EARTHQUAKE PLANNING SCENARIO for A MAGNITUDE 7.5

EARTHQUAKE ON THE HAYWARD FAULT, SAN FRANCISCO BAY AREA

By

KARL V. STEINBRUGGE, Structural Engineer

El Cerrito, California

HENRY J. LAGORIO, Professor of Architecture

University of California, Berkeley

and

JAMES F. DAVIS, State Geologist

JOHN H. BENNETT, Civil Engineer

GLENN BORCHARDT, Geochemist

TOUSSON R. TOPPOZADA, Seismologist

Division of Mines and Geology

A major earthquake within the densely populated San Francisco Bay area, such as the event postulated in this planning scenario or an equivalent event in the urban Los Angeles area, would constitute one of the most devastating natural disasters that could conceivably strike this nation. The damage caused by such an event could well exceed that of a larger earthquake (M 8) in lesser populated regions along the San Andreas Fault.

This article is adapted from Special Publication 78, Earthquake planning scenario for a magnitude 7.5 earthquake on the Hayward fault in the San Francisco Bay area, Division of Mines and Geology, 220 p., 10 plates, in press.

The scenario provides a perspective on many of the plausible consequences of a major earthquake on the Hayward Fault, focusing on the impact to transportation and utility lifelines and critical structures such as hospitals and schools. The scenario will increase public awareness of the threat and the need for coordinated emergency preparedness and response planning to cope with this eventuality . . . . editor.

INTRODUCTION

Following the devastating eruption of Mount St. Helens in 1980, the President requested the National Security Council to consider the implications of the occurrence of a large damaging earthquake in California. One of the major conclusions of that analysis was that although there is a general capability to respond to moderate-size earthquakes, it is unlikely that the collective emergency response capabilities of all levels of government and the private sector would be adequate to cope with the consequences of a major destructive earthquake near a metropolitan area.

The Governor's Emergency Task Force an Earthquake Preparedness was established in February 1981. Working with the Task Force, the Division of Mines and Geology developed two earthquake planning scenarios (Davis and others, 1982a, 1982b). These Scenarios were based upon a repeat of the 1906 San Francisco earthquake (M 8) on the northern San Andreas fault and a repeat of the 1857 Fort Tejon earthquake (M 8) on the southern San Andreas fault.

While these two planning scenarios for great earthquakes on the northern and southern San Andreas fault are basic for emergency planning efforts, it was apparent that similar analyses were needed for other faults in metropolitan areas that are capable of producing earthquakes of equivalent or even greater destruction. Paramount among these were consideration of a M 7.0 earthquake on the Newport-Inglewood fault in southern California and a M 7.5 earthquake on the Hayward fault.

Funded in part by the Earthquake Hazards Reduction Program of the U. S. Geological Survey, the Division of Mines and Geology in collaboration with structural engineer Karl V. Steinbrugge developed a planning scenario for the Hayward fault. Similar scenarios for a M 7.0 earthquake on the Newport-Inglewood fault and for a damaging earthquake in the San Diego Tijuana area are in progress.

While no scenario will prove accurate in detail, this effort provides planners with a regional pattern of the damage and types of problems that will confront emergency response personnel. These scenarios give a more complete understanding of earthquake hazards and provide a basis to develop response plans to cope with them.

HAYWARD FAULT

The Hayward fault is a seismically active major element of the San Andreas fault system, one of several northwest-trending strike-slip faults associated with right-lateral tectonic movement between the North American and Pacific plates. The Hayward fault is the southern segment of an extensive fracture zone consisting of the Hayward, Rodgers Creek, Healdsburg, and Maacama fault segments. The zone extends northwest to Mendocino County (Slemmons and Chung, 1982), a total distance of 280 km (175 miles). The 100 km (62 mile) long Hayward segment extends from San Pablo Bay to an obscure convergence with the Calaveras fault near Mount Misery east of San Jose.

The Hayward earthquake of October 21, 1868 of Richter magnitude about 7 (M 7) was one of the largest earthquakes to occur in this area causing widespread damage throughout the then sparsely populated Bay area. An event of similar destructive magnitude in 1836 also occurred on the Hayward fault. Future earthquakes of comparable magnitude (M 6.5-7.5) are a reasonable expectation and could occur at any time. A large earthquake on either the Hayward or the San Andreas fault poses a major threat to the entire Bay area. While the effects of these earthquakes may differ from place to place, a major earthquake on the Hayward fault is not an exclusive East Bay concern and a San Andreas event is not an exclusive San Francisco concern. The threat to San Francisco from the Hayward fault was recognized by A. C. Lawson in 1908 in Report of the State Earthquake Commission, The California earthquake of April 18, 1906, p. 447: "The foot of Market Street, San Francisco, is about midway between the San Andreas rift and the fault scarp upon which movement occurred in 1868. The city has, therefore, to reckon with the latter as well as the former in its future career, and, consequently, should be doubly prudent in the location and structure of its important buildings".

