from California Geology, January 1977, Vol. 30, No. 1.

LIVING IN EARTHQUAKE COUNTRY

A SURVEY OF RESIDENTS LIVING ALONG THE SAN ANDREAS FAULT

By

RAYMOND SULLIVAN, DAVID A. MUSTART, AND JON S. GALEHOUSE.

Department of Geology

San Francisco State University

In 1970 a survey of residents of the San Francisco peninsula who resided in the area of the San Andreas fault zone was initiated. The objective of the survey was to evaluate the awareness of, and attitudes toward, earthquake hazards in a densely populated area close to the San Andreas fault.

As government agencies and the news media focused increasing attention on the potential destructiveness of a major earthquake, a second study objective became the evaluation of any effect that this publicity had in changing public awareness and attitudes (Algermissen, 1972; Wallace, 1974; and Turner, 1975). Hence, additional surveys were made in 1972, 1974, and 1976 in the same residential area.

SURVEY AREA

The area selected for the survey covered a narrow strip of land in San Mateo County approximately mile wide and 6 miles long, extending northwest-southeast through parts of Daly City, Pacifica, South San Francisco, and San Bruno, along the San Andreas fault zone from Mussel Rock on the north to San Andreas Lake on the south (figures 1, 2). The area includes parts of the communities of Westlake, Fairmont, Edgemar, Westview, Serramonte, and Westborough. This is the most extensive residential development to straddle them 600-mile long San Andreas fault.

Although the boundaries of the study area were defined in 1970, they closely correspond in this region to limits of the "special studies zones" that were established alter passage of the Alquist-Priolo Special Studies ones Act of 1972 (Hart, 1976). The "special studies zones" enclose faults, such as the San Andreas, which have been determined to be active and, therefore, present potential fault-related hazards (front cover).

SEE FIGURE ON FRONT COVER

Residential Development

The development in the study area is almost completely residential (figure 3). About 80% of the buildings are single family units with the remaining 20% made up of apartments and townhouses. Residences are chiefly 2 story wooden frame buildings with stucco exterior and with a built-in garage beneath the living area. Most of the buildings were constructed during 1956-1973 and some building activity has continued to the present time.

Physical Setting

Prior to suburban development, the topography of the study area was characterized by rolling hills and stream-eroded gullies with a prominent erosional fault valley and numerous sag ponds marking the location of the San Andreas fault zone (figures 1, 4). At its northern end, the fault valley subsequently has been greatly modified by suburban development (figures 2, 4, 5, 6; Sullivan, 1975). Many of the hills were leveled, gullies filled, and marshy sag ponds drained. The geologic formations exposed in the area include poorly consolidated sediments of the Colma and Merced Formations of late Cenozoic age and highly sheared rocks of the Franciscan Formation of Mesozoic age (Jennings and Burnett, 1961).

SEE FIGURE 1

Figure 1. Location of the study area and distribution of sag ponds along the San Andreas fault in northern San Mateo County. (From San Mateo quadrangle map, 15 minute series, 1899 edition, reprinted 1913, U. S. Geological Survey).

Earthquake History

The only detailed historic record of a major earthquake in the area is that of the 1906 San Francisco earthquake estimated to have been Richter magnitude 8.3. At that time, the study area was sparsely populated, and only a few farms were scattered over the rolling terrain. Records of the 1906 event indicate violent ground shaking and an almost continuous line of rupturing along the San Andreas fault valley. Maximum local displacement occurred in the vicinity of what is now Skyline Boulevard and Berkshire Drive (figure 1) where fences and water supply pipelines were offset up to 13 feet (Lawson, 1908, v. 1, p. 94). In addition, small scale landsliding was reported throughout northern San Mateo county with most severe sliding concentrated along the coastal bluffs near Mussel Rock where the San Andreas fault enters the Pacific Ocean (Lawson, 1908, p. 92).

