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SURFACE-FAULT-RUPTURE-HAZARD MAP FOR THE ST. GEORGE–HURRICANE METROPOLITAN AREA

Basemap consists of National Agricultural Imagery Programnatural color aerial photography.

Universal Transverse Mercator Projection, zone 12.North American Datum 1983.

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DISCUSSION

USING THIS MAP

MAP LIMITATIONS

SYMBOLS

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An earthquake is the abrupt rapid shaking of the ground caused by sudden slippage of rocks deep beneath the Earth surface. Earthquakes occurwithout warning and can cause great injury and death, major economic loss, and social disruption. The surface along which the rocks slip is called afault. Several faults in the St. George – Hurricane metropolitan area are capable of producing earthquakes. Among the potential effects of largeearthquakes is surface faulting, which occurs when movement at depth on a fault during an earthquake propagates to the surface. The resultingdisplacement at the ground surface produces ground cracking and typically one or more “fault scarps.” Depending on the size of the earthquake, faultscarps can range from a few inches to many feet high and extend for many miles along the fault trace. Local ground tilting and graben formation bysecondary faulting may accompany surface faulting, resulting in a zone of deformation along the fault trace that can be tens to hundreds of feet wide.Surface faulting, while of limited aerial extent when compared to other earthquake-related hazards such as ground shaking, can have seriousconsequences for structures or other facilities that lie along or cross the rupture path.

For additional information about surface faulting and other earthquake-related hazards in the St. George – Hurricane metropolitan area, refer to theEarthquake-Hazards and Earthquake-Ground-Shaking Hazard text documents in this report.

Because surface faulting is typically confined to relatively narrow zones along the surface trace of a fault, early recognition and avoidance is the mosteffective strategy for mitigating this hazard. Once the activity class of the fault is determined, we recommend that facilities be set back from the fault traceand any associated zone of deformation in accordance with the UGS guidelines (Christenson and others, 2003). In the absence of practical designtechniques for mitigating surface faulting, carefully locating all potentially active fault traces on a site, assessing their level of activity and amount ofdisplacement, and establishing an appropriate setback distance from the fault remain the most reliable procedures for mitigating damage and injury due tosurface faulting.

In Utah, earthquake-resistant design requirements are specified in the seismic provisions of the International Building Code (International Code Council,2006a) and International Residential Code (International Code Council, 2006b), which are adopted statewide. IBC Section 1802.2.7 requires that aninvestigation be conducted for all structures in Seismic Design Categories C, D, E, or F (see Earthquake-Ground-Shaking-Hazard text document) to evaluatethe potential for surface rupture due to faulting.

Buried or approximately located Holocene or suspected Holocene fault: surface-fault-rupture-hazard studies recommended for allstructures intended for human occupancy and all critical facilities.

Well-defined Holocene (movement within the past 10,000 years) or suspected Holocene fault: surface-fault-rupture-hazard studiesrecommended for all structures intended for human occupancy and all critical facilities .

PLATE 1

by

William R. Lund, Tyler R. Knudsen, Garrett S. Vice, and Lucas M. Shaw

Well-defined late Quaternary (movement within the past 130,000 years) fault: surface-fault-rupture-hazard studies recommended for allcritical facilities. Studies for other structures for human occupancy remain prudent, but should be based on an assessment of whetherrisk-reduction measures are justified by weighing the probability of occurence against the risk to lives and potential economic loss.

Well-defined Quaternary (movement within the past 1,800,000 years) fault: surface-fault-rupture-hazard studies recommended for allcritical facilities. Studies for other structures intended for human occupancy are optional because of the low likelihood of surfacefaulting, although surface rupture along the fault is still possible.

Buried or approximately located Quaternary (movement within the past 1,800,000 years) fault: surface-fault-rupture-hazard studiesrecommended for all critical facilities. Studies for other structures intended for human occupancy are optional because of the lowlikelihood of surface faulting, although surface rupture along the fault is still possible.

Well-defined fault with unknown activity level: paleoseismic data are lacking, recommend treating as a Holocene fault until provenotherwise.

Buried or approximately located fault with unknown activity level: paleoseismic data are lacking, recommend treating as a Holocenefault until proven otherwise.

1

Surface-fault-rupture-hazard special-study area. The special-study areas established for well-defined faults extend for 500 feet on thedownthrown side and 250 feet on the upthrown side of each fault. Because their location is uncertain, the special-study areas aroundburied or approximately located faults are broader, extending 1000 feet on each side of the suspected trace of the faults.