SCENARIO EARTHQUAKE

Description

This planning scenario is based on the maximum credible earthquake that could occur on the Hayward fault. The assumed characteristics of this earthquake are (1) a Richter magnitude of 7.5 (M 7.5) that results from rupture of the entire length of the Hayward fault from San Pablo Bay to east of San Jose, (2) surface faulting that produces horizontal offsets of up to 10 feet, (3) potentially damaging shaking that continues for 30-40 seconds within 25 km (16 miles) of the fault, and (4) frequent aftershocks that continue for many weeks, including events of M 6 or larger.

While this planning scenario is based upon a maximum credible event for the Hayward fault, damage patterns would in many respects be similar for an event of smaller magnitude. A magnitude 6.5 to 7.0 event similar to the 1868 earthquake, for example, would most probably result from rupture along only one-half the length of the fault and would produce up to about 3 feet of surface offset. The resulting damage to lifelines, critical facilities, and local utility distribution systems, while not as severe, would affect most of the same facilities along the ruptured segment of the fault. Shaking near the rupture zone would be as severe, but presumably, not as prolonged. Ground failures would occur in the same general areas.

Predicted Effects

Fault rupture

Horizontal fault offset of up to 10 feet along the 62-mile length of the fault would cause major damage to structures located on active fault traces. Throughout most of its length the fault traverses residential and commercial areas, posing the threat of widespread damage to buildings, utility lifelines and distribution systems, and transportation routes.

Shaking intensity

The area subject to shaking of Modified Mercalli intensity VIII (strong enough to cause considerable damage in ordinary substantial buildings; great damage in poorly built structures) extends from near Petaluma and Napa in the north San Francisco Bay to south of San Jose. The region encompasses most of the populated areas of eastern Contra Costa County and Livermore Valley on the east, most of the heavily populated greater San Jose area, the communities north along the San Francisco Peninsula to and including much of San Francisco, and the low-lying urban areas of bayside Marin County.

Predicted shaking of Modified Mercalli intensity IX (strong enough to cause considerable damage in specially designed structures; great damage in substantial buildings with partial collapse; buildings shifted off foundations) encompasses an area of some 5 miles in width lying generally west of the Hayward fault, an area that includes virtually all the developed urban area of the East Bay from San Pablo southeast to and including the eastern half of San Jose.

Intensities greater than IX will most commonly occur along the zone of surface rupture and in those areas having a high potential for ground failure, notably around the Bay margins.

Ground failures

Secondary ground failures, notably differential settlements and shifting of the land surface due to liquefaction will be common, particularly on filled ground around the Bay margins. These movements will damage various major structures and lifeline facilities, notably highways, railroads, airport runways, port facilities, and some utility pipelines. Seismically induced landslides pose an additional threat, particularly in the East Bay hills, with the probability of failure being highest in the rainy season.

EARTHQUAKE IMPACT

Deaths and Injuries

Deaths resulting from this scenario earthquake are estimated to range from 1,500 to 4,500 depending upon the time and day of occurrence. Hospitalized casualties are estimated to be three times the number of deaths; significant non-hospitalized casualties are estimated at 30 times the number of deaths.

Hospitals near Fault

Eight of the 26 general acute care hospitals (99 beds or more) in Alameda and Contra Costa counties are located within one mile of the Hayward fault. This represents a bed capacity of 2,300 (about 35 percent) of a total of 6,200 beds available in these major facilities. Almost all buildings at these eight sites were constructed prior to adoption of more stringent hospital building requirements in 1972. Direct damage, restricted access, prolonged loss of public utility services and reduced public confidence in structures near the fault, may necessitate closure of some of these facilities. Thus, one or more hospitals could become an added post-earthquake burden.

Public Schools

Earthquake resistant public school buildings are generally well distributed throughout populated areas and are normally in a safe condition following earthquakes. These structures provide a major resource for mass shelter and feeding. Some substantial damage to several schools can be anticipated, however, because of close proximity to the fault. Also, schools located in the hills east of the fault will be functionally impaired due to disrupted utility services. The Hayward fault traverses the University of California campus where about 20 percent of the floor space is in buildings classified as seismically poor or very poor, some of which can be expected to partially or totally collapse.

