ANTICIPATING THE BIG ONE / Best preparation for quake assumes worst-case scenario
Published 4:00 am, Monday, September 19, 2005
Remember this about future Bay Area earthquakes: Regardless of what geoscientists forecast, prepare for the worst.
To those Bay Area residents who are in denial about the inevitability of another big quake, or who assume a big quake strikes only once in a lifetime, or who think scientists will -- like scientists in Hollywood movies -- issue an alert a few hours before the quake, science and history offer zero reassurance.
According to an authoritative 2003 analysis by the Working Group on California Earthquake Probabilities, an expert panel co-chaired by USGS seismologists Michael Blanpied and David Schwartz, the next big Bay Area quake will probably come within a few decades, and its expected magnitude could be at least 6.7. It could also be much more severe than magnitude 6.7.
And don't be misled by "a few decades": Statistically speaking, the coming quake is literally as likely to strike, say, next month or next Christmas -- or in two minutes -- as it is in 2031, the final year of the time window analyzed by the probability study. The group's overall forecast was that there is a 62 percent probability of at least one 6.7-or-stronger quake between 2002 and 2031.
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Worse, recent research hints that big quakes might attack in swarms, one after another.
Mother Nature indulged Northern Californians for most of the 20th century. No devastating quakes hit the Bay Area between 1906 and 1989. However, that might have been an anomalously calm epoch, seismically speaking -- an epoch that future generations may recall with envy.
By contrast, three quakes that were 6.5 or greater struck the Bay Area in the 30-year period between 1838 and 1868. In the 151-year period encompassing 1838 to 1989, five quakes of 6.5 or higher -- on average, 1 every 30 years -- have jolted the region (in 1838, 1865, 1868, 1906 and 1989).
In addition, recent research by two top quake experts showed that between 1680 and 1776, a 96-year period, severe quakes struck six of the seven major earthquake fault systems in the Bay Area.
At the moment, there's no specific reason to think the 21st century will bring another such epoch of big-quake swarms. But there's no specific reason to think it won't, either. Despite a century of intensive research, we still know so little about quakes that it's anyone's guess whether such a swarm will occur again -- or how it would affect Bay Area tourism, property values, the safety of bridges and other infrastructure, and so on.
Quakes are inevitable: This is simple physics.
"There is ongoing strain accumulation" in the crust because of the inexorable movements of the giant Pacific and North American crustal plates, said seismologist Mary Lou Zoback of the U.S. Geological Survey in Menlo Park. The strain is intermittently unleashed in the form of quakes.
The 1906 quake released an enormous amount of crustal stress, thereby casting a "stress shadow" over much of the 20th century. That's probably why there were so few major Northern California quakes from 1906 to 1989, many experts have long maintained.
"The real question," Zoback asks, "is: Have we emerged from that stress shadow? There's not any specific measurement we can make to determine that."
In any case, Bay Area residents shouldn't be lulled into a false sense of calm by the Working Group's projection of a 6.7 or greater quake between now and 2031.
To laypeople, the magnitude "6.7" might sound deceptively similar to the 6.9 magnitude of the Loma Prieta quake of 1989. Loma Prieta killed almost 70 people and caused billions of dollars in damage. Yet for many locals, the quake's severity has faded with time. For most people, after all, the worst that happened was that their cat got scared or books tumbled off shelves.
They've forgotten how lucky they were. While Loma Prieta's epicenter was some 60 miles to the south, the Working Group says the next quake's epicenter is likely to be directly under an East Bay community. The likeliest epicenter, according to the Working Group, is somewhere along the northern Hayward fault in the East Bay and the adjacent Rodgers Creek fault in the North Bay. The two faults nearly join up to create one extended, roughly linear crustal fissure; their ends are a short distance apart under the waters of San Pablo Bay.
The Loma Prieta quake of 1989 would have wrought "substantially worse" consequences if its epicenter had been in San Francisco or the East Bay rather than in the Santa Cruz mountains, Zoback says.
