On Shaky Ground

Any future modifications or updates of these maps are likely to be available on the Internet prior to "hard copy" publication.

These intensity maps are not intended to be site-specific. Rather, they depict the general risk within neighborhoods, and the relative risk from community to community. Individual intensities can easily be incorrect by plus or minus one intensity unit.

Several cities have copies of the critical earthquake scenario intensity maps for their area. You can also examine or obtain copies of maps for individual areas through ABAG.

Better maps can only be truly better if they lead to actions which save lives, reduce suffering and economic hardship, and help protect our environment. What you do to prepare for shaking can minimize or completely eliminate these effects.

We believe that it is imperative that these revised maps be used for earthquake hazard mitigation and disaster response planning now. They depict a significantly larger hazard than the maps previously published.

Each of the following sections contains general information and recommends additional reports to help you prepare mitigation programs.

ABAG staff are already working to ensure that these maps are used fully by the city and county governments in the Bay Area, by those planning for a better more earthquake-resistant transportation system including the California Department of Transportation (CalTrans), and by relief agencies such as the American Red Cross. But more is needed. It is in this spirit of a call for more mitigation that the following maps showing the extent of our hazard are provided to you, the local governments, business owners, homeowners and residents of the San Francisco Bay Area. These maps are also being published on the Internet, at the abagOnline site at http://www.abag.ca.gov, so that those with access to viewing software such as Mosaic or Netscape on their personal computers can view the regional maps and many local maps.

We all need to take responsibility for making our own homes and workplaces safer so that we can better prepare for the ride of our lives.

As a resident or homeowner:

Most residents must understand that the risk of dying in an earthquake is extremely low. However, the risk of damage to your home can be significant, particularly if it is located in a high intensity area.

The most common damage from earthquakes to single-family homes built prior to the 1940s is foundation damage. The "fix" to prevent this damage is to bolt the foundation sill plate to the concrete foundation and to brace or put plywood sheathing on the inside of the "cripple" walls between the foundation and the first floor.

Mobile homes typically fall off the jacks that support them when exposed to intense ground shaking. Again, earthquake bracing systems can be installed to mitigate this problem.

For those who live in apartments or condominiums, the problems are more complex and the solutions more costly. Again, if the buildings were constructed prior to the 1940s, the potential problems are far greater than for newer construction. The spectacular failures to the apartments in the Marina District of San Francisco occurred largely in pre-1940s vintage buildings with parking on the first floor. Parking can even be a problem in buildings built after 1940. For example, many apartments with ground floor parking collapsed or failed in the Northridge earthquake. The tragic deaths at the Northridge Meadows Apartments occurred in a three-story apartment building in which a large portion of the ground floor was parking. When this ground floor parking collapsed, the adjacent ground floor apartments collapsed as well.

Falling objects within your home will likely result in the greatest losses and pose a significant threat of injury. Just because you cannot push something over easily does not mean that the heavy object will not topple, causing injuries.

• Cupboard doors (including sliding doors) can open in earthquakes. Cupboards containing cleaning solutions or other chemicals, or dishes that are valuable, should be fitted with positive latches.
• Heavy bookcases and tall furniture should be screwed to wall studs.
• Your gas hot-water heater should be secured to the wall studs. All gas appliances should be connected to the gas supply with a short piece of flexible tubing. Propane gas storage tanks should be designed not to overturn and roll.
• Mirrors, bookcases and other heavy objects should be anchored or removed from near your beds.

Everyone should have emergency water, food, and prescription medications on hand. First aid and CPR training is also useful.

Recommendations for an "Earthquake Kit" are available from your local Red Cross chapter, local or County Office of Emergency Services, or by looking in the front of your telephone book.

For More Information:

From the American Red Cross:

Employee Earthquake Preparedness for the Workplace and Home. American Red Cross, 1988. 12 pp. ($1.00 from your local Red Cross Office or by mail from Red Cross Disaster Services, 1550 Sutter Street, San Francisco, CA 94109)

Safety and Survival in an Earthquake. American Red Cross, 1989. 52 pp. ($3.00 from you local Red Cross Office or by mail from American Red Cross, Los Angeles Chapter, 2700 Wilshire Boulevard, Los Angeles, CA 90057, $3.00 plus $1.00 postage and handling)

The Emergency Survival Handbook. American Red Cross, 1989. 63 pp. ($3.00 from you local Red Cross Office or by mail from American Red Cross, Los Angeles Chapter, 2700 Wilshire Boulevard, Los Angeles, CA 90057, $3.00 plus $1.00 postage and handling)

From the Association of Bay Area Governments:

An Ounce of Prevention: Strengthening Your Wood Frame House for Earthquake Safety. California Office of Emergency Services, 1993. 36 pp. (Available from ABAG, as Pub. No. P93001BAR, $3.00 plus $1.00 postage and handling)