HAZARD REDUCTION

The Surface-Fault-Rupture-Hazard Map is based on 1:24,000-scale geologic mapping. We believe that the inventory of potentially active faults obtainedfrom that mapping and shown on the map is complete or nearly so at that scale. However, some smaller faults may not have been detected during themapping or are concealed beneath young geologic deposits. Additionally, buried and approximately located faults by definition lack a clearly identifiablesurface trace, and therefore their location is imperfectly known. Site-specific fault-trenching studies should be preceded by a careful field evaluation of thesite to identify the surface trace of the fault, other faults not evident at 1:24,000-scale, or other fault-related features at a site-specific scale prior to trenching.

Fault - see Explanation for description of colorcodes; ball and bar on downthrown side of fault.

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1Critical facilities are Occupancy Category III and IV structures as defined in the International Building Code (IBC, table 1604.5, p. 281; International Code Council, 2006a), and include schools, hospitals, fire stations, high occupancy buildings, water-treatment facilities, and facilities containing hazardous materials.

Buried or approximately located late Quaternary (movement within the past 130,000 years) fault: surface-fault-rupture-hazard studiesrecommended for all critical facilities. Studies for other structures for human occupancy remain prudent, but should be based on anassessment of whether risk-reduction measures are justified by weighing the probability of occurence against the risk to lives andpotential economic loss.

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 anycircumstances for any direct, indirect, special, incidental, or consequential damages with respect to claims byusers of this product.

For use at 1:24,000 scale only. The Utah Geological Survey does not guarantee accuracy or completeness ofdata.

The Surface-Fault-Rupture-Hazard Map shows potentially active faults along which surfacefaulting may occur. A special-study area is shown around each fault, within which the UGSrecommends that a site-specific, surface-fault-rupture-hazard study be performed prior toconstruction. These studies can resolve uncertainties inherent in generalized hazardmapping and help ensure safety by identifying the need for fault setbacks.

Utah Geological Survey Guidelines for Evaluating Surface-Fault-Rupture Hazards in Utah(Christenson and others, 2003) include a detailed rationale for performing surface-fault-rupture-hazard studies, minimum technical requirements for conducting and reporting thosestudies, recommendations regarding when surface-fault-rupture-hazard studies should beconducted based on fault activity class and the type of facility proposed, and procedures forestablishing safe setback distances from active faults. City and county officials, planners, andconsultants should refer to the guidelines regarding the details of conducting and reviewingsurface-fault-rupture-hazard investigations.

For well-defined faults color-coded red, orange, and green (Holocene, late Quaternary, andQuaternary, respectively), we recommend that surface-fault-rupture-hazard studies beperformed in accordance with the UGS guidelines. Because paleoseismic data are lacking forthe purple-coded faults (fault activity class unknown), we recommend that those faults beconsidered Holocene faults until paleoseismic studies demonstrate otherwise.

Because buried and approximately located faults lack a clearly identifiable surface trace, theyare not amenable to trenching, which is the standard surface-fault-rupture-hazard evaluationtechnique used to study well-defined faults (McCalpin, 1996). Where development isproposed

proposed in a special-study area for a buried or approximately located fault, we recommendthat at a minimum the following tasks be performed to better define the surface-fault-rupturehazard in those areas.

If the results of these studies reveal evidence of possible surface-faulting-related features,those features should be trenched in accordance with the UGS guidelines. Following theabove-recommended studies, if no evidence of surface faulting is found, development at thesite can proceed as planned. However, we recommend that construction excavations andcuts be carefully examined for evidence of faulting as development proceeds.

Review of published and unpublished maps, literature, and records concerninggeologic units, faults, surface and ground water, previous subsurfaceinvestigations, and other relevant factors.

Stereoscopic interpretation of aerial photographs to detect any subtle fault-related features expressed in the site topography, vegetation or soil contrasts,and any lineaments of possible fault origin.

Field evaluation of the proposed site and surrounding area to observe pertinentsurface evidence for faulting; map geologic units as necessary to define criticalgeologic relations; evaluate geomorphic features such as springs or seeps(aligned or not), sand blows or lateral spreads, or other evidence ofearthquake-induced features; and excavation of test pits to evaluate the age ofthe deposits onsite to constrain the time of most recent surface faulting.

1.

2.

3.

UTAH GEOLOGICAL SURVEYa division of

Utah Department of Natural Resources

Plate 1

Utah Geological Survey Special Study 127

Geologic Hazards and Adverse Construction Conditions,

St. George−Hurricane Metropolitan Area, Washington County, Utah