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GUIDELINES FOR EVALUATING
SURFACE-FAULT-RUPTURE
HAZARDS IN UTAH
by
Gary E. Christenson, L. Darlene Batatian, and Craig V Nelson
MISCELLANEOUS PUBLICATION 03-
UTAH GEOLOGICAL SURVEY
a division of Utah Department of Natural Resources
Fault scarp of the 1954 M6.8 Dixie Valley, Nevada, earthquake (photo by K.V. Steinbrugge).
Fault scarp of the 1983 M7.3 Borah Peak, Idaho, earthquake (photo by Gary E. Christenson).
Fault scarp of the 1959 M7.6 Hebgen Lake, Montana, earthquake causing damage to barn (photo by I.J. Witkind).
Fault scarp of the 1934 M6.6 Hansel Valley, Utah, earthquake (photo by Frederick J. Pack, from the collection of R.B. Smith).
UTAH GEOLOGICAL SURVEY
Christenson, Batatian, Nelson
GUIDELINES FOR EVALUATING SURFACE-FAULT-RUPTURE HAZARDS IN UTAH
UGS MP 03-
GUIDELINES FOR EVALUATING
SURFACE-FAULT-RUPTURE
HAZARDS IN UTAH
by
Gary E. Christenson, Utah Geological Survey
L. Darlene Batatian, Salt Lake County Geologist
Craig V Nelson, Western GeoLogic, LLC
Although this product represents the work of professional scientists, the Utah Department of Natural Resources, Utah Geological Survey, makes no warranty, expressed or implied, regarding its suitability for a particular use. The Utah Department of Natural Resources, Utah Geological Survey, shall not be liable under any circumstances for any direct, indirect, special, incidental, or consequential damages with respect to claims by users of this product.
MISCELLANEOUS PUBLICATION 03-
UTAH GEOLOGICAL SURVEY
a division of Utah Department of Natural Resources
ISBN 1-55791-696-
TABLE OF CONTENTS
- ABSTRACT.
- INTRODUCTION.
- Purpose.
- Background.
- Fault Activity Classes.
- Study Requirements.
- Risk-Reduction Measures.
- HAZARD EVALUATIONS.
- Minimum Qualifications of the Preparer.
- Investigation Methods.
- Surface Investigations.
- Subsurface Investigations.
- Trench location.
- Depth of excavation.
- Trench logging and interpretation.
- Field Review.
- Fault Setbacks.
- Downthrown Block.
- Upthrown Block.
- Report Review and Applicability.
- REPORT GUIDELINES.
- ACKNOWLEDGMENTS.
- REFERENCES.
- APPENDIX.
- Figure 1. Three possible fault configurations from exposures in only two trenches.. FIGURES
- Figure 2. Fault trench length and orientation to investigate a building footprint..
- Figure 3. Potential problems caused by improper trench locations..
- Figure 4. Formulas and diagram showing variables used to determine setbacks..
- Table 1. Study and setback recommendations and criticality factors (U) for IBC occupancy classes.. TABLE
downthrown side. Such surface faulting commonly recurs along existing fault traces. If a normal fault were to break the ground surface through the foundation of a building, signifi- cant structural damage or collapse could occur, causing injuries and perhaps loss of life. Engineering design to reduce damage from surface fault rupture to an acceptable level may be impractical; therefore, site-specific investiga- tions are needed to accurately locate Quaternary faults, deter- mine their level of activity and paleoseismic characteristics, and establish appropriate building setbacks and other risk- reduction measures prior to development. Maps designating special-study areas within which surface-fault-rupture-haz- ard studies are recommended have been prepared for Weber, Davis, Salt Lake, Utah, Cache, western Wasatch, and eastern Tooele Counties. The maps are available at each county planning department, and many local governments in these areas have adopted these maps in ordinances requiring spe- cial studies. The purpose of surface-fault-rupture-hazard studies is to use the characteristics of past surface faulting at a site as a scientific basis for providing recommendations to reduce the potential for damage and injury from future, presumably similar, surface faulting. However, performance of these studies and adherence to their recommendations do not guar- antee safety because significant uncertainty remains due to our limited understanding of surface-faulting processes, the possibility of future ruptures in previously unfaulted loca- tions, and practical limitations common to investigations. Also, these guidelines address only hazards related to surface faulting. Other earthquake hazards and non-earthquake- related geologic hazards must also be addressed as part of a comprehensive geologic-hazards study. A site-specific surface-fault-rupture-hazard evaluation typically includes a literature review, aerial photograph analysis, and field investigation, usually including surficial geologic mapping and subsurface investigations consisting of excavating and logging trenches. These guidelines outline appropriate study methods, report content (map and trench- log scales, setback recommendations), and expectations of the reviewer. The guidelines are based largely on minimum standards adopted by Salt Lake County (2002), which were developed from existing guidelines and standards in Utah and elsewhere in the western U.S., including: California Division of Mines and Geology (1986a, 1986b); Association of Engineering Geologists, Utah Section (1987); Robison (1993); Hart and Bryant (1997); and Nevada Earthquake Safety Council (1998). These guidelines represent the recommended minimum acceptable level of effort in conducting surface-fault-rupture- hazard studies in Utah. Adherence to these guidelines will help ensure adequate, cost-effective studies and minimize review time. Considering the complexity of evaluating sur- face faulting, additional effort beyond the minimum outlined in these guidelines may be required at some sites to ade- quately address the hazard. These guidelines are mainly designed for siting new buildings for human occupancy. They are not designed for use in siting lifelines (highways, utilities, pipelines), which commonly must cross faults, or water impoundment and storage facilities (dams, water tanks, lagoons). Investigation methods are the same for these facil- ities, but setbacks and other hazard-reduction techniques may vary.