An Ounce of Prevention (Video). California Office of Emergency Services, 1993. (Available from ABAG, as Pub. No. P93002BAR, $10.00 plus $5.00 postage and handling)

Organizing Neighborhoods for Earthquake Preparedness. California Office of Emergency Services, 1993. 64 pp. (Available from ABAG, as Pub. No. P93006BAR, $4.00 plus $3.00 postage and handling)

From the Califoirnia Office of Emergency Services:

General HOME Preparedness Information Kit. (Set of leaflets covering personal preparedness, emergency supplies, foundation bolting, sheathing of cripple walls, water heater strapping, emergency food supplies.) California Office of Emergency Services, 1988. (OES, Oakland, single sets free -- phone 510-286-0873)

Home Buyer's Guide to Earthquake Hazards. California Office of Emergency Services, 1989. 13 pp. (OES, Oakland, single copies free -- phone 510-286-0873)

Reducing the Risks of Nonstructural Earthquake Damage -- A Practical Guide. Third Edition. By Wiss, Janney, Elstner Associates for FEMA, 1994. (Available from OES, Oakland, single copies free-- phone 510-296-0873)

From the California Seismic Safety Commission:

The Homeowners Guide to Earthquake Safety. 1992. Calif. Seismic Safety Commission Rpt. SSC 92-02, 28 pp. (Avail. from SSC, 1900 K St., Suite 100, Sacramento, CA 95814, $2.25)

From the U.S. Geological Survey:

Reducing Losses from Earthquakes through Personal Preparedness. By W.J. Kockelman, 1984. U.S. Geological Survey Open-File Report 84-765, 21 pp. (USGS, $2.75).

As a school or business owner or worker:

Businesses should improve their knowledge by conducting:

• audits of general earthquake vulnerability; and
• risk assessments to point out ways to avoid risk through avoiding areas of problem soils/geologic conditions, use of "buffer" zones around hazardous areas, and hazardous materials inventory control.

Schools and businesses can mitigate risk by :

• use of specially-designed shelving systems or containers;
• use of special structural designs for buildings, tanks and pipeline supports;
• use of seismic restraints on suspended ceilings, light fixtures, equipment or computer floors (such as bracing, anchors, and straps); and
• control of hazardous materials risk through secondary containment or chemical isolation and separation of incompatible chemicals.

Mitigation measures can be improved. Requirements in OSHA standards and the Uniform Building Code are minimums. Just because you cannot push something over easily does not mean that the heavy object will not topple, causing injuries.

• Cupboard doors (including sliding doors) can open during earthquakes. They should be fitted with positive latches.
• 1" to 1-1/2" shelf lips may not be adequate to restrain chemical reagent bottles in violent ground shaking. Lip designers should look at the size and shape of what is on the shelf.
• Some shelf anchors can fail. They should be designed to restrain full, rather than empty, shelves.
• Tanks and pipelines can have problems due to inadequate attention to details. For example, anchoring thin-walled tanks may make leakage more likely by contributing to the puncturing of the vessel. Proper detailing is essential.
• Gas cylinders cause problems. For example, C-clamp restraints for gas cylinders can slip off lab benches. They can slip out from under single chains. Cylinders in use should be anchored with two chains (one high, one low).

Emergency capabilities should be improved and maintained:

• use back-up systems for power, communications and water;
• improve evacuation routes through (1) avoiding placing shelving in major exit corridors, (2) reducing the risk of window breakage, and (3) structural retrofit of exit areas;
• improve the usefulness of emergency personnel through (1) training of special response and inspection teams, (2) anchoring furniture and equipment in their offices; and (3) using databases to track those personnel and their training; and
• develop emergency response and warning systems.

Employee training programs are essential. If not trained, employees can be very "innovative" in defeating mitigation -- and in designing inadequate solutions. As many employees as possible should be trained in first aid, CPR and fire safety. Drills should be held to exercise the emergency response plan on a regular basis.

For More Information ...

From the American Red Cross:

Employee Earthquake Preparedness for the Workplace and Home. American Red Cross, 1988. 12 pp. ($1.00 from you local Red Cross Office or by mail from Red Cross Disaster Services, 1550 Sutter Street, San Francisco, CA 94109)

From the Association of Bay Area Governments for Businesses and Hospitals:

Business Resumption Planning Guidelines. California Office of Emergency Services, 1993. 28 pp. (Available from ABAG, as Pub. No. P93008BAR, $5.00 plus $2.00 postage and handling)

Earthquake Preparedness Guidelines for Hospitals. California Office of Emergency Services, 1987. 151 pp. (Available from ABAG, as Pub. No. P88001BAR, $10.00 plus $3.00 postage and handling)

