Chapter 14.100
SEISMIC (EARTHQUAKE) HAZARD AREAS
Sections:
14.100.020 Seismic hazard areas.
14.100.030 Seismic hazard area review procedures.
14.100.040 Seismic hazard area standards.
14.100.050 Buffer requirements.
14.100.010 Purpose.
Earthquakes have historically occurred throughout the Puget Sound region. Large earthquakes have caused loss of life and over a billion dollars in property damage. The purpose of this chapter is to protect the public health, safety, and general welfare of the citizens of Edgewood from the damaging effects of earthquakes. This chapter provides standards to ensure life safety and minimize public and private losses that may occur within a seismic hazard area. (Ord. 17-513 § 3 (Exh. A)).
14.100.020 Seismic hazard areas.
A. General. Seismic hazard areas are areas subject to severe risk of damage as a result of earthquake induced landsliding, seismic ground shaking, dynamic settlement, fault rupture, or soil liquefaction.
B. Potential Seismic Hazard Areas. Potential seismic hazard areas are those areas where the suspected risk of earthquake induced landsliding, dynamic settlement, fault rupture, ground deformation caused by soil liquefaction, or flooding is sufficient to require a further seismic hazard area review. These potential seismic hazard areas are determined using the following criteria:
1. Earthquake Induced Landslide Hazard Areas. Areas identified as potential landslide hazard areas in EMC 14.90.020.
2. Liquefaction or Dynamic Settlement Hazard Areas. Areas identified as high and moderate liquefaction and dynamic settlement hazard areas on the geologically hazardous areas map.
3. Fault rupture hazard areas.
C. Seismic Hazard Area Categories.
1. Earthquake Induced Landslide Hazard Areas. Earthquake induced landslide hazard areas include slopes that can become unstable as a result of strong ground shaking, even though these areas may be stable under nonseismic conditions.
2. Liquefaction and/or Dynamic Settlement Hazard Areas.
a. Liquefaction hazard areas are areas underlain by unconsolidated (corrected Standard Penetration Test blow counts, [(N1)60] less than 30) sandy or silt soils (Unified Soil Classification System S or M soil types) and a shallow groundwater table (static groundwater depth less than 30 feet) capable of liquefying in response to earthquake shaking.
b. Dynamic settlement hazard areas are areas underlain by a significant thickness (more than 10 feet) of loose or soft soil not susceptible to liquefaction (e.g., peats or organic silts and clays, unsaturated loose sands or silts), but that could result in vertical settlement of the ground surface in response to earthquake shaking.
3. Fault Rupture Hazard Areas. Fault rupture hazard areas include:
a. Active fault rupture hazard areas are areas where displacement (movement up, down, or laterally) of the ground surface has occurred during past earthquake(s) in the Holocene epoch; and
b. Areas adjacent to the active fault rupture hazard area that may be potentially subject to ground surface displacement in a future earthquake. (Ord. 17-513 § 3 (Exh. A)).
14.100.030 Seismic hazard area review procedures.
A. General Requirements.
1. The city’s geologically hazardous areas map provides an indication of where potential seismic hazard areas are located within the city.
2. The department will complete a review of the Critical Areas Atlas – Seismic Hazard Area Map for any regulated activity to determine whether the site for a proposed regulated activity is located within a seismic hazard area.
3. When the department’s maps indicate that the site for a proposed regulated activity is located within a potential liquefaction or dynamic settlement hazard area, the department shall require the submittal of a geological assessment as outlined in subsection (B) of this section.
4. When the department’s maps indicate that the site for a proposed regulated activity is located within a potential fault rupture hazard area, the department shall require the submittal of a geological assessment as outlined in subsection (B) of this section. The requirement to submit a geological assessment may be waived at the department’s discretion when it is determined that the proposed project area for the regulated activity is located outside the potential fault rupture hazard area.
5. When the department’s maps indicate that the site for a proposed regulated activity is or may be located within a potential earthquake induced landslide hazard area, the department shall conduct a review pursuant to the requirements set forth in Chapter 14.90 EMC.