Background
Consideration of surface-faulting hazards in land-use planning in Utah was greatly strengthened in the early 1970s when Cluff and others (1970, 1973, 1975) completed their investigations and maps of faults along the Wasatch Front in northern Utah. These aerial-photograph-based maps present- ed the first comprehensive compilation of fault locations usable by local governments, and increased awareness of the hazard posed by the Wasatch, East Cache, and West Cache fault zones. Early paleoseismic trenching studies (Swan and others, 1980, 1981a, 1981b) further highlighted the hazard by documenting multiple Holocene ruptures on the Wasatch fault. Recognizing the earthquake risk, local governments, particularly in northern Utah, began adopting ordinances requiring fault and other geologic-hazard investigations prior to development. Local government staff relied heavily on developers’ consultants as professional experts responsible for evaluating surface-fault-rupture hazards and recommend- ing adequate risk-reduction measures for proposed develop- ments. Consultants’ reports would sometimes be sent to the Utah Geological Survey (UGS) for review, but in general technical regulatory reviews were not systematically per- formed prior to 1985. This informal review process lasted until June 1985 when the UGS initiated the Wasatch Front County Hazards Geologist Program, funded through the U.S. Geological Sur- vey's National Earthquake Hazards Reduction Program (Christenson, 1993). County geologists hired in Weber, Davis, Salt Lake, Utah, and Juab Counties began preparing surface-fault-rupture and other hazard maps and assisting city and county planning departments in requiring and reviewing site-specific studies. Since then, various pub- lished guidelines for surface-fault-rupture-hazard studies have included those of the Association of Engineering Geol- ogists, Utah Section (1987); Nelson and Christenson (1992); Robison (1993); Christenson and Bryant (1998); Batatian and Nelson (1999), and Salt Lake County (2002). Many Wasatch Front cities and counties have enacted geologic-hazards ordinances that adopt surface-fault-rup- ture-hazard special-study-area maps that define areas where site-specific studies are required prior to approval of new development. The primary objective in these ordinances is to protect life safety and reduce economic loss in an earthquake causing surface faulting. An earthquake along one of the major known Quaternary faults in Utah can result in 6 feet ( m) or more of displacement of the ground surface (Machette and others, 1992; Hecker, 1993; Black and others, 2003). To address surface-fault-rupture hazards, most local government ordinances prohibit construction of habitable structures and critical facilities across “active” faults. Ordinances typically define active faults by a simple age criterion: active faults have evidence for displacement during Holocene time (about 10,000 years ago to the present). Some ordinances expand the active-fault definition to include older Quaternary faults when siting critical facilities, and some exclude avoidance of faults with 4 inches (100 mm) or less of displacement. Practical engineering measures are used to reduce risks for many geologic hazards such as landslides and liquefac- tion. However, designing a building to withstand significant fault displacement at the ground surface is usually not prac-
2 Utah Geological Survey
tical from an economic, engineering, and architectural stand- point. Avoiding construction on fault traces is generally the most practical risk-reduction measure. Fault locations there- fore should be considered in early phases of site design when property is subdivided and buildings sited. The primary pur- pose of a surface-fault-rupture-hazard evaluation is to evalu- ate the presence or absence of Quaternary faults and deter- mine their level of activity. If faults are found and are suffi- ciently active to pose a threat, zones of deformation and amounts and directions of displacement must be determined and appropriate avoidance strategies such as building set- backs recommended.
Fault Activity Classes
A critical step in evaluating surface-fault-rupture hazards is to determine the age of most recent surface rupture on the fault to indicate its level of activity (activity class) and result- ing need for site-specific studies (see Study Requirements below). Fault activity classes in the Basin and Range Phys- iographic Province, which includes western Utah and the Wasatch Front, are defined by the Western States Seismic Policy Council (WSSPC) in WSSPC Policy Recommenda- tion 97-1 (Lund, 1998) as:
- Holocene fault - a fault that has moved within the past 10,000 years.
- Late Quaternary fault - a fault that has moved in the past 130,000 years.
- Quaternary fault - a fault that has moved in the past 1,600,000 years.