The Working Group report projects a 27 percent probability of a Hayward-Rodgers Creek quake (magnitude 6.7 or worse) between 2002 and 2031.
The next-highest probability quake (21 percent) is an event of similar magnitude or larger on the San Andreas fault.
The third-highest probability (14 percent) is a quake on what they call a "background" fault -- one that is unknown at this time or poorly understood.
The report notes that while the Hayward-Rodgers Creek and San Andreas faults face the highest probability of a quake, "the Hayward fault is of particular concern because of the dense urban development along and directly adjacent to it and the major infrastructure lines (water, electricity, gas, transportation) that cross it."
Large areas of scientific uncertainty cloud the Working Group's forecasts, including:
-- Uncertainty about how a quake on one fault generates stresses that possibly trigger quakes on adjacent faults.
-- Uncertainty about how many unknown faults litter the Bay Area, particularly vertical-moving "blind thrust" faults -- which can't be identified before they break loose because they don't leave any telltale evidence on the surface.
-- Uncertainty over how often quakes occur on the San Andreas and adjacent fault systems over time. Most written historical records of the region don't start till after the 18th century.
For a time, scientists had hoped to ease the latter uncertainty via a moderate-sized project called BAPEX, for "Bay Area Paleoearthquake Experiment." Run by Schwartz, BAPEX was funded for a projected $3 million as part of a cooperative research agreement between PG&E and the U.S. Geological Survey.
The project aimed to dig dozens of trenches along earthquake faults across the Bay Area. By digging such trenches, scientists hoped to detect geological clues that would reveal how often quakes have struck different faults in the Bay Area over the last 2,000 years. In that way, they reasoned, they could learn many new things -- whether quakes tend to strike quasi-cyclically, for example.
But PG&E soon encountered financial woes that would bankrupt the utility. In 2000, after just one year of research, PG&E canceled further funding of
Nonetheless, during that one short year -- during which Schwartz and his colleagues dug 28 trenches -- they uncovered evidence of a possible quake "cluster" from the late 17th to the late 18th century.
In an interview last week, Schwartz told The Chronicle that BAPEX revealed that six of the seven major fault systems in the Bay Area were hit by large quakes between 1680 and 1776. The exact timing of each quake, he said, remains uncertain.
"I think that people have been really lulled into a false sense of security because of the absence of large earthquakes in the 20th century," Schwartz warned. "We have a lot of faults that have not done anything since the late 1700s.
"But we know from geological records that they've moved repeatedly in the past -- and they have to move again."
Scientists estimate a 62 percent chance* of at least one earthquake of magnitude-6.7 or greater occurring somewhere in the Bay Area between 2002 and 2031. This map shows each faults individual probability of causing a magnitude-6.7 (or greater) quake through 2031.
* Estimate incorporates the odds 14 percent of a quake happening on faults not shown on the map.
-- WHAT THESE NUMBERS MEAN
How are these probabilities calculated, and what do they really mean?
Scientists begin by considering the balance between two factors: the motion of plates making up the Earths outer shell, and the amount of slippage on faults. Over time, these two processes must balance each other out. Plate motions load strain onto faults, which builds up over time and is released primarily during earthquakes.
Scientists use geologic data, historical records and various statistical models to inform their predictions.
And yet, because the geologic processes are not fully understood, there is a fair degree of uncertainty in the predictions.
That is why scientists use probabilities expressed as the likelihood that a quake will occur on a given fault in a certain length of time. One way to think of probability is to imagine a scale of 0 to 100, with 0 meaning there is no chance of a given event occurring and 100 meaning it definitely will happen.
A 50 percent chance would represent a toss-up (like flipping a coin, for example). A 62 percent chance almost two-thirds means the event is nearly twice as likely to happen as not to happen.
About the series
Sunday: Is the Bay Area ready? Yes, but ... Read this installment, which includes suggestions on home and car disaster kits, at SFGate.com.
Today: How big will the Big One be?