In the Bay Area, retaining housing is crucial to expediting and ensuring an effective disaster recovery. Limiting catastrophic housing damage and keeping residents in their homes not only helps people who may lack the resources to effectively recover from a disaster, but keeps communities intact. Understanding where the most vulnerable housing types are located, especially those that house vulnerable community members, is a crucial first step to gain a more comprehensive understanding of multi-level risk within the region and to better understand where mitigation needs to be prioritized.

For the region as a whole to become more resistant to disasters, jurisdictions need comprehensive, actionable strategies to reduce vulnerabilities and build more resilient communities. The Bay Area Housing and Community Multiple Hazards Risk Assessment is a multi-agency project led by the Association of Bay Area Governments (ABAG) and the Bay Conservation and Development Commission (BCDC) designed to better characterize both the structural and community components of vulnerability, as well as develop strategies to address these specific vulnerability characteristics. The purposes of this project has been to:

  • Understand the characteristics of San Francisco Bay Area housing and communities that increase vulnerability to earthquakes and sea level rise related flooding
  • Identify and assess housing and community vulnerability at regional and community scales, and
  • Develop strategies that reduce housing and community vulnerability to help the region meet its resilience, sustainability, prosperity, and equity goals.

The analysis was conducted with a sole focus on housing and the residents who live in it. There are many other factors aside from housing integrity that influence a resident’s ability to stay in a home, including impacts to infrastructure and availability of utilities, availability of jobs, and access to resources that fulfill daily needs, such as grocery stores, hardware stores, medical and childcare facilities. While these factors are extremely important, they are only touched upon briefly in this project and may be included in future analysis.

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Housing and Community Vulnerability

This chapter describes the assessment methodology to identify places where housing stock and portions of the community are particularly vulnerable to natural disasters. The approach concentrates on three aspects of vulnerability: identifying areas subject to hazards that have known potential to create damage at a level that could displace residents from their homes, housing types that are vulnerable to the natural hazard events identified, and community characteristics that makes it less likely that the population will be able to prepare for, respond to, or recover from a disaster. Following are the key considerations for each of these vulnerability types.


The vulnerability analysis considered three hazards: ground shaking, liquefaction, and flooding. The specific hazard scenarios used in the analysis are summarized here.

Different earthquakes cause differing levels of ground shaking throughout the region. We selected shaking scenario maps from two previously modeled earthquake scenarios – a Magnitude 7.9 scenario on the San Andreas Fault and a Magnitude 7.0 scenario on the Hayward fault – and determined areas likely to experience ground shaking hazard levels of MMI VIII or above in these scenarios. The ground shaking hazard analysis only includes homes that are likely to be exposed to MMI VIII and greater ground shaking, as they are the most likely to be significantly damaged, thus displacing residents.

Liquefaction hazard levels were determined based on liquefaction susceptibility combined with shaking intensity (MMI). For the purpose of this project, moderate or high liquefaction hazard areas were examined using MMI from the future earthquake shaking scenario maps for the two scenarios outlined above (a San Andreas or Hayward event), as they are the most likely to cause major building damage that displaces residents from their homes.

Any amount of flooding has the potential to displace residents from their homes, as even short duration flooding can undermine building structures or create unsafe living conditions due to mold growth and contamination. Current flooding scenarios are based on published National Flood Insurance Program (NFIP) rate maps.

Future flooding scenarios are based on three regional inundation maps developed by NOAA Office for Coastal Management. These three inundation maps are used to represent future flooding from different combinations of sea level rise and tide levels.

Key Considerations

Hazards can have significant impacts on communities that live in high hazard areas Much of the Bay Area is exposed to natural hazards that have the potential to cause significant impacts on the region and its residents. Seismic events may cause ground shaking or liquefaction, and many shoreline areas are vulnerable to existing flooding and may experience increased flooding in the future due to sea level rise.

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Housing Vulnerability

Regional housing vulnerability was determined based on the eight potentially fragile housing types commonly found in the Bay Area. The presence of vulnerable housing is indicated if 30% or more of housing units in a block group are a fragile housing type located in an area of ground shaking, liquefaction, or flooding hazard.

The fragile housing typology is designed to identify subsets of the Bay Area housing stock that are likely to possess characteristics that increase their vulnerability. This method identifies only what are deemed as the most fragile common housing structure types found within the Bay Area due to likely poor structural performance in an earthquake (i.e., those conditions most likely to cause housing to be red-tagged, requiring either demolition or extensive and lengthy repairs). This method considers critical combinations of material, system, etc. that indicate high fragility. As key data such as structure type (wood frame, concrete, etc.) is not widely available, proxies such as size, age, number of stories, and location that are associated with the most common fragile housing types are used. As different hazards interact with building types differently, hazards including liquefaction, ground shaking, and flooding are examined separately.

