of the standing committee, the Working Group
is now charged with advising USSC and UGS on issues related to active
faulting in Utah. The Working Group met in March 2005 during the 2005 Utah
Earthquake Conference to (1) review recently completed and ongoing
paleoseismic studies in Utah, (2) consider a variety of technical issues
related to Quaternary faults in Utah, including a process for annually
updating the Working Groups consensus fault parameter database, and (3)
identify and discuss possible 2006 NEHRP proposals and partnerships.
Obstacles Other States
Might Encounter
Other
Basin-and-Range states contemplating establishing a Quaternary Fault
Parameters Working Group will likely face two principal obstacles: (1)
finding a sufficient number of technical experts willing to serve in a
volunteer capacity on the Working Group (this is a time-consuming process),
and (2) finding funding for travel and meeting costs.
