Uncertain Lesson in Mexico City Quake

As a race, we learn slowly, but the rate at which we learn seems to speed up as we go along. We probably have learned more about earthquakes in the last 50 years or so than we did in the several preceding millennia, and we still have a lot to learn.

According to the currently accepted theory, called plate tectonics, the earth’s shell (the stony mantle and the crust above it) is cracked like the shell of an egg. The pieces, called plates, float around on the central core of melted iron, made as rigid as a solid by pressure although still a liquid which will flow--with almost incredible slowness.

These plates pull apart from each other, bump into each other and rub against each other, causing both earthquakes and volcanoes.

It’s undoubtedly closer to the facts than the ancient Greeks’ notion that the sea god Poseidon causes earthquakes by striking the earth with his trident. Or that volcanoes take place when Hephaestus, god of metalworking, fires up the forge in his workshop deep in the bowels of a mountain. He was Vulcan to the Romans, hence “volcano.”

So we have, or at least believe we have, a pretty good idea of what causes earthquakes and a fair idea, getting better all the time, of the details of how they work. The quake in Mexico in mid-September was a good candidate to teach us a good bit more.

Among those who feel that way seems to be Assemblyman Richard Alatorre (D-Los Angeles), chairman of the Governmental Organization Committee, who held a hearing, “California Building Safety in Light of the Mexican Earthquake,” on Monday in the State Building here. It was in connection with Senate Bill 547, introduced by state Sen. Alfred E. Alquist (D-San Jose) and dealing with seismic safety.

Four Damage Categories

Most of the testimony was basic information that has been generally publicized, but a number of things were said that have not been widely disseminated.

Chris Arnold, San Mateo architect who went to Mexico City for the California Council, American Institute of Architects, as part of a four-member study team, testified that damage to buildings could be broken down into four categories, which he illustrated with slides taken in Mexico City.

The most prevalent damage was to building tops, which “whipped around like a flagpole.” In many buildings, sound at street level, the top floor, or two or more of the topmost floors, completely crumpled.

An unusual effect appeared in those buildings that suffered damage only, or primarily, in the middle. He had pictures of eight- and 10-story structures with the middle floor or floors wrecked while those above and below were relatively unharmed; the mechanism of that particular type of damage is not understood.

Others suffered damage at the bottom, perhaps the first or the first two or three floors. And there were the “layer cake” collapses, when the floors came down one on top of another and the wreckage looked like a giant--and lethal--stack of pancakes.

Buildings Survived

Arnold also talked about steel-framed structures without sheer walls, usually 30 or 40 years old, whose damage was due to ground motion. Usually they didn’t contain enough steel and were brittle.

One of the effects commented on by almost everyone who has surveyed Mexico City’s damage is the number of unreinforced masonry buildings which survived with no significant damage. They included churches with towering spires and many-storied office buildings. They also included smaller structures of two or three stories.

The latter were unaffected by the ground motion, which was long and slow, Arnold said, “unlike Coalinga, where the quake action was short and sharp and most (of the two- and three-story unreinforced masonry buildings) fell down.”

And there were the tall--30 and 40 stories--buildings that received “far less damage than many of us expected.” Many were glass-walled or had extensive windows; some glass facades were undamaged, others had only a few windows broken. “They show that (such buildings) can ride out a quake.”

Structural engineer John Kariotis, representing the Structural Engineers Assn. of California, pointed out that “the type of soil in Mexico City does not exist in California. Mexico City is unique in being on a 3,000-meter-deep lake bed.

Different Conditions

“Those buildings which survived in Mexico City would not necessarily survive a California earthquake without damage.”

Geologist George Brogan of Applied Geosciences, described how “there were pockets of damage where there were weak soil conditions and most of the loss of life was in those pockets.”

But it was simpler back in the days when Poseidon was whacking the earth with his trident. . . .