The latter two classes are inclusive; that is, Holocene faults are included within the definition of Late Quaternary faults, and both Holocene and Late Quaternary faults are included in Quaternary faults. The activity class of a fault is the youngest class based on the age of most recent surface faulting. The UGS recommends use of these fault activity classes statewide in Utah, and recommends investigators consider all Quaternary faults to be Holocene unless data are adequate to preclude Holocene displacement and assign a Late Quater- nary or Quaternary activity class. Unfortunately, studies to determine fault activity classes have not been performed on many faults in Utah, particularly outside the Wasatch Front. A statewide compilation summarizing existing fault data and giving estimates of the timing of most recent surface rupture on known Quaternary faults in Utah is found in Black and others (2003; updated from Hecker, 1993). However, Black and others (2003) was not prepared for use in assigning activity classes for purposes of land-use regulation. The tim- ing of the most recent event given in Black and others (2003) represents a best (non-conservative) age estimate based on data in existing studies. These estimates, particularly for many pre-Holocene (Late Quaternary and Quaternary) faults, are typically based on limited reconnaissance studies and thus are not adequate to determine the activity class to assess the need for site-specific studies. Faults for which paleoseismic studies at various levels of detail have been performed are listed in table 1 of Black and others (2003). In many cases, these paleoseismic studies are sufficiently detailed to determine the activity class of a fault.
For example, paleoseismic studies (and numerous surface- fault-rupture-hazard evaluations) on the central segments of the Wasatch fault and the West Valley fault zone have shown them to be Holocene faults. However, in some cases, exist- ing paleoseismic studies may not be adequate to assign an activity class to the fault. Thus, faults for which paleoseis- mic studies are inadequate or have not been performed must either be studied in more detail to determine the age of most recent surface faulting, or must be considered to be Holocene until adequate studies demonstrate otherwise. Paleoseismic techniques typically used in studies to determine the age of most recent surface faulting are outlined in McCalpin (1996). Such studies may involve at least a reconnaissance of the entire length of the fault or fault seg- ment to find evidence of the most recent surface faulting because investigations at a single site may be inconclusive and insufficient to assign an activity class. Also, Black and others (1996, figure 7, p. 14) show that not all traces within a fault zone with multiple traces reactivate in every event, and no pattern is evident to suggest which traces may reacti- vate. Therefore, at a site with multiple traces, the fault activ- ity class of all traces should be taken to be that of the youngest trace.
Study Requirements
Once the activity class of the fault is established, the UGS recommends special studies be performed for various facilities (defined below) as follows (table 1):
- Holocene faults - studies are recommended for all structures for human occupancy and all critical facilities.
- Late Quaternary faults - studies are recommended for all critical facilities. Studies for other struc- tures for human occupancy remain prudent (dePolo and Slemmons, 1998) but local govern- ments should base decisions on an assessment of whether risk-reduction measures are justified by weighing the probability of occurrence against the risk to lives and potential economic loss. Earthquake risk-assessment techniques are sum- marized in Reiter (1990) and Yeats and others (1997).
- Quaternary faults - studies are recommended for all critical facilities. Studies for other structures for human occupancy are optional and local govern- ments need not require studies because of the low likelihood of surface rupture, although sur- face rupture is still possible.
Critical facilities are Category II and III structures as defined in the 2000 International Building Code (IBC, table 1604.5, p. 297; International Code Council, 2000) and Cat- egory III and IV structures in the 2003 IBC (table 1604.5, p. 272; International Code Council, 2003), and include schools, hospitals, fire stations, high-occupancy buildings, water- treatment plants, and facilities containing hazardous mater- ials (IBC building occupancy classes E, H, and I structures; see table 1). Surface-fault-rupture-hazard special-study areas have been defined for most Quaternary faults along the Wasatch
Guidelines for evaluating surface-fault-rupture hazards in Utah 3
based on a risk assessment considering the time of the most recent event and average recurrence between events to cal- culate the probability of rupture within a particular time frame. However, paleoseismic data in Utah are generally insufficient, particularly for Late Quaternary and Quaternary faults, to make such calculations. Also, large uncertainties in fault behavior exist because of documented irregular recur- rence intervals, possible clustering and triggering (conta- gion), and poor constraints on timing of prehistoric events, even where isotopic or radiogenic dating methods are used. For these reasons, the UGS has not attempted to estab- lish a rigorous probability-based criterion and recommends a simple time-of-most-recent-rupture criterion to identify faults for risk-reduction measures. Other states that address surface-fault-rupture hazards such as California (Hart and Bryant, 1997) and Nevada (Nevada Earthquake Safety Coun- cil, 1998) have similarly adopted a time-of-most-recent-rup- ture criterion, recommending risk-reduction measures for all facilities for human occupancy along Holocene faults. Neva- da also recommends that critical facilities not be built strad- dling Late Quaternary faults, in part because most historical surface-faulting events in the Basin and Range Physiograph- ic Province have been on Late Quaternary rather than Holo- cene faults (dePolo and Slemmons, 1998). The most common surface-fault-rupture risk-reduction measure is avoidance using setbacks. Consistent with neigh- boring western states, most local government ordinances in Utah prohibit placing buildings in positions that straddle Holocene faults (for example, the Salt Lake County Geolog- ic Hazards Ordinance; Salt Lake County, 2002). The UGS concurs with this requirement, and recommends setbacks from Holocene faults for all structures for human occupancy (occupancy classes A, B, F, M, R) and critical facilities (occupancy classes E, H, I) as shown in table 1. The UGS also recommends that critical facilities be set back from Late Quaternary faults, and that, for other build- ings for human occupancy, setbacks are prudent but deci- sions regarding setting back should be based on a risk assessment. For Quaternary faults, the UGS recommends that studies for critical facilities provide information needed for prudent decisions that weigh the probability of occur- rence and need for setbacks and other risk-reduction meas- ures against the risk to lives and potential economic loss. For other structures for human occupancy, the UGS believes set- backs from Quaternary faults are optional and need not be required by local governments because of the low likelihood of surface rupture. The UGS recommends appropriate dis- closure at all sites potentially subject to surface fault rupture near any Holocene, Late Quaternary, or Quaternary fault, including disclosure of the existence of reports of studies that assessed the surface-fault-rupture hazard. Some local government ordinances exempt faults having less than 4 inches (100 mm) of displacement from setback requirements, based largely on Youd (1980) who concludes that up to 4 inches (100 mm) of displacement generally caus- es damage that is likely not a life-safety threat. Although we do not categorically exempt small-displacement faults from setback requirements, certain structural risk-reduction options such as foundation reinforcement may be acceptable for some small-displacement faults in place of setbacks. Fault studies must still identify faults and fault displacements (both net vertical displacements and horizontal extension
across the fault or fault zone), and consider the possibility that future displacement amounts may exceed past amounts. If structural risk-reduction measures are proposed, a struc- tural engineer must provide appropriate designs and the local government should review the designs.