6. Unless otherwise stated in this chapter, the critical area protective measure provisions contained in Chapter 14.10 EMC shall apply.
B. Geological Assessments. A geological assessment is a site investigation process to evaluate the on-site geology affecting a subject property and define the extent and severity of potential seismic hazards.
1. A geological assessment shall be required when the department’s maps, sources, or field investigation indicate a site contains a potential liquefaction, dynamic settlement, or fault rupture hazard area. Geological assessments shall be submitted to the department for review and approval together with a seismic hazard area application.
2. A geotechnical professional(s) shall complete a field investigation and geological assessment to determine whether or not the site for a proposed regulated activity is located within a liquefaction or dynamic settlement hazard area.
a. The geological assessment shall be submitted in the form of a geotechnical verification when the geotechnical professional(s) finds that no liquefaction or dynamic settlement hazard area exists within the proposed project area.
b. The geological assessment shall be submitted in the form of a geotechnical report when the geotechnical professional(s) finds that a liquefaction or dynamic settlement hazard area exists within the proposed project area.
3. A geotechnical professional shall complete a field investigation and geological assessment presented in the form of a geotechnical report to determine whether or not the site for a proposed regulated activity is located within a fault rupture hazard area. Any structural recommendations proposed to mitigate the fault rupture hazard that are included in the geotechnical report shall be prepared by an engineer.
4. All geological assessments for seismic hazards submitted under this chapter shall include, at a minimum, the following items identified in subsections (B)(4)(a) through (i):
a. All of the items required per EMC 14.10.080(C).
b. The parcel number(s) of the subject property.
c. Site address, if the city has assigned one.
d. A brief description of the project (including the proposed land use) and the area to be developed.
e. A map showing the property lines for the site, existing two-foot contours of the existing site topography, and the location of any existing structures, utilities, wells, stormwater or septic systems, or other developments.
f. A site plan delineating the limits of the proposed development and the location of all areas of the site subject to potential seismic hazards based on the geologically hazardous areas map and, if applicable, limits of associated buffers.
g. A description of the surface and subsurface geology, hydrology, soils, and vegetation of the site.
h. A detailed overview of the field investigations, published data and references, data and conclusions from past geological assessments or geotechnical investigations of the site, site-specific measurements, tests, investigations, or studies, as well as the methods of data analysis and calculations that support the determination regarding whether liquefaction and/or dynamic settlement hazards are present on the site.
i. The results, conclusions, and recommendations resulting from the geological assessment of the liquefaction and/or dynamic settlement hazards on the subject property as prepared by a geotechnical professional(s).
5. Geological assessments shall be prepared, signed, stamped, and dated by the appropriate geotechnical professional(s) and the format shall be pre-approved by the department.
6. Geological assessments that do not contain the minimum required information will be returned to the geotechnical professional(s) for revision.
7. The department shall review the geological assessment and either:
a. Accept the geological assessment and approve the application; or
b. Reject the geological assessment and require revisions or additional information.
8. A geological assessment for a specific site may be valid for a period of up to five years when the proposed land use activity and surrounding site conditions are unchanged. However, if any environmental conditions associated with the site change during that five-year period, the applicant may be required to submit an amendment to the geological assessment. (Ord. 17-513 § 3 (Exh. A)).
14.100.040 Seismic hazard area standards.
A. Earthquake Induced Landslide Hazard Areas. All standards set forth in Chapter 14.90 EMC shall apply to earthquake induced landslide hazard areas.
B. Liquefaction or Dynamic Settlement Hazard Areas.
1. All building structures shall conform to the standards set forth in EMC Title 15, Buildings and Construction.
2. Utility Lines. Utility lines, except for gas pipelines, which are prohibited, will be permitted when no other conveyance alternative is available. The line shall be located above ground and properly anchored and/or designed so that it will continue to function in the event of seismically induced ground deformation. Provision for automatic shutoff of utilities in a ground-rupturing event will be required.