SEE FIGURE 2

Figure 2. Aerial view taken in 1964 looking eastward over the study area (outlined). Residential development in the suburbs south of San Francisco has expanded to the vicinity of the San Andreas fault. In the lower right-hand corner of the photograph is the city of Pacifica situated along the ocean. In the upper half is San Francisco Bay and the communities along its margin. Photo by Raymond Sullivan.

In 1957, before most of the present homes were built (figure 5), this same area near Mussel Rock marked the epicenter of a moderate earthquake measuring 5.3 on the Richter Scale (Oakeshott, 1959). During this earthquake no significant rupture occurred on the land surface, but ground shaking and landsliding above the coastal bluffs in the Westlake Palisades area caused an estimated $1 million damage. Since that time, several earthquakes with distant epicenters have been felt in the area but none has resulted in significant damage. This portion of the San Andreas fault has remained locked for about 20 years. Some earth scientists consider that temporarily locked areas of the San Andreas fault are more likely to produce major earthquakes than portions of the fault that are creeping (Allen, 1968, p. 77).

SEE FIGURE 3

Figure 3. Patterns of residential development in northern San Mateo County. From San Francisco South and Montara Mountain quadrangles, 7.5 minute series, 1956 edition, photo-revised 1968 and 1973, U. S. Geological Survey.

SEE FIGURE 4

Figure 4. Aerial view taken in September 1960 looking northwest over the study area (outlined) before widespread residential development. The locations of former sag ponds within the San Andreas fault zone are identified as in figure 1. The main highway extending approximately northwest-southeast is Skyline Boulevard, and the road extending east to west between sag ponds P-4 and P-5 is King Drive. Photo courtesy of the California Division of Highways (Caltrans).

QUESTIONNAIRES

During the surveys, San Francisco State University students enrolled in a course entitled "Earthquakes and the San Andreas fault", interviewed residents and recorded their responses to a series of questions related to earthquake hazards (table 1). Results of over 1400 questionnaires were transferred to computer cards and tabulated on the CDC Model 3150 computer at San Francisco State University. Two questionnaires were used during the course of the 6-year study. In 1970 and 1976, a detailed set of questions was asked; whereas, in the intervening years, an abbreviated form was used. Information collected in the survey can be summarized into the following categories: characteristics of respondents; awareness of, and attitudes toward, earthquake hazards; attitudes toward earthquake prediction; and awareness of public agencies' information policy.

DATA ANALYSES

The data represent an average over the entire study area because no well-defined local characteristics were apparent. In discussing the data, percentages are always given for 1976 responses (indicated by 2 asterisks). In most cases percentages for 1970 responses (indicated by 1 asterisk) are given for comparison. Information for the intervening years is not presented because those responses are generally consistent with, or intermediate between, those of 1970 and 1976. The awareness of, and attitudes toward, various earthquake-related information, legislation, and action are discussed below and are summarized in table 1.

Table 1. Summary of Earthquake hazards survey, 1970 and 1976.

* Results of 1970 survey. ** Results of 1976 survey.

Respondents

The persons interviewed were generally adults (94%**) with about equal representation of females (56%**) and males (44%**). Many residents owned their homes (93%*, 82%**) and had lived in the San Francisco Bay area for much of their adult lives. Approximately half of those interviewed had lived in the Bay area for more than 20 years (51%*, 48%**). A large percentage of residents had occupied their current dwelling for less than four years (67%*, 50%**), indicating the mobility of residents in the survey zone.

Awareness of, Attitudes toward, Earthquake Hazards

Almost all the residents (96%*, 93%**) of the study area had at least heard of the San Andreas fault, although about one-third (31%*, 38%**) were unaware of the direct relationship between fault movement and earthquakes. Residents were generally aware (77%*, 77%**) that the fault was situated within 1 mile of their homes. A somewhat lower percentage (72%*, 63%**) knew of the fault location before taking up residence. A common attitude seemed to be that the entire San Francisco Bay area is vulnerable to earthquake damage and most people (82%*, 74%**) would feel no safer if they lived 5 miles further away from the fault. Only about one-fourth of those interviewed (20%*, 26%**) felt that the building industry was doing all it could to make their homes safe from earthquake damage.

