Known as the most dangerous earthquake fault in North America, the Hayward Fault, in San Francisco, is well known as a time bomb to those who live and work along its 150 mile track. To begin with, it is pretty obvious that something is on the move. The Hayward fault has a creep mechanism, where the surface shows signs of a steady movement while the tectonic plates are locked together deep below the surface of the earth.
Cracks appear in masonry and paving along the length of the fault, and the walls of the Memorial Stadium at Berkeley are now offset by about a foot. The stadium is bisected along its length by the fault. The stadium owners are retaining the damaged wall for historical reasons, but incorporating floating sections and earthquake safe structural support during repairs, because the bomb is ticking. When the fault builds enough strain to suddenly unlock underground, there could be a 6 foot horizontal displacement, accompanied by an earthquake of magnitude 6.5 to 8.
History and expectation
Information about the last jolt from the Hayward, in 1868, is widely publicized. Geologists constantly monitor the fault, digging trenches to discover that the fault ruptures, on average, every 140 years. Geologists are conservative with their predictions of when the fault will leap again. Accustomed to dealing with vast spans of time, they usually give a thirty year window as a fairly accurate timespan for an earthquake prediction. When the Hayward Fault leaps, many will be prepared for it, but do they know what to be prepared for?
Infrastructure at risk
The path of the fault is easily discernible in some places but hidden in others. Does it run under the dam at Lake Temescal? It certainly crosses the Claremont Water Tunnel. Carrying around 150 millions of gallons of water a day, the tunnel supplies domestic water to over 800,000 people and is a vital water supply for firefighting. Built in the 1920s, the tunnel has been fitted with a bypass structure in case the fault line causes rupture or blockage. Many buildings along and around the Hayward Fault have been retrofitted, especially after the 1989 Loma Prieta earthquake. Roads and railways also cross the fault line, and hundreds, if not thousands, of homes have been built right over it.
Is San Francisco prepared for a Hayward leap? Yes. Are they ready? They need to be, the next Hayward Fault earthquake could be the big one they’ve been waiting for.
Tafur, V., San Francisco Chronicle, Remodeling Cal’s Memorial Stadium is a bear, May 11th 2011.
Sloan, Wells, Borchardt, and others, 2006, The Hayward fault: GSA Field Guide 7: 1906 San Francisco Earthquake Centennial Field Guides. Accessed May 12th 2011
Berkeley Seismological Laboratory. The Hayward Fault. Accessed May 12, 2011.
U.S. Geological Survey. Active Traces of the Hayward Fault. Accessed May 12, 2011.
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