HAZARD EVALUATIONS
Minimum Qualifications of the Preparer
Surface-fault-rupture-hazard evaluation is a specialized discipline within the practice of engineering geology, requir- ing technical expertise and knowledge of techniques not commonly used in other geologic or geotechnical investiga- tions. Fault investigations must be performed by or under the direction of engineering geologists specifically trained and experienced in such investigations, and the final report should be signed and sealed by the licensed Utah Profession- al Geologist conducting or directing the study and include a statement of their qualifications outlining their education and experience conducting similar studies. Minimum qualifica- tions of the engineering geologist who performs a fault study include all of the following:
- An undergraduate or graduate degree in geology, engineering geology, geological engineering, or a related field with a strong emphasis on geolo- gic course work, from an accredited college or university.
- Three full years of experience in a responsible posi- tion in the field of engineering geology in Utah, or in a state having similar geologic hazards and regulatory environment. This experience must include the application of technical expertise, including familiarity with local Quaternary geol- ogy, and knowledge of appropriate techniques in performing surface-fault-rupture-hazard studies.
- A current license as a Utah Professional Geologist.
Geologists preparing surface-fault-rupture-hazard stud- ies are ethically bound first and foremost to protect public safety and property, and as such must adhere to the highest ethical and professional standards in their investigations. The geologists’ conclusions, drawn from information gained during the investigation, must be consistent, objective, and unbiased. Relevant information gained during an investiga- tion may not be withheld. Differences in opinion regarding conclusions and recommendations and perceived levels of acceptable risk may arise between consulting geologists per- forming studies and agency-employed or retained geologists working as reviewers for a public agency. Adherence to these guidelines should reduce these differences of opinion and simplify the review process.
Investigation Methods
Inherent in fault study methods is the assumption that future faulting will recur along pre-existing faults (Bonilla, 1970, p. 68; McCalpin, 1987, 1996) and in a manner gener- ally consistent with past displacements (Schwartz and Cop-
Guidelines for evaluating surface-fault-rupture hazards in Utah 5
persmith, 1984; Crone and others, 1987). The focus of sur- face-fault-rupture-hazard investigations is therefore to: 1) determine whether Quaternary faults may exist at a site, 2) accurately identify and locate faults, 3) determine the age of most recent surface rupture and activity class of the faults, and 4) estimate amounts and directions of past displacements to provide a scientific basis for recommending fault setbacks. Special care should be taken in investigations where faults cross landslides. Geomorphic and subsurface features in fault zones and landslides may be similar, and investiga- tions may be inconclusive regarding the origin of such fea- tures. Therefore, report conclusions should address uncer- tainties in the investigation, and recommendations for risk reduction should consider both fault and landslide hazards.
Surface Investigations
The most direct surface method of locating faults and evaluating fault activity is to map fault scarps and surficial geology. Faults may be identified by examining geologic maps and aerial photographs, and by directly observing young, fault-related geomorphic features. Surface investiga- tions include detailed mapping of fault scarps. Topographic profiling of fault scarps can aid in estimating age and amounts of displacement (Bucknam and Anderson, 1979; Andrews and Bucknam, 1987; Hanks and Andrews, 1989; Machette, 1989; McCalpin, 1996). Detailed surface investi- gations help to identify fault scarps and other possible fault- related features such as sag ponds, springs, aligned or dis- rupted drainages, faceted spurs, grabens, and displaced land- forms (terraces, shorelines) or Quaternary geologic units. Site-specific surficial geologic mapping depicts relations between faults and geologic units to help determine the loca- tion and age of faults, and is necessary to identify potential trench sites.