Each fragile housing type was mapped at the block group level to identify block groups with the characteristic combinations associated with each fragile housing type. Only block groups exposed to the identified hazard level for ground shaking, liquefaction, and flooding are flagged; vulnerability is a combination of exposure and fragility.

Key Considerations

Ground shaking can damage cripple wall and house-over-garage single-family homes Many established residential neighborhoods have single-family homes that could be significantly damaged during an earthquake. These include homes with short unreinforced walls that raise the first floor 1-5 feet above ground level (i.e., cripple walls) and those that are two or more stories with garages or other large openings on the first floor. Renters and owners of single-family homes that are not retrofitted may be displaced from their existing neighborhood and could have a difficult time rebuilding or finding a replacement home.
Ground shaking can damage weak story, concrete and cripple wall multi-family housing There are a number of multi-family housing types that can be significantly damaged if not properly retrofitted. This includes those with parking or retail on the ground floor (i.e., weak story or open front), those built from concrete that is not properly reinforced (i.e., non-ductile), or those that have short unreinforced walls that raise the first floor 1-5 feet above ground level (i.e., cripple walls). Depending on the number of units, damage to multi-family housing can displace a large number of residents, many of who are likely renters. In addition, multi-family housing does not always receive an equitable share of state or federal financial and technical assistance during recovery efforts and therefore may not always be rebuilt in a timely manner.
Housing is generally built to life safety standards rather than shelter-in-place standards The current building code is designed to a life safety standard to protect occupant lives during an earthquake event. Newly constructed housing built to life safety standards can still be significantly damaged during an earthquake, displacing residents while the structure is repaired or replaced. The result is that some residents will not be able to shelter-in-place or remain in their homes, and that extensive repairs or rebuilding may be required.
Most foundations cannot withstand liquefaction Homes located where soils are susceptible to liquefaction, (e.g., along the Bay shoreline or on fill) may experience significant enough damage during an earthquake to become uninhabitable. Most single- and multi-family homes under 10 stories are unlikely to have foundations stable enough to withstand liquefaction even if they can withstand ground shaking.
Most houses cannot withstand any amount of flooding If exposed to flooding, most housing built in the Bay Area will be damaged as current construction materials, siting and design standards do not consider potential exposure to either water or salt. As sea level rises, existing and future housing of all types within FEMA identified Special Flood Hazard Areas (SFHAs) will be at greater risk of flooding, and housing in low-lying areas not currently at risk may begin to experience flooding.
Houses with habitable space or critical equipment below-grade are at risk from flooding Homes with habitable living space or critical building equipment below-grade are likely to be significantly damaged by flooding. Neighborhoods with existing drainage issues, for example that Bay Area housing that experiences street or basement flooding during current rainfall events or when groundwater levels are high, will be at even greater risk as the Bay rises.

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Community Vulnerability

Community vulnerability was determined using ten indicators that represent characteristics of individuals and households that affect their ability to prepare for, respond to, and recover from a disaster. These indicators collectively present a picture of a community’s vulnerability. A concentration of these indicators is assumed to influence the recovery of a community. Key themes that emerged included age-related vulnerabilities, language and ethnicity vulnerabilities, cost-burdened residents, housing tenure issues, and access to resources.Indicators were measured and scored using the method developed by the Metropolitan Transportation Commission (MTC) to identify Communities of Concern (CoC). This is meant to identify block groups with higher than average concentrations of the particular indicator and therefore may have higher concentrations of vulnerability.

Key Considerations

Many community members have limited access to resources Many Bay Area residents that live in areas at risk from natural disasters are resource-constrained. This includes households that are low- and very low-income, households of all income levels that are housing and transportation cost-burdened, and transit-dependent households that do not own a car. Resource-limited households are less able to prepare for natural disasters, and if displaced from damaged homes, will likely struggle to find housing that is affordable and near to the jobs, schools, medical facilities, and other services on which they rely.
Housing affordability is an existing challenge that could hinder recovery Housing affordability for both renters and owners is an existing challenge in the Bay Area that will compound the number of community members displaced by a natural disaster. Much of the region is cost-burdened with regard to housing already, spending 30% or more of income on housing. After a disaster, if many housing units are lost, a constrained market may drive up the cost of housing even further. Loss or damage of housing that results in increased costs to either renters or home-owners will likely increase the number of permanently displaced Bay Area residents as finding housing that is affordable and near jobs, schools, medical facilities, and other services on which they rely will be challenging.
Renters have limited ability to improve their housing resilience Many Bay Area residents that live in areas at risk from natural disasters are renters. Renters have a limited ability to improve the housing in which they live and often do not have hazard insurance to protect themselves and their belongings in case of a disaster. Communities with a large number of renters, and in particular resource-limited renters, will need to assist these residents both during a disaster (e.g., with shelter-in-place facilities), as well as post-disaster with finding interim, affordable housing to avoid the permanent displacement of renters from communities due to damaged housing.
Many community members have limited or inadequate information about hazards Access to timely, correct, and meaningful information both before and after a natural disaster can be challenging in all communities and can be a particular challenge in communities that are ethnically and culturally diverse, and where there is a large number of households in which English is not the primary language spoken. Additionally, in the Bay Area many of these same community members are resource-constrained renters who are often living in overcrowded housing. Damage to housing during a natural disaster can lead to a significant amount of displacement and a struggle to find housing that is affordable and near enough to jobs, schools, medical facilities, and other services.
Information on elderly and very young community members is limited Up-to-date and easily accessible information about the number of elderly and very young living in a community can be challenging to find, particularly during a disaster when it is most needed. It can be difficult to evacuate these community members, especially if they need specialized equipment or supervision, and shelter-in-place facilities need to be prepared to both house them safely and maintain communication with concerned family members.

