As San Francisco commemorates the 25th anniversary of its last serious quake this week, a new report has warned that the city could once again be sitting on top of a ticking seismic time bomb.

Scientists tracking the movements of four highly stressed seismic faults that form part of the Bay Area’s densely populated San Andreas system have discovered worrying surface mobility which suggests that they could burst forth in a major quake at any time.

A Fault Monitoring Program has been measuring tiny movements in the faults using surveying tools called theodolites and GPS instruments.The faults under particular scrutiny comprise the Calaveras, which spans from Hollister (San Benito County) to Danville; the Hayward, located in between Suisun Bay and San Jose; the Rodgers Creek Fault in southern Sonoma County; and the Green Valley Fault, which runs from Vallejo to Fairfield.

“The extent of fault creep controls the size and timing of large earthquakes, and measuring that creep rate helps tell us how much strain is building up on the faults underground — although it can’t tell us when a fault will rupture in a quake,” said geophysicist James J. Lienkaemper of the U.S. Geological Survey in Menlo Park, who led the study.

The scientists’ report, which was published Monday in the Bulletin of the Seismological Society of America, predicts that the impending quake could rival the 6.9 magnitude Loma Prieta quake of 25 years ago, which killed 63 people and caused $6 billion in damage. Lienkaemper and his colleagues said all of the faults measured had the potential to rupture and trigger quakes of between 6.8 and 7.1 magnitude, and though the Hayward was considered to be the most likely to crack, all of the faults were considered to be long overdue for a major event.

“Given how long ago they had their last earthquakes, they are more than ready to produce a major earthquake again now,” said Roland Burgmann, a geophysicist and expert on crustal deformation at UC Berkeley who was not involved in Lienkaemper’s report.

The last quake was not centred in a very built up area but still caused immense destruction:

“It was a big quake,” notes University of California, Berkley’s Richard Allen. “But it was a big quake 60 miles south of much of the urban population — San Francisco.”

The Hayward fault runs directly through some of San Francisco’s suburbs, and the prospect of a sizeable earthquake occurring in this area poses a greater level of concern about the potential impact it could have. David Schwartz of the U.S. Geological Survey considered it to be “probably the most seismically dangerous fault in the Bay Area.”

Disaster predictors believe a 7.9-magnitude quake could cause 7,000 deaths, while leaving 300,000 people homeless and 1.8 million homes without drinkable water. It is a sobering thought, but being forewarned and making plans can save lives and damage, says Anne Kronenberg, executive director of San Francisco’s Office of Emergency Management. Even a nine second warning could give enough time to take appropriate action.

“The more prepared you are at home, the better it is for our emergency responders to be able to actually do what they have to do once the big one hits,” she explained. " “As an individual you can do the normal things — duck, cover and hold on under a sturdy table.”

Mother Nature will always have the upper hand on this precarious planet that we call home, but if we learn to predict and prepare for the effects of her wrath, then the odds of survival after such an event could be much greater.
 

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