3. Roads, Bridges, and Trails. Roads, bridges, and trails shall be allowed when mitigation measures are provided that ensure the roadway prism or bridge structure will not be susceptible to damage from seismic induced ground deformation. Mitigation measures shall be designed for static and seismic loading conditions in accordance with the most recent version of the American Association of State Highway and Transportation Officials (AASHTO) Manual and also for an estimated range of ground surface offset presented in the geotechnical report.
C. Fault Rupture Hazard Areas. Any development, encroachment, clearing and grading, or building structures shall be prohibited within fault rupture hazard areas and associated buffers except as specified in the following standards:
1. Utility Lines. Utility lines, except for gas pipelines, which are prohibited, will be permitted when no other conveyance alternative is available. The line shall be located above ground and properly anchored and/or designed so that it will continue to function in the event of seismically induced ground deformation. Provision for automatic shutoff of utilities in a ground-rupturing event will be required.
2. Roads, Bridges, and Trails. Roads, bridges, and trails shall be allowed when all of the following conditions have been met:
a. Mitigation measures are provided that ensure the roadway prism and/or bridge structure will not be susceptible to damage from seismically induced ground deformation. Mitigation measures shall be designed for static and seismic loading conditions in accordance with the most recent version of the American Association of State Highway and Transportation Officials (AASHTO) Manual and also for an estimated range of ground surface offset presented in the geotechnical report.
b. The road is not a sole access for a development. (Ord. 17-513 § 3 (Exh. A)).
14.100.050 Buffer requirements.
A. Determining Buffer Widths.
1. The buffer width shall be measured on a horizontal plane from a perpendicular line established at the edge of the fault rupture hazard area limits.
2. A buffer is an area that is adjacent to a fault rupture hazard area that may be potentially subject to ground surface displacement in a future earthquake. No development shall be permitted within a fault rupture hazard area and its associated buffer. The required buffer width is the greater amount of the following distances:
a. Fifty feet from all edges of a fault rupture hazard area, except for high occupancy or essential facilities, where the minimum buffer distance shall be 100 feet; or
b. The required buffer width is the minimum distance recommended by the geotechnical professional(s).
B. Modification of Buffer Widths. The department may require a larger buffer width than the buffer distance, as determined in subsection (A) of this section, if the department determines the standard or proposed buffer is not adequate to protect the health, safety, or welfare of any proposed development. (Ord. 17-513 § 3 (Exh. A)).
14.100.060 Appendices.
A. Geological Assessments – Liquefaction or Dynamic Settlement Hazard Areas.
B. Geological Assessments – Fault Rupture Hazard Area Geotechnical Report.
APPENDIX A
GEOLOGICAL ASSESSMENTS – LIQUEFACTION OR DYNAMIC SETTLEMENT HAZARD AREAS
Article I. Geotechnical Verification
A. A geotechnical verification shall, at a minimum, include the following:
1. The general critical areas report requirements.
2. The geotechnical verification shall include all mandatory items listed in this chapter.
3. The geological assessment must include a determination that no liquefaction or dynamic settlement hazard exists within the proposed project area.
4. The verification shall include an accurate site plan drawn at a scale of either one inch equals 20 feet, 30 feet, or 50 feet, unless otherwise approved by the department. The department may require that the site plan information be based on a field survey by a licensed surveyor. The site plan shall include:
a. Property lines for the site, and the location of any existing structures.
b. The full geographical limits of the proposed project area or conceptual project area (i.e., area to be developed) and the location of any proposed structures, on-site septic systems, wells, and stormwater management features or facilities associated with the development, if known.
B. The geotechnical professional(s) who prepared the geotechnical verification shall stamp the verification with their license stamp or seal.
C. Hold harmless clauses, disclaimers, and limitations are not allowed within a geotechnical verification.
Article II. Geotechnical Report
A. A geotechnical report shall, at a minimum, include the following:
1. The general critical areas report requirements.
2. The document shall include all mandatory items listed in this chapter. The report shall be prepared by an engineer and shall be co-written by an engineering geologist where geological interpretations and conclusions critical to the assessment of liquefaction and/or dynamic settlement hazard and potential effects are necessary or prudent. The report shall specify the desired performance level of the structures and other development facilities, e.g., safety to building occupants, minimal damage to structure, post-earthquake serviceability for pre-earthquake operations, or no damage.