Earthquake Preparedness Training for Businesses. California Office of Emergency Services, 1990. 88 pp. (Available from ABAG, as Pub. No. P90001BAR, $25.00 plus $5.00 postage and handling)

Hazardous Materials Problems in Earthquakes: A Guide to Their Cause and Mitigation. By J.B. Perkins, E. Wyatt, J.H. Schmidt, and G. Selvaduray, 1990. ABAG, 72 pp. (Available from ABAG as Pub. No. P90002EQK, $12.00 plus $3.00 postage and handling)

Toxic Gas Releases in Earthquakes: Existing Programs, Sources and Mitigation Strategies. By J.B. Perkins, E. Wyatt, and G. Selvaduray, 1991. ABAG for the South Coast Air Quality Management District, 374 pp. (Available from ABAG as Pub. No. P91002EQK, $20.00 plus $5.00 postage and handling)

From the Association of Bay Area Governments for Schools and Childcare Providers:

Earthquake Preparedness Activities for Child-Care Providers. California Office of Emergency Services, 1989. 54 pp. (Available from ABAG, as Pub. No. P89002BAR, $8.00 plus $2.00 postage and handling)

Earthquake Preparedness: What Every Childcare Provider Should Know (Video). California Office of Emergency Services, 1993. (Available from ABAG, as Pub. No. P93003BAR, $10.00 plus $5.00 postage and handling)

Earthquake Ready: Preparedness Planning for Schools. California Office of Emergency Services, 1990. 76 pp. (Available from ABAG, Pub. No. P90002BAR, $7.00 plus $3.00 postage and handling)

From the California Office of Emergency Services:

Reducing the Risks of Nonstructural Earthquake Damage -- A Practical Guide. Third Edition. By Wiss, Janney, Elstner Associates for FEMA, 1994. (Available from OES, Oakland, single copies free-- phone 510-296-0873)

From the California Seismic Safety Commission:

The Commercial Property Owners Guide to Earthquake Safety. 1993. Calif. Seismic Safety Commission Rpt. SSC 93-01, 32 pp. (Avail. from SSC, 1900 K St., Suite 100, Sacramento, CA 95814, $3.25)

As a local government elected official or staff member:

Large variations in the level of ground shaking hazard exist in the San Francisco Bay Area. Thus, local government programs dealing with earthquake hazards should recognize these differences.

Mitigation options include:

• land use/zoning controls, particularly for critical or hazardous facilities;
• requirements for soils and geotechnical studies;
• special building design requirements;
• special requirements for nonstructural components;
• hazardous building retrofitting and abatement programs;
• programs to strengthen housing;
• special requirements related to hazardous materials;
• infrastructure and lifeline requirements;
• disclosure requirements and posting of signs;
• disaster response planning;
• reconstruction and redevelopment planning; and
• public information and education programs.

Policy statements on all of these strategies can become a part of the safety element of a jurisdiction's general plan. However, these general plan policies must be backed by programs, ordinances and regulations to have any meaningful impact on our safety.

A land use control might be to avoid particular problem areas in the siting of new critical facilities, such as fire stations. Zoning controls might include restrictions on facilities handling hazardous materials "on shakier ground."

Geotechnical studies and environmental reviews in areas of strongest shaking should be required to be performed by licensed professionals and go beyond statements of fact to include the conclusions and recommendations for appropriate mitigation.

Building codes should be recognized as only minimum standards. Construction supervision by a structural engineer can prove effective for critical facilities in high intensity areas. In addition, local governments should improve the qualifications and encourage specialized training and continued education for building department personnel responsible for structural review.

Although unreinforced masonry buildings have become a symbol for hazardous buildings, they are not the only potential problem. For example, many older cities are dominated by wood-frame houses over 50 years old which were built before such buildings were required to be bolted to their foundations. Cities should consider requiring bolting and strengthening when such buildings are sold or within a fixed period. Because of the large number of such buildings and limited local resources, cities may find it appropriate to target neighborhoods "on shakier ground."

For More Information ...

From the Association of Bay Area Governments:

Earthquake Recovery: A Survival Manual for Local Government. California Office of Emergency Services, 1993. 488 pp. (Avail. from ABAG, as Pub. No. P93007BAR, $14.00 plus $6.00 postage and handling)

Earthquake Vulnerability Analysis for Local Governments. California Office of Emergency Services, 1989. 16 pp. (Avail. from ABAG, as Pub. No. P89003BAR, $4.00 plus $2.00 postage and handling)

Liability of Local Governments for Earthquake Hazards and Losses -- A Guide to the Law and Its Impacts in the States of California, Alaska, Utah and Washington. By J.B. Perkins and K. Moy, 1989. ABAG, 52 pp. (Available from ABAG as Pub. No. P88003PLN, $12.00 plus $3.00 postage and handling)

