How Does the Seismic Hazards Mapping Act Affect Site Development?

In 1990 the California State Legislature passed the Seismic Hazards Mapping Act to protect public safety from the effects of strong ground shaking, liquefaction, landslides, or other ground failure, and other hazards caused by earthquakes.

Landscape architects practicing in California need to be familiar with the Act and how the regulations will affect site development.

What is A Seismic Hazard Zone?

The California State Geologist identified “seismic hazard zones” which have been mapped to identify potential seismic hazards.

California’s Geological Society published guidelines to help developers mitigate harmful effects of seismic events. The purpose of the Seismic Hazards Mapping Act is to:

  • Assist in the evaluation and mitigation of earthquake-related hazards for projects within designated zones of required investigations
  • Promote uniform and effective statewide implementation of the evaluation and mitigation elements of the Seismic Hazards Mapping Act.

“The Seismic Hazards Mapping Act and related regulations establish a statewide minimum public safety standard for mitigation of earthquake hazards. This means that the minimum level of mitigation for a project should reduce the risk of ground failure during an earthquake to a level that does not cause the collapse of buildings for human occupancy, but in most cases, not to a level of no ground failure at all.”

“Effective evaluation and mitigation ultimately depends on the combined professional judgment and expertise of the evaluating and reviewing professionals.”

“Nothing in these Guidelines is intended to negate, supersede, or duplicate any requirements of the California Environmental Quality Act (CEQA) or other state laws and regulations.”

What Types of Projects are Regulated by The Seismic Hazards Mapping Act?

According the the state of California, a project “is defined by the Seismic Hazards Mapping Act as any structures for human occupancy, or any subdivision of land that contemplates the eventual construction of structures for human occupancy.”

A “structure for human occupancy” is any structure used or intended for supporting or sheltering any use of occupancy, which is expected to have a human occupancy rate of more than 2,000 person-hours per year.

The following projects may be exempt for these regulations:

  • A single-family dwelling otherwise qualifying as a project may be exempted by the city or county having jurisdiction of the project.
  • A single-family wood-frame or steel-frame dwelling not exceeding two stories when that dwelling is not part of a development of four or more dwellings.
  • “Project” does not include alterations or additions to any structure within a seismic hazard zone which do not exceed either 50 percent of the value of the structure or 50 percent of the existing floor area of the structure.

How To Determine If A Site Is Located In A Seismic Hazard Zone

“Seismic Hazard Zones” are areas shown on Seismic Hazard Zone Maps. All projects that fall within a Seismic Hazard Zone  require investigation to determine if mitigation for potential liquefaction and/or earthquake-induced landslide ground displacements is required.

The State Geological Society publishes Seismic Hazard Zone Maps on their website.

Northern California Seismic Hazard Areas

Northern California Seismic Hazard Areas

Southern California Seismic Hazard Areas

Southern California Seismic Hazard Areas


The Seismic Hazard Maps may not accurately identify all areas that may be subject to liquefaction, earthquake-induced landsliding, strong ground shaking, and other earthquake and geologic hazards.

If The Site is Located on A Seismic Hazard Area, Is Further Investigation Needed?

The information on the Seismic Hazard Zone Map is not a sufficient substitute for the required site-investigation reports required by the Act.

Can lead agencies or local municipalities impose more stringent requirements?

“Nothing in the Act, the regulations, or these Guidelines precludes lead agencies from enacting more stringent requirements, requiring a higher level of performance, or applying these requirements to developments other than those that meet the Act‘s definition of “project.”

What is the “Worst Case” Scenario?

The worst case scenario that has the greatest potential for seismic damage is a strong earthquake during or after period of heavy rainfall. As of February 2012, this has not occurred in California.

Public Buildings Have More Stringent Requirements

“More stringent requirements are prescribed by the California Building Code (CCR Title 24) for hospitals, public schools, and essential service buildings. For such structures, the requirements of the Seismic Hazards Mapping Act are intended to complement the CCR Title 24 requirements.”

What is the Criteria for Project Approval?

The following criteria is the State’s minimum standards for approval. Lead agencies can add to these minimum requirements.

“A project shall be approved only when the nature and severity of the seismic hazards at the site have been evaluated in a geotechnical report and appropriate mitigation measures have been proposed.”

“The geotechnical report shall be prepared by a registered civil engineer or certified engineering geologist, having competence in the field of seismic hazard evaluation and mitigation. The geotechnical report shall contain site-specific evaluations of the seismic hazard affecting the project, and shall identify portions of the project site containing seismic hazards. The report shall also identify any known off-site seismic hazards that could adversely affect the site in the event of an earthquake. The contents of the geotechnical report shall include, but shall not be limited to, the following:

  1. Project description.
  2. A description of the geologic and geotechnical conditions at the site, including an appropriate site location map.
  3. Evaluation of site-specific seismic hazards based on geological and geotechnical conditions, in accordance with current standards of practice.
  4. Recommendations for appropriate mitigation measures as required in Section 3724(a), above.
  5. Name of report preparer(s), and signature(s) of a certified engineering geologist and/or registered civil engineer, having competence in the field of seismic hazard evaluation and mitigation.

“Prior to approving the project, the lead agency shall independently review the geotechnical report to determine the adequacy of the hazard evaluation and proposed mitigation measures and to determine the requirements of Section 3724(a), above, are satisfied. Such reviews shall be conducted by a certified engineering geologist or registered civil engineer, having competence in the field of seismic hazard evaluation and mitigation.”

What Consultants Are Required for the Geotechnical Report?

According to state law and regulation, landscape architects may not prepare the geotechnical report. Therefore, landscape architects need to assemble a team of sub-consultants to prepare the report for project approval.

The site-investigation report needs to be prepared by a certified engineering geologist or registered civil engineer. After the initial report is prepared, a certified engineering geologist or registered civil engineer must review and approve the report. All these consultants are required to be “competent in the field of seismic hazard evaluation and mitigation.”

“The State Mining and Geology Board recommends that engineering geologists and civil engineers conduct the assessment of the surface and subsurface geological/geotechnical conditions at the site, including off-site conditions, to identify potential hazards to the project. It is appropriate for the civil engineer to design and recommend mitigation measures. It also is appropriate for both engineering geologists and civil engineers to be involved in the implementation of the mitigation measures– engineering geologists to confirm the geological conditions and civil engineers to oversee the implementation of the approved mitigation measures.”

State Resource Guidelines:

The quoted sections of text came directly from California Geological Society SPECIAL PUBLICATION 117A. Read the entire document for more in-depth information.


John is a landscape architect who is currently preparing to take the California Supplemental Exam to become licensed in California. He is currently a licensed professional landscape architect in Georgia and Florida. John graduated from California State University, Pomona with a BSLA degree in landscape architecture in 1997 and has extensive practice experience in residential and commercial landscape design.

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