3. The results, conclusions, and recommendations resulting from the geological assessment of the liquefaction or dynamic settlement hazards on the subject property as prepared by the geotechnical professional(s).
4. The geological assessment geotechnical report shall include:
a. A statement that the proposed project area falls within a liquefaction and/or dynamic settlement hazard area.
b. A detailed engineering evaluation of expected ground displacements or other liquefaction or dynamic settlement effects, e.g., bearing failures, flotation of buried tanks, or similar, and proposed mitigation measures to ensure an acceptable level of risk for the proposed structure type or other development facilities, as well as the proposed land use type or occupancy category. The minimum level of acceptable risk for any proposed structure or development facility shall ensure the life safety of any occupant. Where appropriate, a range of mitigation options should be considered depending on site conditions, the intended use of the structures, and acceptable levels of settlement.
5. The report shall include a site plan drawn to scale. The department may require that the site plan information be based on a field survey by a licensed surveyor. The site plan shall include:
a. Property lines for the site and the location of any existing structures.
b. The limits or location of any liquefaction or dynamic settlement hazard area(s).
c. The full geographical limits of the proposed project area or conceptual project area (i.e., area to be developed) and the location of any proposed structures, on-site septic systems, wells, and stormwater management features or facilities associated with the development, if known.
d. Location and unique identifier of geotechnical explorations used to characterize subsurface conditions.
6. The geotechnical study shall include field exploration sufficient to assess the potential for liquefaction or dynamic settlement hazards and options for mitigation of those hazards. Copies of the exploration logs shall be provided in the report. The geotechnical study shall include field exploration sufficient to assess the potential for liquefaction or dynamic settlement hazards and options for mitigation of those hazards. Copies of the exploration logs shall be included in the report. The project geotechnical professional must provide justification for the scope of the field exploration program. The city’s geotechnical professional reserves the right to request additional exploration if deemed appropriate. If a dispute arises between the city’s geotechnical professional and the project geotechnical professional regarding the scope of the field exploration, the city reserves the right to require an independent, third party review to be paid for by the applicant to resolve the dispute.
7. If beneficial to the assessment of seismic hazards for the project, the three-dimensional subsurface conditions at the site shall be presented using one or more cross-sections showing location and depth penetration of borings or CPT soundings, interpretation of the geometry of major soil units, and projected location of the static groundwater surface determined from the subsurface exploration. The cross-sections shall be presented at a scale of one inch equals 20 feet, one inch equals 30 feet, one inch equals 50 feet (or other scale deemed appropriate by the department). Each cross-section shall have a legend with a description of the various major soil units. The city’s geotechnical professional reserves the right to request inclusion of one or more cross-sections in the geotechnical report. If a dispute arises between the project geotechnical professional and the city’s geotechnical professional regarding this issue, then the city reserves the right to require an independent, third party review to be paid for by the applicant to resolve the dispute.
8. All assessments of liquefaction or dynamic settlement hazards and effects will be based on a design earthquake using ground motion parameters consistent and equivalent to those specified in the most current version of the International Building Code. These assessments shall use the shallowest groundwater table observed during or inferred from subsurface exploration and characterization, e.g., the measured depth of static groundwater immediately prior to abandonment of borings, or observation of iron oxide mottling of soils samples.
9. Results of laboratory testing of samples retrieved during drilling and sampling shall be presented in order to support the values of fines contents used in subsequent analysis of liquefaction and/or dynamic settlement hazard. Where only CPT methods are used in site assessment, the correlation between fines content and CPT measurements will be discussed and documented. This documentation will require rigorous correlation of CPT and fines content measurements from similar geological deposits within the Puget Sound region.
10. The geotechnical report shall include a detailed assessment of the liquefaction and/or dynamic settlement hazard based on analysis of available subsurface data using state-of-the-practice methodologies. The results of the analysis shall be documented, and all results of intermediate and final calculations and results, including factors of safety, shall be included.
