Twenty years ago, American seismologist John G. Anderson bet that the west coast of Mexico was due for a catastrophic earthquake.
He and colleagues used money from a National Science Foundation grant to buy 30 seismic monitors and install them in the states of Guerrero and Michoacan, aiming to be the first to digitally record a great quake.
Most of the monitors were in place on the morning of Sept. 19, 1985, when strong-motion sensors began recording a magnitude 8.1 earthquake.
Roberto Quaas, the engineer who installed the seismic monitors, recalled his horror as the earth shook his Mexico City home that morning, then his fascination with the seismic data that streamed into the laboratory. The readings provided, for the first time, an accurate recording of the intensity and type of movements the Earth experiences in a great quake.
"It was a moment of extraordinary professional satisfaction," he said. "Then we started hearing about all the deaths and the destruction, and it turned to an overwhelming sadness."
Quaas, who now heads the country's National Disaster Prevention Center, was among the legions of Mexico City residents who formed ad-hoc rescue and relief teams as government help stalled.
He estimates that at least 6,000 people died and tens of thousands were injured, although other tallies put the death toll at 10,000 or more. The quake inflicted about $4 billion worth of damage, and nearly 250,000 people were left homeless.
These days, there is little to see of the scars left by the quake. The rubble has long been cleared, and new office and apartment buildings have sprung from the ruins.
But scientists such as Anderson, Quaas and MIT urban sociologist Diane E. Davis say the effects of the quake continue to reverberate two decades later.
"It was a tragedy, but it also created an opportunity," Quaas said. "It triggered a national system of civil protection, similar to FEMA."
The center Quaas now runs was built five years after the quake; it keeps watch over a metropolitan population of more than 20 million. From a building in the south of Mexico City, scientists and technicians monitor hurricanes, earthquakes and Popocatepetl, the 17,886-foot volcano that looms 40 miles to the southeast.
Mexico City is in a high-risk area, Quaas noted. Not only was it built on a filled-in lake bed that amplifies temblors, but it is endangered by its proximity to Popocatepetl. In December 2000, a spectacular eruption forced the evacuation of thousands of nearby villagers.
But you can't move the city, he said. You can only try to minimize its vulnerability.
"New Orleans is built between the ocean and the Mississippi. Hurricanes happen every year, yet it was built there," he said. "Mexico is no exception. People need to see their own risk and ask, 'What can I do to prepare?' When one learns the risk, one can take responsibility."
Since the 1985 quake, communications and warning systems have been improved. The local, state and federal governments are better coordinated. Building codes are stronger. Schools hold earthquake evacuation drills, which are distinguished from fire drills by students exiting buildings holding books on their heads to protect against falling objects.
Even so, Quaas said, "are we ready for the next great disaster? No, we are never ready."
Besides revealing Mexico's vulnerability, the quake disclosed the ineffectiveness of its government, said Davis, who has studied the event's social and political aftermath. Authorities seemed paralyzed in the hours and days after the quake, forcing residents to take up the slack.
That resulting loss of face by the ruling Institutional Revolutionary Party, or PRI, led to a series of defeats in Mexico City mayoral campaigns, said Davis, and may have even contributed to its loss of the presidency in 2000.
It is a lesson, commentators here say, that American politicians should be wary of in the aftermath of Hurricane Katrina.
Mexico's government was able to eventually find its feet, Davis said, and used the quake's destruction to establish clear property title regulations. A legitimate title system, taken for granted in the United States, laid the groundwork for the city's redevelopment, she said, creating incentives for investment.
"People had lost sight of who owned properties," Davis said. "With the formal distribution of titles, a whole new urban land market was put into play."
During the redevelopment that followed, Mexican authorities used the seismic data recorded by Anderson's monitors to improve building codes here. The data also helped to advance seismic design in the Pacific Northwest and Japan.
Anderson -- who was at UC San Diego in 1985 -- was expecting a quake along a stretch of the Pacific coast dubbed the Guerrero Gap. Instead, it hit farther north.
"But fortunately, we had four monitors directly above the fault that broke," said Anderson, now director of the Nevada Seismological Laboratory at the University of Nevada, Reno. "Nobody likes an earthquake disaster, but we were happy about getting the data and learning from it. We ended up with the first digital record of an 8 magnitude quake."
Anderson said he was astonished that the quake's intensity was about the same at the epicenter as in Mexico City, more than 200 miles away. Researchers are still seeking the full explanation, he said.
For now, Mexico City -- like Los Angeles and San Francisco -- remains vulnerable to the next big shaker.
"I hate to say this, but we are still waiting for the Guerrero Gap to fail," Anderson said. "I'm concerned that buildings that survived 1985 won't survive the next one. If there is a weakness anyplace, the earthquake will find it."
Times researcher Cecilia Sanchez contributed to this report.