With the passage of the Alquist-Priolo Act in 1972, the study area was designated as a "special study zone" for fault-related hazards. Relatively few persons interviewed (16%**) claimed that they were informed of any possible earthquake hazards by the previous owner, developer, or landlord. However, this percentage had doubled from the number who knew of these hazards in 1970 (8%*), which suggests that recent legislation and publicity may be having some small effect on public awareness of these hazards. An increasing percentage of people (22%*, 51%**) responded that they would inform future residents of potential earthquake hazards.

About half of those interviewed (52%*, 44%**) reported having felt small tremors while living in their current residence. In the event of a future earthquake, residents indicated a wide range of reactions. The most common response was to stand under a doorway (43%*, 43%**) whereas some would seek safety under a desk, table, or bed (8%*, 15%**) and others would stay away from windows, mirrors, and chimneys (5%*, 3%**). A lesser percentage indicated they would stay where they were and remain calm (20%*, 16%**) and some suggested they would run outside (12%*, 14%**). The remainder were unsure of their reaction (10%*, 5%**), or gave another response (2%*, 4%**).

One response which showed a strongly defined trend was the number of residents who carry special earthquake insurance on their homes. The cost of such insurance in the Bay area is independent of proximity to active faults, and, at the present time, the annual premium is typically about $2.00 per $1,000 of coverage. In 1970, very few of the families (5%*) in the area had coverage, but, by 1976, the percentage had more than quadrupled (22%**). The chief reasons given by those not covered by insurance were: (a) too expensive (59%*, 42%**), (b) not needed (29%*, 28%**), or (c) unaware of availability (9%*, 14%**).

Attitudes Toward Earthquake Prediction

Earthquake prediction once relegated to the realm of cultism, has in recent years become a respected scientific pursuit and has been given considerable attention in the news media. This is reflected by the large percentage of respondents (61%**) who think that earthquakes can be predicted. The vast majority (94%**) feel that earth scientists rather than astrologers or psychics will make the most reliable estimates.

Many people involved in prediction of earthquakes are concerned about the attitude of the public toward an announcement of an impending earthquake. In the questionnaire, residents were invited to give their initial reaction to a prediction of a major earthquake expected to occur in 20 years, 1 year, or 1 week. It was surprising to discover that a large number of persons would do nothing in the event of any such prediction, although the proportion of such responses has decreased somewhat over the 6-year study period. In the case of a prediction of a major earthquake occurring 20 years in the future, most persons would do nothing (94%*, 85%**), whereas a small percentage would move (2%*, 5%**). For a earthquake warning 1 year in advance, a smaller percentage of residents (77%*, 61%**) indicated they would take no action, whereas a growing percentage (11%*, 19%**) indicated they would move or leave the area temporarily. Some were unsure of what steps to take (9%*, 8%**) and the remainder had a varied response (table 1). Finally, with a warning of only one week, a surprisingly large percentage of residents (52%*, 38%**) continued to indicate that they would do nothing, while over a third (36%*, 40%**) would move or leave the area temporarily. The remaining responses were similar to those for the 1-year prediction (table 1).

SEE FIGURE 5

Figure 5. Aerial view taken in October 1957 looking south toward Pacifica across the northern end of the study area (outlined) before extensive residential development. Highway construction to realign Skyline Boulevard (Route 35) and the Cabrillo Freeway (Route 1) intersection is underway (lower center portion of the photo). Sag pond, P-1, is in the process of being drained and filled (left center). Franklin Delano Roosevelt Elementary School, located in the San Andreas fault zone, is under construction (right center). Photo courtesy of the California Division of Highways (Caltrans).