Transportation Lifelines

Trans-bay bridges

The trans-bay bridges will be temporarily closed due to ground and structural failures at the bridge approaches. Roadway clearance, emergency repairs, de-tours, and bridge inspections will preclude or severely restrict use of these structures during the initial post-earthquake hours. The Oakland Bay Bridge will be effectively closed due to major damage at the east approach interchange and northward along Interstate 80/Route 17; the Richmond-San Rafael, San Mateo, and Dumbarton crossings should be available to limited emergency traffic in less than 36 hours. The Golden Gate Bridge will remain open, but traffic will be severely limited by damage at the southerly bridge approaches.

Major freeway routes

All of the major freeway routes to the East Bay from the east and south either cross the fault or are otherwise vulnerable to damage by strong shaking and ground failures. Major routes subject to surface fault offset (up to 10 feet) include Interstate 80 at San Pablo, Interstate 580 in East Oakland, Interstate 680 at Fremont and south to Milpitas, Route 24 west of the Caldecott Tunnel and most of Route 13 (Warren Freeway). Ground failures due to liquefaction and strong ground shaking may cause major damage along Route 17 from Richmond to San Jose.

Virtually all older freeway bridges in the area have been refitted to increase their resistance to shaking. Nevertheless, damage to and collapse of some of these structures is to be expected. Access to and travel within the East Bay will be difficult and limited to emergency traffic. Most principal routes on the San Francisco and Marin peninsulas and western portion of the greater San Jose area will be open subject to major delays and detours.

Airports

Runways at the major Bay area airports are generally constructed of fill placed over Bay mud of varying depths. Their performance when subjected to prolonged shaking is questionable, and liquefaction and differential settlement may render all or portions of many runways unusable by larger aircraft. For planning purposes, San Jose Municipal Airport is assumed to be available for larger transport aircraft. San Francisco and Oakland International, Hayward Municipal, and other secondary Bay area airports should be available for limited use by small aircraft and helicopters. Alameda Naval Air Station will be closed.

BART

BART will be shut down due to the lack of electrical power and the need to assess and repair damage. Principal damage will be to the Berkeley Hills tunnels which will be closed indefinitely as a result of fault rupture. Damage to a few elevated spans is postulated in the East Bay. The trans-bay tube and the subway systems are expected to survive with no major damage.

Railroads

Rail service to the Bay area from the east and south will be curtailed due to fault rupture, ground failures at various locations around the Bay perimeter, and structural damage to numerous bridges. Rail service via the coast route from southern California to San Francisco will be restored rapidly but all other lines to and from the Bay area will be blocked for at least the initial 72-hour post-earthquake period.

Port facilities

Most of the docks in the Bay area are pile supported and are not expected to be greatly affected. Port facilities at San Francisco are, therefore, expected to generally remain functional, though initially the loss of power and impaired access to the area will curtail operations.

In the East Bay, the major Port of Oakland and other smaller commercial port facilities at Richmond and in the Carquinez Straits will generally be nonfunctional as a result of prolonged power loss and damage to truck and rail access routes. Within the port areas filled land will settle disrupting both rails and streets. Damage to oil pipeline and storage facilities at the Richmond and Carquinez facilities poses a threat of contamination and fire.

Utility Lifelines

Communications

Telephone communications will be overloaded by post-earthquake calls within the area and from the outside. This situation will be further complicated by physical damage to equipment due to ground shaking and loss of electrical power. Moreover, not all of the systems in the region are set up to process emergency calls automatically on a previously established priority basis. Thus, overloading of equipment still in service could be very significant.

The East Bay and San Jose areas have a substantial number of telephone facilities located in areas subject to severe shaking and high probability of ground failure. Access for repairs will be a major problem.

The lack of emergency power has been the primary cause of communications failure in past disasters. Poor installation practices and inadequate preventative maintenance of backup power equipment contribute to a high failure rate. The presumed scarcity of propane and gasoline after a major earthquake will strictly limit the viability of surviving communications sites.

Electrical power

During some portion of the first 72-hour period following the earthquake, all portions of the planning area will experience some loss of power. It is reasonable to consider about one-third of the service connections in the area to be without power for 24 hours. In the urban sections of Oakland and other East Bay cities, the power outage should be considered at 100% for the first 24 hours and 75% for an additional 24 hours. This means that 75% of customers will have no power and 25% will have only limited service. The power outage for San Francisco should be considered at 50% for the initial 24 hours and at 25% for an additional 24 hours.