Subsurface Investigations
Trenching is generally required for surface-fault-rup- ture-hazard studies to accurately locate faults, determine the fault activity class, document the nature and extent of fault- related deformation, and measure fault displacements and orientations. Trenches are usually excavated perpendicular to fault traces. Because fault displacements may vary along strike, the investigation should determine the maximum dis- placement along a fault trace at a site and at least one trench should be excavated into the highest part of a scarp. Zones of deformation are common along major fault traces. Such deformation typically consists of multiple dis- crete displacements on secondary shears and is particularly common in graben floors. The trench investigation must define the zone of deformation, and for sites in a graben, trenches must be excavated perpendicular to the bounding faults across the entire part of the site within the graben. Additional subsurface methods such as drilling and geophys- ical surveys may be used and should be clearly described in the report. Geophysical methods may be used to help iden- tify faults in the subsurface to target trench sites, but do not provide sufficient information to preclude trenching. Taylor and Cluff (1973); Sherard and others (1974); Slemmons (1977); Wallace (1977); Hatheway and Leighton (1979); Bonilla (1982); Association of Engineering Geologists, Utah
Section (1987); McCalpin (1987, 1996); and Slemmons and dePolo (1992) summarize investigation methods. Trench location: The purpose of a trenching study and objectives in locating trenches vary depending on the type of development and design phase during which studies are per- formed. When studies are performed prior to site design, such as for multi-unit subdivisions, trenches are used to locate faults and recommend setbacks so that buildings can be placed outside the setback zones. Multiple trenches may be necessary to accurately delineate faults as they cross the property (figure 1).
When studies are performed after building locations have been laid out, trenches may be used to identify faults trending through the proposed building footprints (figure 2). The trenches must be oriented perpendicular to the trend of mapped fault traces at or near the site, and of adequate length to intercept faults projecting toward the proposed building footprint and any potential setback (figure 3). Trenches should therefore extend beyond the building footprint by at
6 Utah Geological Survey
Figure 1. Three possible fault (dashed lines) configurations from fault exposures (X) in only two trenches (A, B) showing the need to measure fault orientations and excavate additional trenches (C), particularly when fault traces are not mappable at the surface.
Trench investigations should be performed in compli- ance with current Occupational Safety and Health Adminis- tration (OSHA) excavation safety regulations (Occupational Safety and Health Administration, 1989; website: osha- slc.gov/SLTC/constructiontrenching/index.html). Trench logging and interpretation: In preparation for log- ging, trench walls should be cleaned of backhoe soil smear to permit direct observation of the geology. Logging at a min- imum scale of 1 inch equals 5 feet (1:60) is recommended, and accepted fault trench investigation practices (McCalpin,
- should be followed. Some form of vertical and hori- zontal logging control must be used and shown on the log. The log should document all pertinent information from the trench, including geologic-unit contacts and descriptions, faults and other deformation features, and sample locations. The engineering geologist interprets the ages of sedi- ments exposed in the trench and, when necessary, obtains samples for radiocarbon or other age determinations to con- strain the age of most recent surface fault rupture. In the Lake Bonneville basin of northwestern Utah, the relation of deposits to latest Pleistocene Bonneville lake-cycle sedi- ments is commonly used to infer ages of sediments, and thus estimate ages of surface-faulting events. Unfaulted Bon- neville lake-cycle sediments in a trench therefore provide evidence that Holocene faulting has not occurred at that site. Outside the Lake Bonneville basin and in the Lake Bon- neville basin but above the highest shoreline, determining the age of surficial deposits is generally less straightforward and commonly requires advanced knowledge of local Quaternary stratigraphy and geomorphology, and familiarity with appro- priate geochronologic techniques. At sites lacking deposits of known and sufficiently old ages, particularly to assess Holocene activity, radiocarbon or other age determinations of deposits that constrain the age of the most recent surface- faulting event may be required (McCalpin, 1996).
Field Review
Field reviews of trenches and trench logs by the Salt Lake County Geologist are required in Salt Lake County, as outlined in appendix A of the Salt Lake County Geologic Hazards Ordinance (Salt Lake County, 2002). Elsewhere in Utah, the UGS is commonly the reviewing agency and, although not required, the UGS appreciates being afforded the opportunity to perform field reviews of trenches. Because the UGS is interested in determining earthquake timing, activity classes, and recurrence intervals on all Qua- ternary faults in Utah, geologists performing surface-fault- rupture-hazard studies are requested to inform the UGS if trenches encounter stratigraphic relations and datable materi- al that may be used to estimate the age of a paleoearthquake.
Fault Setbacks
The UGS recommends that Salt Lake County’s fault set- back calculation method for normal faults (Batatian and Nel- son, 1999; Salt Lake County, 2002) as presented below be used throughout the state. The method should be used to establish the recommended fault setback for structures, depending on the fault activity class, as outlined in table 1. Variables used in the equations are shown in figure 4, and an
example of a setback calculation is given in the appendix. This calculation method is for use with normal faults only. If reverse, thrust, or strike-slip faults are present, the geologist should provide the geologic justification in the report for the method used. Faults and fault setbacks should be clearly identified on the fault map (see Report Guidelines below). Minimum setback recommendations are based on the proposed IBC building occupancy class (table 1). The cal- culated setback using the formulas presented below is com- pared to the minimum setback given in table 1, and the greater of the two is used. Minimum setbacks given in table 1 apply to both the downthrown and upthrown blocks. These setbacks apply only to surface faulting; greater setbacks may be necessary for slope, property boundary, or other consider- ations.