Awareness of Public Agencies Information Policy

This area of questioning was a brief survey of the role played by government agencies in informing the residents of earthquake hazards and concerns about pursuing earthquake research. In general, it was found that a relatively low percentage of residents (22%*, 28%**) had received earthquake hazard information directly from their local government. The majority of those interviewed (81%*, 77%**) supported legislation which would make it a requirement that all prospective occupants be informed of the potential earthquake hazards at the site. Only a small percentage (16%**), however, was aware of the passage in 1972 of the Alquist-Priolo Act which was designed to encourage such disclosures and limit building of residential structures on seismically active faults. In light of the recent efforts of Senator Alan Cranston of California and others to increase significantly the funding of earthquake research, it is interesting that the vast majority of residents (75%*, 88%**) feel that such research should be sponsored by State and Federal governments.

USE OF SURVEY RESULTS

The presentation of these results of a 6-year survey is intended as a preliminary study of the public's knowledge and concerns about earthquake hazards in a narrow zone along the San Andreas fault. It is apparent from the study, that although those interviewed were well aware of their proximity to the fault, they were for the most part unconcerned about the potential destructiveness of a major or catastrophic earthquake. Such an attitude may be understandable in view of the fact that the area has been subjected to only minor earthquakes over the past 70 years, and also because of a lack of experience among residents with other natural disasters.

Further contributing factors to the lack of concern by residents may be the failure of many to understand the direct relationship between faults and earthquakes, and the lack of knowledge of the potential geologic hazards in the immediate vicinity of the fault zone. The evidence presented by Lawson (1908), Oakeshott (1959), Algermissen (1972), and Borcherdt and others (1975) has shown that maximum ground shaking and fault rupture will typically be greatest along the fault zone (front cover). It may be concluded, therefore, that although this area astride the San Andreas fault is predominately residential and does not have high-rise structures like those of the large cities of the San Francisco Bay area, widespread damage to structures will probably result if a major earthquake of the magnitude and location of the 1906 tremor occurs.

EARTHQUAKE EDUCATION

The need for greater community earthquake preparedness is receiving increasing attention, particularly since an accurate and dependable system of prediction may be developed in the near future. This increased likelihood of reliable earthquake prediction makes a continued assessment of public attitudes a necessary step in defining the role of government agencies in hazard reduction and disaster preparedness. Recent experiences in China indicate that a key to reducing life loss is an understanding of the hazard and willing cooperation of citizens (Press, and others 1975). It is apparent from the survey that at present citizens are generally uninformed as to the proper action to take in the event of such a warning. Even with a prediction that a major earthquake will occur in one week, more than one-third of those interviewed would take no action.

SEE FIGURE 6

Figure 6. View similar to that in figure 5 taken in April 1968 showing the extent of residential development between 1957 and 1968 in the northernmost part of the study area. The sag pond P-1 is the site of newly constructed homes. The highway interchange and the Franklin Delano Roosevelt School have been completed. Photo courtesy of the California Division of Highways (Caltrans).

A program of public education should begin immediately, even though there may be concerns about continually alarming the public about earthquake hazards while sustaining their awareness over a period of time. It is recommended that an established California governmental agency undertake to organize and coordinate this program. Along with advice on disaster preparedness and hazard abatement, residents should also be familiar with the longer term social and economic disruptions which are inevitable as a result of a tremor of the magnitude of the 1906 earthquake. These and other topics can be discussed most logically at community meetings involving representatives of appropriate government agencies along with members of the news media, and specialists in scientific, financial, and business matters. A community-centered program should be effective and have optimum impact because many of the hazards will be unique to each region. Without a long-range campaign, the public may not accept the credibility of scientific predictions and the necessity for inconvenient and sometimes costly hazard reduction measures.