Electrical power facilities in the East Bay are particularly vulnerable to damage from the scenario earthquake, and the time required to restore full power will be prolonged. While the resources may be available to rapidly deal with repairs to the system, the general confusion and damage to other lifelines such as communications and highways will complicate restoration efforts. Realistically, power is unlikely to be restored to many areas for several days or longer. Those concerned with emergency planning for power-dependent systems such as communications, water supply, fire fighting, and waste treatment should be cognizant of this likelihood.

Water supply

Water supply systems in the East Bay will be severely crippled in this scenario earthquake. Displacement along the Hayward fault will heavily damage all major tunnels, aqueducts and the many distribution systems that cross the fault. The flow of water crossing the fault will be reduced to 10% for the first 24 hours. The public will need to conserve available supplies (as an example, water in hot water heaters) and to take safety measures against contamination.

Restoration of water service to all areas east of the fault in the East Bay hills will be greatly delayed. In the Alquist-Priolo zone, temporary pipelines, similar to those provided to many residences after the 1971 San Fernando earthquake may be used. Restoration of full service could take months.

Within the past 10 years the East Bay Municipal Utility District (EBMUD) has rebuilt the older, weaker dams in their system to improved seismic standards. Consequently, a major dam failure is not considered a credible element in this scenario.

Waste water

Waste water pipelines from the hillside areas that cross the Hayward fault will be sheared and unable to carry sewage. Open trenches may be necessary to carry sewage for short distances. Alternatively, planners will have to provide for emergency housing or temporary sanitation facilities.

Treatment plants will shut down due to lack of power. EBMUD's electric power system which uses methane gas from its treatment plant will be unable to support full plant function. It may be necessary for emergency treated raw sewage to be discharged into the Bay for up to one month.

Natural gas

Horizontal displacement of up to 10 feet across the fault zone will cause hundreds of breaks in mains, valves, and service connections. Secondary ground failures resulting from high intensity shaking will result in many additional breaks in the system in the proximity of the fault zone. Some fires will occur in streets due to broken gas mains; structural fires will occur as a result of broken service connections.

Fault rupture will also cause damage to the larger diameter transmission pipelines where they cross the fault at San Pablo and Fremont. As a result of damage to these transmission facilities, natural gas will be unavailable to all of the East Bay from San Pablo on the north to Milpitas on the south.

While gas supplies to most of the East Bay will be restored rapidly, some areas in the hills immediately east of the fault could be without gas for several weeks.

Damage to facilities serving the south Bay and San Francisco Peninsula should be minimal. Where poor ground conditions result in substantial damage to distribution systems, restoration of service will be prolonged. Throughout the north Bay, only minimal damage to isolated segments of the distribution system is anticipated.

Petroleum refineries and products

The six major refineries in the San Francisco Bay area are all subject to shaking and damage from ground failure. Refineries may also suffer damage by fire and operations will be curtailed by loss of utility services. Pipelines and storage facilities located on unstable and landslide sensitive ground along the Bay margin are vulnerable to damage, particularly those at marine terminals. All major pipelines transporting petroleum fuels to the Bay area (including the south Bay) cross the Hayward fault either at San Pablo or Fremont. They all are vulnerable to damage by surface fault rupture.

Lifeline Corridors

The major transportation corridors that serve the East Bay area, such as the routes through San Pablo and Fremont, are commonly shared by various other lifeline facilities, all of which are vulnerable to major damage where they cross the fault. Multiple failures of several major lifelines within these restricted corridors have the potential to complicate emergency response efforts. These corridors warrant special attention by emergency planners.

REFERENCES

Davis, J. F., Bennett, J. H., Borchardt, G. A., Kahle, J. E., Rice, S. J., and Silva, M. A., 1982a, Earthquake planning scenario for a magnitude 8.3 earthquake on the San Andreas fault in the San Francisco Bay area: Division of Mines and Geology, Special Publication 61, 160 p.

Davis, J. F., Bennett, J. H., Borchardt, G. A., Kahle, J. E., Rice, S. J., and Silva, M. A., 1982b, Earthquake planning scenario for a magnitude 8.3 earthquake on the San Andreas fault in southern California: Division of Mines and Geology, Special Publication 60, 128 p.

Slemmons, D. B., and Chung, D. H., 1982, Maximum credible earthquake magnitudes for the Calaveras and Hayward fault zones, California in Hart, E. W., Hirschfeld, S. E., and Schulz, S. S., editors, Proceedings of the Conference on Earthquake Hazards in the Eastern San Francisco Bay Area: Division of Mines and

Geology, Special Publication 62, p. 115-124