ABAG Shaken Awake! Report
SUMMARY
from the 1996 report (NOT updated with 2003 data)

The statistics are numbing. The northern Hayward fault, extending from San Pablo Bay to San Leandro, suddenly ruptures, generating a magnitude 7.1 earthquake.

Almost 88,000 housing units are made uninhabitable.
Over 211,000 people are forced from their homes.
Over 63,000 people require publicly provided shelter.

In another earthquake, the peninsula segment of the San Andreas fault, located in San Mateo County, ruptures, generating a magnitude 7.1 earthquake. Fewer homes and apartments are located nearby.

"Only" 46,000 housing units are made uninhabitable.
Almost 109,000 people are forced from their homes.
Almost 29,000 people require publicly provided shelter.

Yet each of these earthquakes are reasonably likely. The northern Hayward earthquake and the peninsula San Andreas earthquake each has about a one-in-four chance of occurring in the next 30 years.

ABAG examined the effect of eleven earthquakes on housing and shelter demand. Six of those earthquakes are expected to have more of an impact than the Loma Prieta earthquake in 1989 which caused a total of over 16,000 units to be uninhabitable throughout the Monterey and San Francisco Bay areas (including almost 13,000 in the Bay Area). In fact, three of those earthquakes will probably have a greater impact than the more recent 1994 Northridge earthquake in the Los Angeles area, in which over 48,000 housing units were made uninhabitable in the affected area.

These past earthquakes emphasize the importance of creating housing loss estimates useful for emergency response planning in the San Francisco Bay Area.

These housing loss estimates were produced using models designed to provide estimates of the number of uninhabitable dwelling units. Uninhabitable is defined as unable to be occupied due to structural problems. It is equivalent of the Applied Technology Council (ATC) "red" tagging for unsafe buildings where entry is prohibited for single family homes. For multi-family units, the structure can be either "red" tagged or "yellow" tagged where entry is restricted. (See ATC-20, 1991.) Building departments in California uniformly use the ATC definitions in their post-earthquake tagging. Uninhabitable dwellings are not necessarily destroyed; in fact, most are repairable.

The approach used in developing these estimates of uninhabitable dwelling units uses a five-step process:

estimating intensity for each of eleven future earthquakes;

developing an inventory of dwelling units by construction type (based on construction material, age, number of stories, and single vs. multi-family);

estimating dwellings in each intensity category by assigning the residential units to residential land in each intensity category;

relating intensity directly to habitability using a matrix for red and yellow tagging percentages for each combination of intensity and construction type; and

aggregating estimates of uninhabitable dwellings to obtain census tract, city (or community), county and regional totals.

The intensity maps are based on the most recent version of ABAG's ground shaking models, described in On Shaky Ground (Perkins and Boatwright, 1995). The scenarios examined include:

entire Hayward fault (both northern and southern segments);
southern segment of the Hayward fault;
northern segment of the Hayward fault;
Healdsburg-Rodgers Creek fault;
Maacama fault;
peninsula segment of the San Andreas fault;
San Gregorio fault;
northern Calaveras fault;
Concord-Green Valley fault;
Greenville fault; and
West Napa fault.

The estimate of the percentage of each combination of intensity and construction type that is expected to be uninhabitable is based on actual statistics on residential damage in the Loma Prieta and Northridge earthquakes collected by ABAG, as well as additional data from prior earthquakes from earlier researchers (Dunne and Sonnenfeld, 1991).

The results of this modeling effort, by county, are shown in a table. The results of the models for the Loma Prieta earthquake, as well as the actual data from that earthquake, are provided for comparison. To view this table, click here.

Another way of expressing the model results is in terms of construction type, rather than by county area. These numbers estimate ways in which building retrofits can be targeted to reduce the number of uninhabitable dwelling units predicted. The results of this analysis are also shown in a table. Again, the results of the models for the Loma Prieta earthquake, as well as actual data from that earthquake, are provided for comparison To view this table, click here.

In both these tables, it is important to note the huge number of predicted uninhabitable dwellings in the entire Hayward, northern Hayward, and southern Hayward scenarios relative to the other eight scenarios.

The emphasis of this effort is on determining damage due to ground shaking, not on potential additional lost housing due to fire following an earthquake, liquefaction, and earthquake-induced landsliding. However, estimates of such additional uninhabitable units could be made in the future based on available models, together with ways to improve those estimates with additional research.

Local governments have a number of different options for using these results, as well as the more detailed information given later in this report and in the Appendices.

  1. Data on the number of dwelling units predicted to be red- or yellow-tagged can alert building departments on the relative man-power needs for post-earthquake safety evaluations associated with various scenario earthquakes. The absolute man-power requirements are obviously related to other factors, such as the extent of minor damage which causes housing renters or owners to request inspections, which are not addressed in this document.

  2. Housing and community development departments may be interested in the type of buildings being red- or yellow-tagged. For example, those living in some types of buildings (such as mobile homes and unreinforced masonry single-room occupancy apartments) tend to have demographic characteristics of significance in emergency shelter planning, as well as in estimating long-term housing assistance requirements.

  3. Community education campaigns intended to reinforce the message to neighborhood groups of the need to retrofit and seismically strengthen pre-1940 wood-frame dwellings located in areas that have greater exposure to high intensity ground shaking.

  4. Agencies responsible for providing short-term emergency shelter can use these estimates to predict mass care requirements. To provide information useful for these organizations, George Washington University (GWU) has been working with ABAG to develop models that combine the tagging data with demographic information essential for these forecasts.

All of those households living in uninhabitable dwellings (red-tagged or multi-family yellow-tagged) will be seeking alternative shelter. Many will stay with friends and relatives or in the family car. Some will stay in public shelters provided by the Red Cross or others. Those seeking public shelter can be estimated from past disasters, including both hurricanes and earthquakes. Those seeking shelter typically have very low incomes for these families have fewer options. Other variables, such as ethnicity and housing ownership, have a modifying effect on the results.

The results of this modeling process are shown in a table. To view this table, click here.

Note that this modeling process does not include three important influences on shelter populations:

pre-disaster homeless;
secondary disasters (such as toxic gas releases and fires); and
non-resident populations (such as tourists and commuters).

The research that forms the basis for these loss estimates and mass care requirements is significantly more sophisticated than that used in earlier estimates by ABAG and GWU produced for the California Office of Emergency Services (OES) and the American Red Cross in 1992 (Perkins, 1992; Harrald and others, 1992). Therefore, those earlier documents should be considered superseded by this report.

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ABAG, the Association of Bay Area Governments, is the regional planning and services agency for the nine-county San Francisco Bay Area.

This page was last updated 9/29/03 by jbp.