The following words and terms shall, for the purposes of this appendix, have the meanings shown herein. Refer to Chapter 2 of this code for general definitions.
Earth Fissure. Ground cracks or voids found in the near surface of the earth. Earth fissures are believed to have formed in response to tensional or horizontal stresses from regional land subsidence or to ground shaking from earthquakes resulting in ground deformation or both.
- Fault, Holocene Active: A fault that has moved within the last 11,000 years.
- Fault, Late Quaternary Active: A fault that has moved within the last 130,000 years.
- Fault, Quaternary Active: A fault that has moved within the last 1,600,000 years.
- Fault, Inactive: A fault without recognized activity within the past 1,600,000 years.
Land Subsidence. The gradual downward settling or sinking of the earth's surface.
An evaluation of sites for potential surface rupture or hazards due to differential subsidence and fissuring as described in this appendix shall be performed when any of the following conditions apply:
- A fault has been previously mapped or otherwise documented to exist within 1,000 feet from the site.
- When a fault has been previously mapped within the limits of the property.
- When required by the building official.
The registered design professional performing the evaluation must determine what is appropriate and necessary.
- Dwellings and accessory structures (e.g. casita, patio covers, decks, canopies, etc.) associated with a single lot, single family residence. In this case, the fault location may be historically approximated by the registered design professional through historical research and shall be shown in the Geotechnical Investigation report. A setback of at least fifty (50) feet from each side of the historically approximated fault edge shall be established.
A registered design professional shall perform an evaluation. The evaluation shall include:
- Research of available information, such as geologic maps, technical publications, historical imagery, etc.
- A surface evaluation.
- A subsurface investigation as described in Section P104.1.3 if any Quaternary-age or more recent surface rupture is mapped or otherwise documented to exist within the limits of the property or within 50 feet from the property line as noted in section P188.8.131.52.
The methodology and results of the evaluation must be properly documented in the Geotechnical Investigation report (See section P105 for reporting requirements). Some of the evaluation methods described below should be carried out beyond the site being investigated.
- Historic earthquakes, epicenter locations, and magnitudes in the vicinity of the site.
- Location of fault traces that may affect the site, including maps of faults and a discussion of the tectonics and other relationships of significance to the proposed construction.
- Location and chronology of other earthquake-induced features, such as settlement, landslides and liquefaction.
- Review of local groundwater data (water-level fluctuations, groundwater impediments, water quality variations, or anomalies indicating possible faults).
- Identify and locate any faults, scarps, and fissures in the vicinity of the site.
- Review available land level lines of past ground surface movement in the vicinity of the site, including degree of differential subsidence across nearby faults and proximity of regional subsidence bowls.
- Review groundwater development in the vicinity including the location of nearby high-capacity wells. Review available well maintenance records of nearby wells for signs of possible subsidence-induced damage.
- Review of subsurface units from available well driller's logs for nearby water wells and available historic water level data from nearby wells (e.g. the State of Nevada Department of Water Resources through their website provides free access to Nevada hydrology data, including well logs and historic and current water levels).
- Conduct visual inspections for signs of ground movement (distress) of man-made structures on adjacent developments. Review available geotechnical reports to determine the geotechnical conditions of sites in the area.
- Mapping of surface features, including geologic units and structures and topographic features both on and beyond the site.
If any Quaternary-age or more recent surface rupture is mapped or otherwise documented to exist within the limits of the property or within 50 feet from the property line, the feature(s) shall be further investigated as described in section P104.1.3.
Note: In the event that the subsurface investigation cannot be performed beyond the limits of the property, the registered design professional shall perform the subsurface investigation within the limits of the property, as close as practical to the feature of interest, to disprove the possibility of the fault being present onsite.
- This includes trenching across potentially active fault zones to determine the following: location and recency of movement, width of disturbance, physical condition of fault zone materials, type of displacement, geometry of fault features, slip rate, and recurrence interval.
- Borings or test pits to collect data to evaluate depth and type of materials present, groundwater depth, and to verify fault-plane geometry. Data points should be sufficient in number and adequately spaced to permit correlations and interpretations.
- Geophysical surveys conducted to facilitate the evaluation of the types of site materials and their physical properties, ground water conditions, and fault displacements. When geophysics is utilized for fault mapping, a minimum of two arrays perpendicular to the suspected fault trace shall be performed. The geophysical exploration program, including the number of geophones, type of geophones, spacing and other survey parameters, shall be selected by the registered design professional.
- Purpose and scope of investigation.
- Geologic setting.
- Site description and conditions, including information on geologic units, aquifer conditions, graded and filled areas, vegetation, existing structures, and other factors that may affect the choice of investigative methods and the interpretation of data.
- Methods of investigation utilized.
- Location (or absence) of all surface ruptures on or adjacent to the site.
- Type of faults and nature of anticipated offset: Direction of relative displacement, and maximum possible displacement.
- Statement of relative risk addressing the probability or relative potential for future surface displacement. This may be stated in semi-quantitative terms such as low, moderate, or high, or in terms of slip rates determined for specific fault segments.
- Degree of confidence in, and limitations of, the data and conclusions.
- The minimum Setbacks shall be per section 1808.10. If the recency of movement cannot be determined, then the fault shall be assumed to be Holocene for minimum setback purposes.
- The faults and minimum setback shall be clearly shown to scale on the grading plan, plot plan and final map; no portion of the foundation system shall be constructed within that zone.
- Need for additional studies, or inspection during construction.
- Literature and records cited or reviewed; citations should be complete.
- Aerial photographs or images interpreted including type, date, scale, source, and index numbers.
- Other sources of information, including well records, personal communications, and other data sources.
- Location map - identify site locality, significant faults, geographic features, regional geology, seismic epicenters, and other pertinent data. A 1:24,000 scale is recommended.
- Site development map. Show site boundaries, existing and proposed structures, graded areas, streets, exploratory trenches, borings, geophysical traverses, and other data. Recommended scale is 1 inch equals 200 feet (1:2,400) or larger.
- Geologic map. Shows distribution of geologic units (if more than one), faults and other structures, geomorphic features, aerial photo lineaments, and springs, on topographic map at 1:24,000 scale or larger. Can be combined with items 1 or 2.
- Geologic cross-sections.
- Logs of exploratory trenches and borings. Show details of observed features and conditions; should not be generalized or diagrammatic. Trench logs should show topography and geologic structure at the same horizontal and vertical scale.
- Geophysical data and geologic interpretations.
- Photographs of scarps, surface ruptures, trenches, samples, or other features that enhance understanding of the site conditions.