Downthrown Block The fault setback for the downthrown block should be calculated using the following formula:
S = U [2D + (F/tanθ)] where: S = Setback within which buildings are not per- mitted. U = Criticality factor, based on the IBC building occupancy class (table 1). D = Expected fault displacement per event (use the maximum vertical displacement meas- ured for past events or, if not measurable, estimated based on paleoseismic data). Along main traces where displacement is not measurable, a maximum estimated single-event displacement should be used. F = Maximum depth of footing or subgrade por- tion of the building. θ = Dip of the fault (degrees).
Setback distances on the downthrown block are meas- ured from where the fault intersects the final grade level for the building (figure 4). For dipping faults, if the fault trace daylights in the face of a scarp above final building(s) grade, the setback is taken from where the fault would intersect the
8 Utah Geological Survey
Figure 4. Formulas and schematic diagram showing variables used to determine setbacks.
final grade level for the building(s) rather than where it day- lights in the scarp.
Upthrown Block
Because the setback is measured from the portion of the building closest to the fault, whether subgrade or at grade, the dip of the fault and depth of the subgrade portion of the structure are irrelevant in calculating the setback on the upthrown block. The setback for the upthrown side of the fault should be calculated as:
S= U (2D)
Setback distances on the upthrown block are measured from where the trace daylights at the surface, commonly in a scarp. Minimum setbacks apply as discussed above.
Report Review and Applicability
The UGS recommends review of all reports by a licensed Utah Professional Geologist qualified in surface- fault-rupture-hazard studies and acting on behalf of local governments to protect public safety and reduce risks to future property owners and taxpayers. The reviewer should evaluate the adequacy of the investigation, report, and set- backs, and provide recommendations to the local govern- ment regarding the need for additional work, if warranted. Review requirements are outlined in local government ordinances. Surface-fault-rupture-hazard studies for sites in Salt Lake County that are reviewed by the Salt Lake County Geologist must satisfy minimum standards in the Salt Lake County Geologic Hazards Ordinance (Chapter 19.75, appen- dix A). Other local government ordinance requirements for studies are generally non-specific, and the UGS recommends the guidelines given herein be applied elsewhere throughout the state. If study or setback requirements in a local govern- ment ordinance exceed recommendations given herein, ordi- nance requirements must be met. Other state or federal reg- ulations may supercede these guidelines.
REPORT GUIDELINES
Surface-fault-rupture-hazard reports in Utah are expect- ed, at a minimum, to address the topics below. Site condi- tions may require that additional items be included.
A. Purpose and scope of investigation. Describe location and size of site and proposed type and number of buildings (if known).
B. Geologic and tectonic setting. Reference published and unpublished geologic literature with emphasis on current sources, and discuss Quaternary faults in the area, historical seismicity (particularly earthquakes attributed to area faults), and geodetic measurements where pertinent.
C. Site description and conditions. Include pertinent infor- mation on geologic units, geomorphic features, graded and filled areas, vegetation, existing structures, and other factors that may affect the fault study, site development plan, and choice of investigative methods.
D. Methods of investigation.
- Review of published and unpublished maps, litera- ture, and records concerning geologic units, faults, surface and ground water, and other rele- vant factors, with emphasis on current sources.
- Stereoscopic interpretation of aerial photographs to detect fault-related topography, vegetation or soil contrasts, and all lineaments of possible fault origin. List source, date, flight-line numbers, and scale of aerial photos used (preferably 1:24,000 scale or larger).
- Field observations of pertinent surface features, both onsite and offsite, including mapping of geologic units; geomorphic features such as scarps, springs and seeps (aligned or not), faceted spurs, and disrupted drainages; and geo- logic structures as needed, depending on site complexity. Other possible earthquake-induced features such as sand blows, lateral spreads, and other evidence of liquefaction and ground settle- ment should be mapped, described, and assigned ages. Profiling of fault scarps may provide a basis for estimating the age and amount of verti- cal displacement. Landslides, although they may not be conclusively tied to earthquake causes, should also be mapped and described.
- Subsurface investigations including trenching for direct observation of continuous exposures of geologic units, soils, and geologic structures. Trenches must be of adequate length and depth as discussed above (see Investigation Methods section), and be carefully logged. The strike, dip, and vertical displacement (or minimum dis- placement if total displacement cannot be deter- mined) of faults should be noted. The report should describe the criteria used to determine the age and geologic origin of the deposits encoun- tered in the trenches, and clearly evaluate the presence or absence of Holocene, Late Quater- nary, or Quaternary faults.
- Other methods may be required to supplement trench data when special conditions or require- ments for critical facilities demand a more inten- sive investigation. These may include the fol- lowing methods: a. Test pits, boreholes, geoprobe holes, or cone- penetrometer tests. These may provide data on geologic units and ground water at spe- cific locations. The number and spacing of data points must be sufficient to permit valid correlations and interpretations. b. Geophysical investigations. These are indirect methods (Chase and Chapman, 1976; Shar- ma, 1998) that require knowledge of the geology for reliable interpretation. Geo- physical methods alone cannot prove the presence or absence of a fault or determine the age of faulting. Techniques may in-
Guidelines for evaluating surface-fault-rupture hazards in Utah 9
mation should be included, based on deposition- al sequence. Fault orientation and geometry (strike and dip), and amount of displacement should be measured and noted.
Provide evidence for the age determination of geologic units. For suspected Holocene faults where unfaulted Holocene deposits are deeper than practical excavation depths, clearly state the study limitations.