11. When appropriate, the geotechnical report shall include an assessment of the potential for large lateral spreads or flow failures, bearing failures, settlement, limited lateral displacement, and flotation of buried facilities. The methodologies used must be, at a minimum, state-of-the-practice, and the conclusions regarding the potential and severity of the possible liquefaction and/or dynamic settlement induced failure modes shall be presented.
12. Alternative mitigative measures including structural and foundation design options and/or soil improvement techniques shall be evaluated and compared for their effectiveness in reaching the level of performance specified in the report introduction. Effectiveness of soil improvement techniques shall be specified in terms of post-treatment densification or strength improvement as measured by appropriate subsurface investigation and testing. The extent of the post-treatment verification testing shall be provided on a site map at the same scale as the map presented in Article I. Geotechnical review of all final plans is required and the findings of the review shall be documented in writing.
B. The geotechnical professional(s) who prepared the geotechnical report shall stamp the report with their license stamp or seal.
C. Hold harmless clauses, disclaimers, and limitations are not allowed within a geotechnical report.
APPENDIX B
GEOLOGICAL ASSESSMENTS – FAULT RUPTURE HAZARD AREA GEOTECHNICAL REPORT
A. A geotechnical report shall, at a minimum, include the following:
1. The general critical areas report requirements contained herein.
2. The report shall be prepared by an engineer and shall be co-written by an engineering geologist where geological interpretations and conclusions critical to the assessment of liquefaction and/or dynamic settlement hazard and potential effects are necessary or prudent.
3. The following topics should be considered and addressed in detail where essential to support opinions, conclusions, and recommendations in any geologic report on faults. It is not expected that all the topics or investigative methods would be necessary in a single investigation. In specific cases, it may be necessary to extend some of the investigative methods well beyond the site or property being investigated.
a. Purpose and scope of investigation; description of proposed development.
b. Geologic and tectonic setting. Include seismicity and earthquake history.
c. Site description and conditions, including dates of site visits and observations. Include information on geologic units, graded and filled areas, vegetation, existing structures, and other factors that may affect the choice of investigative methods and interpretation of data.
d. Methods of Investigation.
i. Review of published and unpublished literature, maps, and records concerning geologic units, faults, groundwater barriers, and other factors.
ii. Stereoscopic interpretation of aerial photographs, review of LiDAR based topography, and other remotely sensed images to detect fault-related topography (geomorphic features), vegetation and soil contrasts, and other lineaments of possible fault origin. The area interpreted usually should extend beyond the site boundaries.
iii. Surface observations, including mapping of geologic and soil units, geologic structures, geomorphic features and surfaces, springs, deformation of engineered structures due to fault creep, both on and beyond the site.
iv. Subsurface Investigations.
(A) Trenching and other excavations to permit detailed and direct observation of continuously exposed geologic units, soils, and structures; must be of adequate depth and be carefully logged (Taylor & Cluff 1973, Hatheway & Leighton 1979, McCalpin 1996b).
(B) Borings and test pits to permit collection of data on geologic units and groundwater at specific locations. Data points must be sufficient in number and spaced adequately to permit valid correlations and interpretations.
(C) Cone penetrometer testing (CPT) (Grant et al., 1997, Edelman et al., 1996). CPT must be done in conjunction with continuously logged borings to correlate CPT results with on-site materials. The number of borings and spacing of CPT soundings should be sufficient to adequately image site stratigraphy. The existence and location of a fault based on CPT data are interpretative.
v. Geophysical Investigations. These are indirect methods that require a knowledge of specific geologic conditions for reliable interpretations. They should seldom, if ever, be employed alone without knowledge of the geology (Chase & Chapman 1976). Geophysical methods alone never prove the absence of a fault nor do they identify the recency of activity. The types of equipment and techniques used should be described and supporting data presented (California Board of Registration for Geologists and Geophysicists, 1993).
(A) High-resolution seismic reflection (Stephenson et al., 1995, McCalpin, 1996b).
(B) Ground penetrating radar (Cai et al., 1996).
