Soft Soil Helped Spell Disaster in Mexico City

The extensive earthquake damage in Mexico City last month resulted from unusually strong energy waves that amplified the soft soil underneath the capital. In turn, the soil, acting as a conduit, made many buildings sway in time to the waves to the point where they gave way, scientists here reported Thursday.

In releasing the first evidence gathered by instruments at the earthquake’s epicenter and in Mexico City, scientists called the data the best ever gathered from any earthquake.

Scripps Institution of Oceanography researchers said they do not know yet why so much of the earthquake’s energy appears to have been at an unusually long wave frequency of about two seconds, which so severely jolted the capital’s lake bed sediments.

Until the data is further analyzed, they cautioned against faulting Mexican architects and engineers for not anticipating such ground acceleration in the jelly-like soil underneath much of the Mexico City neighborhoods where damage was most extensive.

“This particular quake was rich in two-second energy,” James Brune, professor of geophysics, said at a press conference. “It traveled (from the Pacific Coast epicenter) through the earth into the valley of Mexico City.”

Brune explained that all structures have a natural frequency of oscillation, like a tuning fork, determined by design and construction.

In the quake last month, those buildings that collapsed had natural periods near two seconds--matching the seismic wave data recorded by Brune and his colleagues from the National Autonomous University of Mexico. Brune spent several days in Mexico City last week with building engineers from UC Berkeley.

In almost all cases, Brune said, damaged buildings were constructed of concrete that either were not reinforced with steel rods through their columns or were inadequately braced with steel bracing and sheer walls. Proper reinforcement stiffens a masonry building, reducing its oscillation rate to the point where the building will not sway in time to the most damaging energy waves.

“On any given block in Mexico City, you could see one building that collapsed and six that weren’t damaged,” Brune said. “And there were no steel-frame buildings that collapsed. They certainly swayed, but steel can be bent very significantly without cracking.”

Brune said that there was no data until this quake to suggest that Mexico City was at risk from such low-frequency waves. UC and Mexico university scientists had placed measuring devices near the site of the 8.1 magnitude epicenter beginning nine months ago in anticipation of a major temblor.

The Pacific Coast region of Mexico is subject to frequent seismic activity because it is the site where an oceanic plate of the Earth’s crust is thrusting under a continental plate, resulting in strain building up to the point where slippage takes place, eventually causing an earthquake. The area of last month’s quake had not slipped for almost a century, leading to what scientists call a seismic gap.

Epicenter Damage Light

Damage was not extensive at the epicenter along the coast, 200 miles west of Mexico City, Brune said, in part because the quake took place deep beneath the surface, from between 12 to 18 miles. The high-frequency waves that usually cause the most damage near an epicenter consequently lost most of their energy by the time they reached the surface, Brune said.

In contrast, most quakes in California occur at depths of six miles or less from the surface, Brune said. A similar magnitude quake along the San Andreas Fault would cause much more damage to inadequately built structures near the epicenter than occurred in Mexico, he said.

“There might be damage as well in areas farther away where the soil is soft, depending on the energy released from low-frequency waves,” he said.

Growing Potential

In general, Brune said, the longer the period between quakes along an active fault zone, the larger the potential for damage from low-frequency waves. Scientists are eager to determine the precise source of the Mexico quake to learn whether quakes along seismic gaps identified on the San Andreas might release such waves.

Brune said engineers believe the Mexico quake will be the most important ever in evaluating construction methods.

“There is a simple lesson here,” Brune said. “Because records are so (scarce), every quake provides a surprise in some way.”