- Borehole Logs. Because boreholes are typically multipurpose, borehole logs should contain stan- dard geotechnical and geologic data such as lithology descriptions, soil class, sampled inter- vals and sample recovery, blow-count results, static ground-water depths and dates measured, total depth of boreholes, drilling and sampling methods, and identity of the person logging the borehole. In addition, borehole, geoprobe hole, and cone-penetrometer logs for fault studies should include the geologic interpretation of de- posit genesis for all layers.
- Geophysical Data and Geologic Interpretations.
- Photographs. Photos of scarps, walls of excava-
tions, or other features may enhance understand- ing of site conditions and report conclusions.
I. Authentication. Include the signature and seal of the investigating licensed Utah Professional Geologist(s); quali- fications giving education and experience in engineering geology and fault studies can be presented in resume format in an appendix (see J. Appendices below).
J. Appendices. Include supporting data relevant to the in- vestigation not given in the text such as cross sections, con- ceptual models, fence diagrams, survey data, water-well data, and qualifications statements.
ACKNOWLEDGMENTS
We thank the Utah Section of the Association of Engi- neering Geologists (Greg Schlenker, Chair), the UGS Geo- logic Hazards Program staff, and Craig dePolo (Nevada Bureau of Mines and Geology) for their helpful comments. Particular thanks go to David Simon (Simon-Bymaster Inc.), Jeff Keaton (AMEC Earth and Environmental), Greg Schlenker (Kleinfelder), and Charles Payton for careful and thorough reviews.
Guidelines for evaluating surface-fault-rupture hazards in Utah 11
12 Utah Geological Survey
Andrews, D.J., and Bucknam, R.C., 1987, Fitting degradation of shoreline scarps by a model with nonlinear diffusion: Jour- nal of Geophysical Research, v. 92, p. 12,857-12,867.
Association of Engineering Geologists, Utah Section, 1987, Guidelines for evaluating surface fault rupture hazards in Utah: Utah Geological and Mineral Survey Miscellaneous Publication N, 2 p.
Batatian, L.D., and Nelson, C.V, 1999, Fault setback require- ments to reduce fault rupture hazards in Salt Lake County: Association of Engineering Geologists Abstracts with Pro- grams, 42nd Annual Meeting, p. 59.
Black, B.D., Hecker, Suzanne, Hylland, M.D., Christenson, G.E., and McDonald, G.N., 2003, Quaternary fault and fold database and map of Utah: Utah Geological Survey Map 193DM, CD-ROM, map scale 1:500,000. Black, B.D., Lund, W.R., Schwartz, D.P., Gill, H.E., and Mayes, B.H., 1996, Paleoseismic investigation on the Salt Lake City segment of the Wasatch fault zone at the South Fork Dry Creek and Dry Gulch sites, Salt Lake County, Utah, in Lund, W.R., editor, Paleoseismology of Utah, Volume 7: Utah Geological Survey Special Study 92, 22 p.
Bonilla, M.G., 1970, Surface faulting and related effects, in Wiegel, R.L., editor, Earthquake engineering: Englewood Cliffs, N.J., Prentice-Hall, Inc., p. 47-74.
—1982, Evaluation of potential surface faulting and other tec- tonic deformation: U.S. Geological Survey Open-File Report 82-732, 58 p.
Bucknam, R.C., and Anderson, R.E., 1979, Estimation of scarp ages from a scarp-height-slope-angle relationship: Geolo- gy, v. 7, p. 11-14.
California Division of Mines and Geology, 1986a (revised), Guidelines to geologic and seismic reports: Division of Mines and Geology Note 42, 2 p.
—1986b (revised), Guidelines for preparing engineering geo- logic reports: Division of Mines and Geology Note 44, 2 p.
Chase, G.W., and Chapman, R.H., 1976, Black-box geology - uses and misuses of geophysics in engineering geology: California Geology, v. 29, p. 8-
Christenson, G.E., 1993, The Wasatch Front County Hazards Geologist Program, in Gori, P.L., editor, Applications of research from the U.S. Geological Survey program, Assess- ment of Regional Earthquake Hazards and Risk along the Wasatch Front, Utah: U.S. Geological Survey Professional Paper 1519, p. 114-120.
Christenson, G.E., and Bryant, B.A., 1998, Surface-faulting hazards and land-use planning in Utah, in Lund, W.R., edi- tor, Western States Seismic Policy Council proceedings volume, Basin and Range Province Seismic-Hazards Sum- mit: Utah Geological Survey Miscellaneous Publication 98- 2, p. 63-73.
Cluff, L.S., Brogan, G.E., and Glass, C.E., 1970, Wasatch fault, northern portion, earthquake fault investigation and evalua- tion, a guide to land use planning: Oakland, California, Woodward-Clyde and Associates, unpublished consultant’s report for the Utah Geological and Mineral Survey, vari- ously paginated.
—1973, Wasatch fault, southern portion, earthquake fault inves- tigation and evaluation, a guide to land use planning: Oak- land, California, Woodward-Lundgren and Associates, unpublished consultant’s report for the Utah Geological and Mineral Survey, variously paginated.