(C) Other methods include: seismic refraction, magnetic profiling, electrical resistivity, and gravity (McCalpin, 1996b).
vi. Age-dating techniques are essential for determining the ages of geologic units, soils, and surfaces that bracket the time(s) of faulting (Pierce, 1986, Birkeland et al., 1991, Rutter & Catto, 1995, McCalpin, 1996a).
(A) Radiometric dating (especially 14C).
(B) Soil profile development.
(C) Rock and mineral weathering.
(D) Landform development.
(E) Stratigraphic correlation of rocks, minerals, and fossils.
(F) Other methods: artifacts, historical records, tephrochronology, fault scarp modeling, thermoluminescence, lichenometery, paleomagnetism, dendrochronology, etc.
vii. Other methods should be included when special conditions permit or requirements for critical structures demand a more intensive investigation.
(A) Aerial reconnaissance overflights.
(B) Geodetic and strain measurements.
(C) Microseismicity monitoring.
e. Conclusions.
i. Location and existence (or absence) of hazardous faults on or adjacent to the site; ages of past rupture events.
ii. Type of faults and nature of anticipated offset, including sense and magnitude of displacement, if possible.
iii. Distribution of primary and secondary faulting (fault zone width) and fault-related deformation.
iv. Probability of, or relative potential for, future surface displacement. The likelihood of future ground rupture seldom can be stated mathematically, but may be stated in semiquantitative terms such as low, moderate, or high, or in terms of slip rates determined for specific fault segments.
v. Degree of confidence in, and limitations of data and conclusions.
f. Recommendations.
i. The recommended increase from the standard buffer distance (50 feet) of proposed structures from fault rupture hazard areas. The recommended buffer distance generally will depend on the quality of data and type and complexity of fault(s) encountered at the site and the proposed land use type (i.e., occupancy). In order to establish an appropriate buffer distance from a fault located by indirect or interpretative methods (e.g., borings or cone penetrometer testing), the area between data points also should be considered underlain by a fault unless additional data are used to more precisely locate the fault. Additional measures (e.g., strengthened foundations, engineering design, and flexible utility connections) to accommodate warping and distributive deformation associated with faulting (Lazarte and others, 1994).
ii. Risk evaluation relative to the proposed development.
iii. Limitations of the investigation; need for additional studies.
g. References.
i. Literature and records cited or reviewed; citations should be complete.
ii. Aerial photographs or images interpreted: list type, data, scale, source, and index numbers.
iii. Other sources of information, including well records, personal communications, and other data sources.
h. Illustrations. The following illustrations should be provided:
i. A location map that identifies site locality, significant faults, geographic features, regional geology, seismic epicenters, and other pertinent data; 1:24,000 scale is recommended.
ii. A site development map that shows site boundaries, existing and proposed structures and limits of the proposed project area, graded areas, streets, exploratory trenches, borings geophysical traverses, locations of faults, and other data; recommended scale is 1:2,400 (one inch equals 200 feet), or larger.
iii. A geologic map that shows the distribution of geologic units (if more than one), faults and other structures, geomorphic features, aerial photographic lineaments, and springs; on topographic map 1:24,000 scale or larger; can be combined with subsection (A)(3)(h)(i) or (ii) of this appendix.
iv. Geologic cross-sections, if needed, to provide three-dimensional picture.
v. Logs of exploratory trenches and borings that show details of observed features and conditions (note: these should not be generalized or diagrammatic). Trench logs should show topographic profile and geologic structure at a 1:1 horizontal-to-vertical scale; scale should be 1:60 (one inch equals five feet) or larger.
vi. Geophysical data and geologic interpretations.
i. Appendix. Attach any supporting data not included above, e.g., water well data, photographs, and aerial photographs.
4. The geotechnical professional who prepared the geotechnical report shall stamp the report with their license stamp or seal.
5. The department may request a geotechnical professional to provide additional information in the geotechnical report based upon existing conditions, changed conditions, or unique circumstances occurring on a case-by-case basis.
6. Hold harmless clauses, disclaimers, and limitations are not allowed to be included, neither expressly nor implied, within a geological assessment. (Ord. 17-513 § 3 (Exh. A)).