The Hayward Fault: Will it trigger the next quake:

What to do if it does

April 10, 1992

Editor's note: LBL geologist Pat Williams examines the probability that the nearby Hayward Fault will produce a major earthquake, and discusses how we can prepare for that possibility, both at work and at home.

By Pat Williams

One day in the future, while many or most of us are still employed at LBL, there will be a catastrophic earthquake in the Bay Area. Many earthquake researchers believe that our very close neighbor, the northern Hayward Fault, is the top candidate to produce the area's next major shock. Modest preparations at home and at work will make a tremendous difference in our comfort, safety, and peace of mind in the aftermath of this event.

Long-term earthquake forecasting leans heavily on history for evaluating earthquake occurrence probabilities. This method relies on three pieces of information: 1) the fault's long-term rate of slip, 2) the time elapsed since its last rupture, and 3) the offset expected in a "typical" fault rupture.

Surprisingly, little of this information can be determined by classical seismological techniques. Earthquake science now relies heavily on geological and historical investigation of past fault behavior. Geological fault studies search for ancient evidence of slip rate, the size of past offsets, and the times of past ruptures.

Investigators scan old newspapers to learn the extent and size of historical ruptures. Studies of the Hayward Fault have provided the following clues: its average slip rate is about 9 mm/yr (0.35 in/yr); the latest rupture of its southern segment (Fremont to San Leandro) occurred in 1868; and rupture of the northern section (San Leandro to Pinole) probably occurred in 1936. Earthquake forecasters estimate an average earthquake recurrence interval of 167 years. Other concepts, particularly the idea that strain of the earth's crust in the Bay Area has slowly "recharged" after being greatly relaxed by the 1906 San Francisco earthquake, suggest that new Hayward Fault earthquakes are likely during the period of the next few years to decades.

LBL's Exploratory Research and Development Fund enabled a direct study of the Hayward Fault's earthquake history. Current results of that study indicate that the fault's past ruptures occurred, on average, every 150-250 years. This appears to support the 167- year average recurrence estimated by earthquake forecasters.

Following a large earthquake, the greatest concern we will probably have, after our personal safety, will be the safety and whereabouts of our families. Due to heavy damage to the transportation infrastructure at the Lab and in the Bay Area, it is likely that most of us will have to leave the site under our own power in order to reunite with our families. This will be more difficult for those of us who live very far from the Lab.

Lab roads will probably be closed by landslides and ground rupture along faults. The accompanying figure shows that ground rupture on the Hayward Fault is likely to close both Centennial Drive and Cyclotron Road for some period of time. Roads closed by fault breaks may be made passable by the Lab's own crews within a few hours. Roads closed by landslides are generally more difficult to repair, and are likely to remain impassable for days to weeks. Even after Lab roads are made passable, use will generally be restricted to emergency vehicles only. Lab earthquake procedures (located on the inside-back cover of the LBL telephone directory) instruct us **not** to leave the Laboratory by car.

After a major seismic event in the Bay Area, bridges and rail systems are likely to remain closed for a few hours to a few weeks while they are inspected, and if necessary, repaired. Those of us who used bridges and rail transit to commute to work may be stranded away from home for a day or more, and when we do go home, we are likely to cover most of the distance on foot.

Reasonable preparations for a long walk home include keeping sturdy shoes, a jacket, a hat, and a backpack, containing some high-energy nonperishable food, a water bottle, and a flashlight, at your work place and/or in your car. Additionally, it is essential that we **write down** a family earthquake plan and in it include as participants teachers, friends, neighbors, and relatives who can help us in reuniting our families and whom we can help during the crisis.

In the plan: 1) make a school/daycare evacuation plan; 2) choose a primary and an alternate family meeting site: 3) identify some person(s) outside the area to coordinate family messages (long distance lines will be the first to be reestablished; and 4) include someone in the plan would could care for your children if the family is separated during an earthquake. Store adequate food, water, batteries and other supplies to last three or more days after the earthquake. Be sure that both the structural and non-structural elements of your residence are earthquake safe. The telephone white pages contain an excellent summary of earthquake emergency information. By preparing for future Bay Area earthquakes, we acknowledge the potency of the active faults of this region, we contribute to our own peace of mind, and we set the stage for a more rapid post-earthquake recovery of LBL and the community.