Two Ways To Investigate Seismic Hazards In California

Identifying sites with potentially damaging seismic conditions is one step of the site inventory process. As landscape architects, we may be called upon to conduct an initial site investigation regarding seismic hazards. When landscape architects are in the lead on a project, they must understand what seismic risks may be present and the procedures for meeting regulatory requirements.

In 1990 the State Legislature passed the Seismic Hazards Mapping Act which requires most development projects to undergo a seismic site investigation report.

While the investigation reports (also called geotechnical reports) are prepared by a Certified Engineering Geologist and Registered Civil Engineer, landscape architects must be familiar with the reports and which sites may need further investigation. Additionally, seismic conditions are a topic on the California Supplemental Exam (CSE) for landscape architects.

Seismic Risk Site Investigations

Most types of development are required by state law to undertake a seismic hazard evaluation.

There are two types of seismic hazard evaluations.

  • Screening Investigation
  • Quantitative Assessment

These screenings should be performed as two different steps in the site inventory process. If the initial screening demonstrates that the site is not subject to appreciable seismic risk, the quantitative assessment may not be required.

Step 1: Seismic Screening Investigation

First, an initial screening investigation is conducted to determine a site’s “seismic potential and consequences” or identify sites with a low potential for seismic hazards.

If you can prove to the satisfaction of the lead agency technical reviewer that the site has a low potential for seismic hazards, then further investigation may not be required. But, if an initial screening cannot rule out that seismic hazards exist on the the site, a quantitative assessment will be required for project approval.

Sources of information should be regional in scope and discuss the types of seismic hazards common in the area. However, if site-specific information exists, this is even better and provides a higher degree of certainty about the potential seismic hazards extant on the site.

  1. Identify if the site is located in within a seismic risk area on a California Seismic Zone Map. Maps can be found at the California Geological Survey website.
  2. Seismic Hazard Evaluation Reports can be used to summarize risk conditions and seismic danger potential based on Seismic Zone Maps. These maps and reports are available from the California Geological Survey.
  3. “Aerial photographs can be useful to identify existing and potential landslide and/or liquefaction features (headwall scarps, debris chutes, fissures, grabens, sand boils, etc.) that suggest or preclude the existence of ground failure hazards that might affect the site.”
  4. Conduct a field evaluation to fill in any gaps in data and to clarify map and aerial photo information.

If your site’s initial investigation reveals potential seismic or geological hazard conditions, further investigation is necessary. Otherwise, submit the initial Seismic Screening Investigation to the lead agency for approval.

Step 2: Quantitative Evaluation of Hazard Potential

If the Seismic Screening Evaluation discovered seismic or geologic conditions that require further investigation, then a detailed field investigation is required.

“The objective of the detailed field investigation is to obtain sufficient information on which the engineering geologist and/or civil engineer can evaluate the nature and severity of the risk and develop a set of recommendations for mitigation.”

“The work should be based upon a detailed, accurate topographic base map prepared by a registered civil engineer or land surveyor. The map should be of suitable scale, and should cover the area to be developed as part of the project, as well as adjacent areas: which affect or may be affected by the project.”

“The detailed field investigation commonly involves the collection of subsurface information from trenches or borings, on or adjacent to the site. The subsurface exploration should extend to depths sufficient to expose geologic and subsurface water conditions that could affect slope stability or liquefaction potential. A sufficient quantity of subsurface information is needed to permit the engineering geologist and/or civil engineer to extrapolate with confidence the subsurface conditions that might affect the project, so that the seismic hazard can be properly evaluated, and an appropriate mitigation measure can be designed by the civil.”

Who Will Review and Approve The Seismic Risk Site Investigation?

A lead agency (such as a municipality or state agency) has the ability to approve projects that are in compliance with all applicable state and local regulations. Conversely, the agency is obliged to reject projects that do not meet the regulations and are a threat to the public’s health, safety, and welfare.

Get Your Project Approved the First Time

Before a seismic hazard investigation begins on large projects, meet with the lead approving agency’s technical reviewer to establish the scope of the seismic screening investigation.

Have your consultant ask the technical reviewer about the area that will be investigated for seismic hazards. Also, determine which investigation techniques are deemed acceptable and appropriate by the agency’s technical reviewer. Other topics include special local requirements and on-site inspection requirements.

Completing the following items to the satisfaction of the technical reviewer will certainly help speed the process:

  • Include a thorough site-specific investigation
  • Your consultants’ seismic and geological hazard findings are valid
  • “Proposed mitigation measures achieve an acceptable level of risk, as defined by the lead agency and CCR Title 14, Section 3721(a)”

Technical experts with the lead agency can probably help direct you and your consultants towards sources of useful information that may save time and money in the long run.


Even though the Seismic Hazard Map shows that your project is not located in a Seismic Hazard Zone, regulators may still require a Seismic Risk Site Investigation if they believe that changes to the site (i.e.for example, cuts, fills, and/or modifications that would significantly raise the water table) would create conditions that would be negatively impacted by a seismic event.

The regulations allow for state and local agencies to require more stringent site investigations and mitigation strategies based on site-specific soil and geologic hazards. It is best to begin a dialog with the lead agency early in the design process to avoid unnecessary surprises later in the project.


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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