Cluff, L.S., Glass, C.E., and Brogan, G.E., 1975, Investigation
and evaluation of the Wasatch fault north of Brigham City and Cache Valley faults, Utah and Idaho, a guide to land use planning: Oakland, California, Woodward-Lundgren and Associates, unpublished Final Technical Report for the U.S. Geological Survey National Earthquake Hazards Reduction Program, contract no. 14-08-001-13665, variously paginat- ed. Crone, A.J., Machette, M.N., Bonilla, M.G., Lienkaemper, J.J., Pierce, K.L., Scott, W.E., and Bucknam, R.C., 1987, Sur- face faulting accompanying the Borah Peak earthquake and segmentation of the Lost River fault, central Idaho: Bulletin of the Seismological Society of America, v. 77, p. 739-770. dePolo, C.M., and Slemmons, D.B., 1998, Age criteria for active faults in the Basin and Range Province, in Lund, W.R., editor, Western States Seismic Policy Council pro- ceedings volume, Basin and Range Province Seismic-Haz- ards Summit: Utah Geological Survey Miscellaneous Pub- lication 98-2, p. 74-83. Forman, S.L., editor, 1989, Dating methods applicable to Qua- ternary geologic studies in the western United States: Utah Geological and Mineral Survey Miscellaneous Publication 89-7, 80 p. Hanks, T.C., and Andrews, D.J., 1989, Effect of far-field slope on morphological dating of scarplike landforms: Journal of Geophysical Research, v. 94, p. 565-573. Hanson, K.L., Kelson, K.I., Angell, M.A., and Lettis, W.R., 1999, Techniques for identifying faults and determining their origins: Washington, D.C., U.S. Nuclear Regulatory Commission NUREG/CR-5503, variously paginated. Hart, E.W., and Bryant, W.A., 1997 (revised), Fault-rupture hazard zones in California: California Division of Mines and Geology, Special Publication 42, 38 p. Hatheway, A.W., and Leighton, F.B., 1979, Trenching as an exploratory tool, in Hatheway, A. W., and McClure, C.R., Jr., editors, Geology in the siting of nuclear power plants: Geological Society of America, Reviews in Engineering Geology, v. IV, p. 169-195. Hecker, Suzanne, 1993, Quaternary tectonics of Utah with emphasis on earthquake-hazard characterization: Utah Geological Survey Bulletin 127, 157 p. International Code Council, 2000, International Building Code- 2000: Falls Church, Virginia, International Code Council, 756 p. —2003, International Building Code-2003: Falls Church, Vir- ginia, International Code Council, 656 p. Lund, W.R., editor, 1998, Western States Seismic Policy Coun- cil proceedings volume, Basin and Range Province Seis- mic-Hazards Summit: Utah Geological Survey Miscella- neous Publication 98-2, 206 p. Machette, M.N., 1989, Slope-morphologic dating, in Forman, S.L., editor, Dating methods applicable to Quaternary geo- logic studies in the western United States: Utah Geological and Mineral Survey Miscellaneous Publication 89-7, p. 30-
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14 Utah Geological Survey
APPENDIX
EXAMPLE SETBACK CALCULATION
Trenching along the Wasatch fault in southern Salt Lake County identified the main trace of the fault and a minor antithet- ic fault crossing a property. Displacement on the main fault could not be determined, except that it exceeded the 18-foot depth of the trench and the fault accommodated the entire down-to-the-west component of displacement (that is, no other parallel down-to-the-west traces exist in the zone). The main fault dips 70 degrees to the west. Total displacement on the antithetic fault was 2 feet, dipping 50 degrees to the east. Single-family dwellings with basements requiring 8-foot foundation depths are planned. Main fault setback. Because the total displacement could not be measured, an average single-event displacement for the main trace of the Salt Lake City segment of the Wasatch fault of 8.5 feet is estimated based on data in Black and others (1996) and McCalpin (2002)*. Using D = 8.5 feet in the calculation:
Downthrown (western) block = U[2D+(F/tanθ)] = 1.5[(2)(8.5)+8/tan70)] = 1.5(17+3) = 30 feet
Upthrown (eastern) block = U(2D) = 1.5(2)(8.5) = 26 feet
Antithetic fault setback. Because we do not know whether the 2-foot displacement on the antithetic fault is an incremen- tal or single-event displacement, we must assume it occurred in a single event.
Downthrown (eastern) block = U[2D+(F/tanθ)] = 1.5[(2)(2)+8/tan50] = 1.5(4+7) = 17 feet
Upthrown (western) block = U(2D) = 1.5(2)(2) = 6 feet Because 6 feet is less than the 15-foot minimum Setback = 15 feet
*Total single-event displacement data for the Salt Lake City segment are poor because in research trenches the total displace- ment has been spread over several traces. Black and others (1996, p. 15) determined that the combined single-event down-to- the-west displacement along all traces at the South Fork Dry Creek site ranged from 4.9-8.2 feet (1.5-2.5 m)/event, depending on the number of events assumed to displace the late Holocene debris flow at the site. McCalpin (2002, p. 23) estimated the total throw across the two major traces at the Little Cottonwood megatrench site and calculated about 8.5 feet (2.6 m)/event, near the upper end of the Black and others (1996) range. For purposes of setback calculations, a conservative estimate of 8.5 feet (2.6 m) is prudent.