Public educational programs have not been highly successful in the past. Nevertheless, a well-planned and imaginatively-constructed campaign, designed to relate to situations of immediate and tangible concerns may have a considerable impact on the residents of a disaster-prone area. The possibility of a potentially destructive earthquake in California in the near future dictates that such a program be designed and implemented as soon as possible.

ACKNOWLEDGMENTS

We wish to thank our colleagues, Raymond Pestrong and Richard Lambert, for help in administering the surveys and for continued discussions and advice, and Charles Bickel for computer assistance. We also thank Hugo Hawkins for drafting assistance and George Strauch of Caltrans for providing most of the photographs. Special thanks go to the many students who conducted the surveys and to the residents of the study area for their courtesy and cooperation.

EDITOR'S NOTE

Readers of California Geology will be interested to know that since this study was accepted for publication, another study forecasting public response to earthquakes was released in December 1976. That study, by J. Eugene Haas and Dennis S. Mileti and entitled "Socioeconomic Impact of Earthquake Prediction on Government, Business, and Community." was conducted and released by the Institute of Behavioral Science, University of Colorado, Boulder, Colorado.

The Haas-Mileti study took a broader approach, involving the expected reactions of key elements of government, business, and citizens, to several detailed earthquake scenarios. California Geology will carry further news of that study.

A basic conclusion of both studies is that as increased public information programs make the people of California more aware of the realities of the earthquake threat in California, their expected responses will change significantly.

References Cited

Algermissen, S. T. (principal investigator), 1972, A study of earthquake losses in the San Francisco Bay area (data and analysis), a report prepared for the Office of Emergency Preparedness: U. S. Department of Commerce, National Oceanic and Administration Environment Research Laboratories, 220 p.

Allen, C. R., 1968, The tectonic environments of seismically active and inactive areas along the San Andreas fault system: in Proceeding of Conference on Geologic Problems of San Andreas Fault System, edited by Dickinson, W. R. and Grantz, A., Stanford University Publication, Geological Sciences, vol. XI, 374 p.

Borcherdt, R. D., Gibbs, J. F., and Lajoie, K. R., 1975, Prediction of maximum earthquake intensity in the San Francisco Bay region, California, for large earthquakes on the San Andreas and Hayward faults: Report to accompany Miscellaneous Field Studies, United States Geological Survey, Map MF-709, 3 maps, 11 p.

Hart, E. W., 1976, Fault hazard zones in California, Alquist-Priolo Special Studies zones Act of 1972 with index to Special Studies zones Maps: California Division of Mines and Geology, Special Publication 42 (Revised edition), 27 p.

Jennings, C. W., and Burnett, J. L., 1961, San Francisco sheet, Geologic map of California: California Division of Mines and Geology.

Lawson, A. C. (Chairman), 1908, The California Earthquake of April 18, 1906, Report of the State Earthquake Investigation Commission, Carnegie Institute of Washington, Washington, D. C., 2 vol., 643 p.

Oakeshott, G. B. editor, 1959, San Francisco earthquakes of March 1957: California Division of Mines and Geology, Special Report 57, 127p.

Press, F., and others, 1975, Earthquake research in China: Eos Transactions, American Geophysical Union, v. 56 (11), p. 838-881.

Sullivan, R., 1975, Geological Hazards along the coast south of San Francisco: California Geology, v. 28, no. 2, p. 27-33.

Thatcher, W., 1975, Strain accumulation on the northern San Andreas fault zone since 1906: Journal of Geophysical Research, 80, p. 4875-4880.

Turner, R. H. (Chairman), 1975, Earthquake prediction and public policy; prepared by the Panel on the Public Implications of Earthquake Prediction of the Advisory Committee on Emergency Planning: Commission on Sociotechnical Systems, National Research Council, National Academy of Sciences. 142 p.

Wallace, R. E., 1974, Goals, strategy, and tasks of the Earthquake Hazard Reduction Program: U. S. Geological Survey Circular 701, 26 p.