Putting Seismic Safety Policies to Work. By M. Blair-Tyler and P.A. Gregory, 1988. California Office of Emergency Services, 44 pp. (Available from ABAG, as Pub. No. P88006BAR, $9.00 plus $2.00 postage and handling)

Seismic Retrofit Incentive Programs-A Handbook for Local Government. By D. Barzel and W. Darragh, ABAG, for the California Office of Emergency Services, 1992. 252 pp. (Avail. from ABAG, as Pub. No. P92001BAR, $20.00 plus $5.00 postage and handling)

From the California Office of Emergency Services:

Reducing the Risks of Nonstructural Earthquake Damage -- A Practical Guide. Third Edition. By Wiss, Janney, Elstner Associates for FEMA, 1994. (Available from OES, Oakland, single copies free-- phone 510-296-0873)

From the California Seismic Safety Commission:

California at Risk--Steps to Earthquake Safety for Local Governments. By G.G. Mader and M. Blair-Tyler, 1988. Calif. Seismic Safety Commission Rpt. SSC 88-01, 92 pp. (Avail. from SSC, 1900 K St., Suite 100, Sacramento, CA 95814, $10)

From the U. S. Geological Survey:

Geologic Principles for Prudent Land Use -- A Decisionmaker's Guide for the San Francisco Bay Region. By R.D. Brown and W.J. Kockelman, 1983. U.S. Geological Survey Professional Paper 946, 97 pp. (Available from USGS, $5.50)

Look Before You Build--Requiring Geologic Studies for Reviewing Building Projects. By M. Blair Tyler, Spangle Associates, 1995. U.S. Geological Survey Circular 1130, 59 pp. (Available from USGS, free of charge)

Seismic Safety and Land Use Planning -- Selected Examples from California. By M.L. Blair and W.E. Spangle, 1979. U.S. Geol. Survey Prof. Paper 941-B, 82 pp. (Available from USGS, $6.50)

We continue to learn more about the ground shaking hazard. Further study of the damage patterns in the 1994 Northridge earthquake should contribute more to our knowledge of thrust faulting and the role of directivity. Further study of the 1995 earthquake in Kobe, Japan should help us confirm the role of directivity in strike-slip earthquakes. It may also help to reduce the discrepancy between the revised model used in this report and the attenuation curve calculated from 1906 data for distances within a mile of the surface trace of the fault. (See Ref. 28.) Finally, study of seismic records from these two earthquakes and the 1992 Landers earthquake should provide instrumental evidence on the role of directivity and the appropriateness of the scaling factors being used in this report.

Two types of maps published in the 1987 version of this document have not been regenerated at this time: maximum intensity (using the highest intensity from any of the individual earthquake scenario maps) and composite risk maps (adding the individual earthquake scenario maps together based on how often the earthquakes are expected to occur and damage characteristics for three "average" building designs). Both of these types of maps need to incorporate information on the hazard from thrust faulting that is not available at this time. In addition, the probability of future earthquakes occurring is critical in any risk mapping. These probabilities are currently only available for a few of the scenarios examined. Finally, additional damage information tied to these intensities should become available during the next several months. We anticipate adding these additional maps to this document in one to two years.

ABAG is also in the process of using these intensity maps to model the expected number of housing units to be "red tagged" by local building departments as "unsafe for occupancy" in each of the scenarios depicted on the following pages. Although the table on page 20 provides an "official" definition for the various categories of shaking intensity, the following table predicting the percentage of single-family homes expected to be "red tagged" also provides sobering data

TABLE: PERCENT OF DWELLING UNITS RED TAGGED

        TYPE                    MODIFIED MERCALLI INTENSITY                                                     

                          V        VI      VII     VIII      IX       X+     

     Mobile Home         0.0      0.0      0.4     12.0     76.0     86.0    

  Wood-Frame,  1-3       0.0      0.0     0.11     3.0      18.0     20.0    
  Stories, <1940,                                                            
    Single Family                                                            

  Wood-Frame,  1-3       0.0      0.0      0.0     0.18     7.5      10.0    
  Stories, >1939,                                                            
    Single Family                                                            

Note: The "0" values provided in these tables are actually greater than zero, but still quite small. Although occasional dwellings are "tagged" in these categories, there are also large numbers of dwellings exposed to these relatively low levels of shaking. The values are based on statistics from several earthquakes. In the 1989 Loma Prieta earthquake, over one-third of the pre-1940 single-family homes exposed to modified Mercalli intensity IX-X+ were "red tagged."

To make your home as safe as one built after 1940, bolt the home to the foundation and brace the "cripple" wall (the wall between the foundation and the floor joists)! Similarly, the installation of "earthquake bracing" to your mobile home will greatly reduce the chances of it falling from its supports.