Doubting Thomas

Will Los Angeles buildings bend, but not break, when the next big earthquake strikes directly under the city? Although most scientists might demur on such a weighty, politically loaded question, Caltech seismology professor Thomas H. Heaton is not among them.

In his usual droll way, Heaton, 45, continues to advance the argument he has been making with great persistence ever since the Northridge earthquake three years ago.

Long-period waves from a major earthquake will one day propagate up the high-rise buildings of Los Angeles or another large California city, “and certain of those buildings will bend,” Heaton told a recent symposium of the American Geophysical Union.

“Once they bend, I do not see how they can straighten out again,” he said.

According to Heaton’s somber vision, earthquakes far more powerful than the Northridge disaster lie in the state’s future, and building codes are inadequate to deal with them.

Some day, he said, there will be high-rise devastation and many casualties when a magnitude 7 to 8 quake strikes directly under a downtown area.

Two other Caltech professors, John Hall and Wilfred Iwan, have also expressed high-rise concerns. But Heaton’s fears and especially his prescriptions for dealing with them are the most sweeping.

Heaton says plainly that California should not have any buildings taller than 10 stories. He is dubious about seismic base isolation systems, which put buildings on springs or rollers and allow them to move to muffle the shaking. And he believes that many mid-rise concrete frame buildings are in jeopardy as well.

“You could certainly improve the situation of these buildings,” he said in a recent interview, “but the problem inherently is the physics.

“Modern buildings are built to be flexible on the thesis that displacements during earthquakes will be small. But in a large quake, the displacements will be large. This is incompatible with these buildings remaining vertical.

“You hear many say, ‘This building is built to withstand a magnitude 8.’ This is nonsense.”

Despite Heaton’s growing stature as an earthquake damage expert, many of the state’s leading engineers say they disagree with him.

Scott Stedman, president of the Structural Engineers Assn. of California, said, “I think newly constructed high-rise structures will respond adequately to major earthquakes. That doesn’t mean there won’t be damage.”

John “Traylor” Martin, a Los Angeles engineer with the firm of John A. Martin & Associates, said Heaton is “a very, very intelligent gentleman, but I do disagree with him. I don’t believe it’s really necessary to limit ourselves to 10 stories in high seismic areas.

“There is nothing to convince me we are building unsafe buildings. When we look at them, we find ourselves more concerned about their movement in strong winds than we do about any seismic weaknesses.”

But Ronald Hamburger of the firm EQE International cautioned: “I think the concerns that have been pointed out by a number of professors at Caltech are real.

“I think there is a potential danger with buildings with certain dynamic characteristics, if they happen to be located within a few kilometers of an earthquake.”

However, Hamburger added, he is more concerned about mid-rise than high-rise buildings.

Heaton, who majored in physics at Indiana University and earned a PhD in geophysics and applied mechanics at Caltech in 1979, has specialized in the physics of the earthquake rupture process and ground motions occurring close to the epicenters.

His PhD thesis focused on near-source ground motions (shaking very close to quake epicenters), and his studies have convinced him that such motions can reach levels that are extremely hazardous to high-rise buildings.

“There is a misunderstanding about the size and nature of earthquakes between the seismological and engineering communities,” he said.

“It’s more common for engineers to discuss these matters in terms of ground acceleration. But as quake magnitudes get above a certain level, acceleration of the ground does not increase that much.”

But, he warned, ground displacements--the size of ruptures moving one side of a fault past the other, or up, over the other--”increase dramatically” in the biggest temblors.

This is why Heaton doubts the survival of many seismic base isolation-protected buildings during the largest earthquakes.

He fears that the amount of displacement allowed these buildings may be insufficient, and the buildings will collide with the ground or other structures when their displacement allowance is exceeded, causing catastrophic damage.

In 1990, Heaton published a paper finding that quake slip--or displacement of the ground--moves as a pulse along the fault during a rupture and occurs very quickly in any one location.

“These displacements happen fast enough so they can compromise large buildings,” he said, explaining that individual quake waves may arrive at such short intervals that they interfere with each other. As one wave propagates up a building, it may collide with the reverberations of others, increasing pressure on the building and possibly doing great damage.

“Each quake does have different characteristics,” Heaton said. “But a very common characteristic within five miles of the fault rupture is that there is a large over-and-back pulse in displacement.”

In the magnitude 7.3 Landers quake of 1992, the displacement exceeded 20 feet.

The displacement in any one place in a large quake can occur at a destructive speed of three or four feet per second, Heaton noted.

Although to many Angelenos the Northridge quake seemed very intense, Heaton warns that most of its energy was directed into the sparsely populated Santa Susana Mountains, and that by no means did it provide a valid test of high-rise buildings in downtown and the Wilshire corridor.

His Caltech colleague John Hall commented:

“Heaton’s main gist is talking about large earthquakes, which we haven’t experienced before in our urban areas, not the small to moderate ones we’ve been having.

“We’ve got the faults that are capable of delivering the larger earthquake. The question is not whether these are possible, but what the interval between them will be.

“There are six or seven of these faults in the Greater Los Angeles area, and even if the interval on each of them is a few thousand years, we still could have a quake on one of them every few hundred years.”