Of all the brutal surprises of the Northridge earthquake, its intense vertical motion has been one of the most discussed. The quake's strong upward heaving has become a popular explanation for the failure of parking garages and other structures that were fortified only against lateral force.
To underscore the unexpected power of the quake's upward motion, some owners and designers of trashed buildings have noted that the Uniform Building Code, adopted as law by California cities and counties, does not significantly address vertical thrust as a cause of structural failure.
But more than two decades ago, an official with the National Oceanic and Atmospheric Administration wrote that the Sylmar earthquake of 1971 "produced evidence of vertical accelerations far in excess of any ever recorded instrumentally."
And a committee of the Oakland-based Earthquake Engineering Research Institute, discussing the effects of the Sylmar quake, cited "strong evidences of severe vertical accelerations which could contribute to increased damage."
Similar descriptions of the Northridge quake may be a case of "short-term engineering memory," as one government seismologist put it.
In the wake of the Northridge temblor, engineers involved in the code-revision process say the significance of vertical motion--on the back burner since the Sylmar quake--deserves another look.
But as if to illustrate that little about an earthquake is simple, some experts still doubt that vertical thrust played a primary role in structural failures. These authorities fear the vertical explanation is being oversold and may divert attention from what they consider to be a more pervasive problem--strong cost-cutting pressures that can lead to inadequate designs.
"It's always nice to blame Mother Nature," but blame is "as much on the human side as the nature side," said Helmut Krawinkler, co-director of the John A. Blume Earthquake Engineering Center at Stanford University.
"I'm not in the group that's ready to jump on the vertical bandwagon," said Wilfred D. Iwan, director of Caltech's Earthquake Engineering Research Laboratory and chairman of the state Seismic Safety Commission. "But everywhere I go I hear people using this to explain the poor performance of certain structures."
As currently written, the building code reflects the prevailing view that the risk of structural collapse in a big quake stems from sideways shaking.
That consensus survived the Sylmar quake, at least partly because the significance--and even the accuracy--of its high vertical-thrust recordings were a matter of dispute.
Centered beneath the San Gabriel Mountains, the Sylmar quake had a magnitude of 6.4. At the time, only a few strong-motion accelerographs were deployed in the immediate quake area. These instruments measure the lateral and vertical ground motion at specific points as a fraction of the force of gravity--expressed as 1.0g.
One instrument at Pacoima Dam above Sylmar recorded extraordinarily sharp vertical peaks of .5g to .75g. But many experts were not impressed, and when the code was strengthened in 1974, the revisions did not reflect this finding.
"People tended to discard that record (at Pacoima Dam) as unrepresentative," said Filip Filippou of the Earthquake Engineering Research Center at UC Berkeley. It "always had some kind of aura of unbelievability to it because it was just one. . . . Maybe it explains a little bit why we haven't done anything, if you want to call it that."
Some questioned the accuracy of the reading, believing that the instrument's perch atop a steep ridge above the dam made it vulnerable to topographic effects. Debate continued for years on whether the readings were valid or "aberrations due to instrumental error . . . because the strong-motion recording instruments don't always work," said Bob Chittendon, chairman of the seismology committee of the Structural Engineers Assn. of California.
Chittendon's organization is largely responsible for the code updates published every three years, drafting changes for the International Conference of Building Officials, which administers the code. Chittendon and others described code revision as a ponderous, consensus-building process in which changes often take years to effect.
"There is a tremendous inertia in the profession," said Vitelmo Bertero, a research engineer with the Earthquake Engineering Research Center in Berkeley. Engineers, like other people, tend "to forget very soon the bad things," so that "if something is not done after two or three years after an earthquake, they forget about it," Bertero said. "This is the real problem."
Bertero and others said vertical motion deserves a new look based on the record of the Northridge quake. Still, several experts noted that the temblor's vertical thrust was not as remarkable as the intensity of its ground motion generally.