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Housing and Community Risk Map

The final mapping and analysis consists of three maps. The final maps represent block groups within the Bay Area that are likely to be exposed to hazards and also have housing and community characteristics that indicate higher vulnerability, or are more likely to be affected to the degree that residents will have trouble preparing for, responding to, and recovering from a major disaster. Local jurisdictions can use this analysis to zoom in on areas that require more nuanced vulnerability assessment, including more accurate fragile housing inventories and a more detailed understanding of community vulnerability that incorporates a qualitative understanding of community capacity.

Community Profiles

The regional analysis supplies a high-level screen to identify which areas are most likely to have multiple vulnerability factors. However, staff wanted to develop methodology to “zoom in” on certain areas to test the assumptions that the regional analysis made as well as to gather more nuanced and qualitative information on community capacity factors that may increase resilience, even if more quantitative measures indicate high levels of vulnerability. Therefore, the team developed nine community profiles to better illustrate vulnerability within specific communities. Community profile areas were chosen based on the following criteria:

  • Contains a mix of both planned development (such as a Priority Development Area) and existing development.
  • Are exposed to a variety of hazards.
  • Contain a range of fragile housing types and community indicators.
  • Are distributed geographically throughout the Bay Area.
  • City staff has an interest in working with the team and/or can utilize the profiles to further existing or future resilience work within the jurisdiction.

To view the Community Profiles click here.

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Strategies for Seismic and Flood Risks

Once vulnerabilities were identified, the next step was to consider how these vulnerabilities could be reduced. A suite of implementation strategies were developed to help local jurisdictions reduce the vulnerability of housing and populations in the areas identified through the analysis, and to plan for future growth in a way that minimizes new vulnerability

To view these strategies and decide which strategies are most useful for your jurisdiction using our interactive strategy selection tool, click here.

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Financing Mechanisms

Consideration was given to identifying the most appropriate financing mechanisms that might be needed to implement the strategies. The strategies broadly fall into two categories. The first category comprises strategies related to planning, programs, and operations. Strategies in the first category can be implemented through existing departments and programs, sometimes at no additional cost, or through new or expanded programs for which a budget must be found. General fund resources, fee-based special purpose funds, or state, federal, or private grants are among the main sources of funds for these types of strategies.

The second category includes strategies related to capital expenditures. Strategies in the second category involve capital projects, which, by and large, require a level of funding that is a few orders of magnitude greater than planning-level, programmatic, or operational strategies. Depending on the strategy, funding may come from the private sector (individuals, a development company, or professional or philanthropic organizations), the public sector, or a cooperative effort among public and private actors.

To view the table of financing strategies, click here.


Improving resilience should focus on the intersection between fragile housing and community vulnerability. The outcomes of this project should also assist the region to actively avoid increasing the number of communities at risk while still meeting ambitious growth and sustainability goals.

Local jurisdictions are encouraged to conduct more in-depth local analysis based on this project, for example by considering the methods and outcomes of the regional analysis in their Local Hazard Mitigation planning process. Local jurisdictions can also begin using the strategies based on the initial regional analysis even without local analysis. The region can use the outcomes of this project to incorporate resilience into region-wide policies on planning for future growth through Plan Bay Area and in helping jurisdictions decide where and how to grow. Assistance implementing strategies will be provided to local jurisdictions by ABAG through its Regional Resilience Plan throughout 2015 and 2016. The suite of strategies developed by this project are not intended as a one-time effort or a complete set of tools. As communities gain more experience with assessing vulnerability and implementing strategies they may have additional insights to offer on potential actions, or recommendations for modifying the strategies recommended here. ABAG’s ongoing Resilience Program is one vehicle through which new lessons at the local level can be communicated to a broader audience.

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Additional Links

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Funding from the U.S. Geological Survey’s (USGS) Earthquake Hazards Program External Research Support program leveraged additional resources from the U.S. Environmental Protection Agency’s (EPA) Smart Growth Implementation Assistance program, Federal Emergency Management Agency (FEMA), and the California Strategic Growth Council (SGC). AECOM provided significant consultant support.

Last modified: 04.01.2015