When researchers abruptly made a threatening "earthquake deficit" vanish in a flurry of technical revisions this week, their about-face reflected the enormous handicaps affecting scientists who must search blindly for the truth hidden in the earth beneath their feet.
Changes in the face of new evidence are in the nature of any scientific inquiry. Yet when the subject is earthquakes--an emotionally charged topic fraught with implications for construction codes, insurance rates, seismic hazard mapping and residential zoning--the demand for certainty may be higher than such an uncertain science can satisfy, say experts who have made Southern California's earthquake hazards their life's work.
Even so, this week's change involved the correction of a number of specific problems that ranged from subtle mathematical errors and flawed technical assumptions to a fundamental misreading of how Northern and Southern California were settled in the 19th century.
All those problems affected the model of seismic activity used to calibrate the idea of an earthquake deficit.
There were, as well, more substantial technical disagreements, arising from conflicting views about how relatively small faults can link together to unleash an earthquake much more powerful than an analysis of any single stretch of the fissured earth would suggest.
Overall, the 1995 assessment that first proposed the earthquake deficit "had some bugs in it," said Thomas Henyey, director of the Southern California Earthquake Center, which helped formulate that assessment. "People attacked the most provocative aspects of the model and discovered it had a bug in it."
Several experts said of the earthquake deficit model that there had been pressure to appear more certain in public than private misgivings might otherwise justify.
"I don't think it was a true consensus," said David P. Schwartz, a U.S. Geological Survey expert on Western earthquake hazards who was among those helping to prepare the report.
"We probably don't always present all the uncertainties because what we say might lose its impact," Schwartz said. 'You have results, and results have uncertainties and you tend to downplay the uncertainties, as a matter of human nature.
"The public looks askance at that," he said.
As much as anything, the experts agreed, the problem has been one of ambition--something that arose from the unprecedented nature of the effort itself: to draw together and integrate for the first time all the different elements of earthquake science into a single comprehensive hazard analysis of Southern California.
That widely publicized 1995 assessment--released around the first anniversary of the 1994 Northridge disaster and within a few weeks of the devastating Kobe earthquake in Japan--concluded that more tectonic energy had built up in the region's faults than had been released through past earthquakes.
At a time when the region seemed to be experiencing an increase in seismic activity after a 25-year lull--with 11 earthquakes of magnitude 6.0 or more occurring from 1983 through 1994--it appeared reasonable to conclude that even more earthquakes were in store to discharge that excess energy, experts said.
"The randomness of nature can result in some very systematic-looking behavior, and we have to be very careful not to over-interpret what we see," said David D. Jackson at UCLA, one of the main authors of the 1995 report. Jackson even proposed the possibility of a truly huge earthquake between magnitude 8 and 9--greater than anything ever recorded in the region--as a possible mechanism to relieve that pent-up energy.
With Edward H. Field and James Dolan of USC, Jackson now has helped prepare what they hope is a more accurate model of regional seismic activity that downgrades the risk by eliminating the earthquake deficit. That new study was presented Wednesday at a meeting of the Seismological Society of America in Boulder.
In retrospect, earthquake researchers agree that much of the 1995 report's authority stemmed from its wide endorsement by so many respected earthquake researchers.
Many of those who participated in the 1995 assessment now readily acknowledge that some of its more provocative conclusions were stated with more authority than the available data may have warranted.
"Some of my colleagues are more inclined to cry wolf than others," said Thomas C. Hanks, a geophysicist with the U.S. Geological Survey, who is among those whose new work has scaled back the estimate of seismic risk at some cost to their profession's public credibility.
"Perhaps there should be a higher level of collective prudence, but there is a tightrope we walk," Hanks said. "You want to get the information out so people are informed of the hazard. In the process, there is a danger the case may be overstated."
In any case, the raw improbability of any model of seismic activity often is underrated by the public, said Hiroo Kanamori, director of the Caltech Seismological Laboratory. "You have to have really accurate data, and all earthquake data has great uncertainty," Kanamori said.
Indeed, the margin of uncertainty is so broad that a scientist could even take the position that both the flawed 1995 assessment and the new models are equally reasonable, Kanamori said. "It is very hard to pin it down even to that level."
Experts said it is remarkably difficult to decipher the region's seismic history, let alone predict its future when so much of the evidence is buried under local mountain ranges or urban asphalt. The 1995 assessment was based, for example, on a review of earthquakes in Southern California since 1850, but the area was so sparsely populated for much of the century that many moderate earthquakes could easily have escaped notice.
The best researchers can do is to offer an educated guess and then brace themselves for inevitable--and necessary--public corrections.
In the end, experts said Wednesday, the newest research reflects a return to a more traditional view of Southern California's seismic hazards.
That means that the region can expect about one earthquake of magnitude 6.0 or more every two years and an earthquake of magnitude 7.0 or more about every decade.
The San Andreas fault can still be expected to one day unleash the infamous "Big One" like the magnitude 7.9 temblor that occurred in 1857. Just when or where any of this might occur, nobody knows.
As for any effort to predict earthquake activity, no matter how carefully calculated, well-researched or well-intentioned, U.S. Geological Survey geophysicist Lucile M. Jones said, "We have no clue what the next 30 years will bring. I don't think a lot of people who use our information realize that."