Mexico City Quake Readiness Includes Alarm System : Temblor: Warning sounded 50 seconds before ground started to shake. Lessons of deadly ’85 tremor have been well-learned.

Seconds after the tectonic plates jerked more than seven miles beneath the mountainous state of Guerrero at 8:04 a.m. Thursday, dozens of alarms went off hundreds of miles to the north in the Mexican capital.

Triggered by 12 solar-powered seismic detectors along Mexico’s Pacific coast, the alarms automatically ripped into the broadcasts of more than 40 radio stations here 50 seconds before the shock waves slammed into Mexico City, warning many of the 20 million residents about the powerful quake on its way.

The sophisticated alarm system did little to curb panic in a city that was devastated by an 8.1 quake nearly 10 years ago to the day. But it was an indication of the level of earthquake preparedness and public consciousness in a city whose entire face has changed since the 1985 earthquake hit.

In the wake of Thursday’s quake, Mexican scientists, intellectuals, labor leaders and analysts said they sensed a world of difference in the city’s response 10 years after Black Thursday--the Sept. 19, 1985, earthquake that killed at least 10,000 people and razed many parts of the city.

“In 1985, nobody understood the meaning of the magnitude of an earthquake,” said environmentalist Homero Aridjis, an outspoken critic of the government. “This time . . . people were prepared.”

There are still reminders here of the official failures that followed the ’85 quake. Damaged and abandoned structures remain on the city’s landscape, many further eroded by Thursday morning’s temblor. But most Mexico City intellectuals now view the devastation of 1985 as a powerful wake-up call that shook this capital into activism.

The ’85 quake gave rise to hundreds of independent civic-action groups and labor unions after it became clear that many of the dead were virtual prisoners in illegal structures, dilapidated housing and sweatshop factories throughout the city.

Though many are now splintered, those activist groups became powerful voices that helped create laws regulating construction, requiring earthquake drills and improving workplace safety during the past 10 years.

Neighborhood committees rose up amid the government’s post-quake neglect, and some are still influential lobbying groups. Over the years they have taught earthquake safety door to door in the city’s poorest neighborhoods. And their public clamor, analysts said, was largely responsible for Thursday’s fast response.

At the 15th of September Building in the capital’s Tlatelolco district--one of the worst-hit areas in 1985--a terrified but well-informed Hilda Gutierrez said she knew exactly what to do when she heard the radio announcer shout, “An earthquake is coming! It’s an earthquake!”

The 62-year-old woman shut off the gas heater and the stove, grabbed her 4-year-old grandson and ran to the doorway, “which they told us was the safest place in an earthquake.”

Asked who had told her that, Gutierrez said it was a doctor in the building--a physician who became a fierce activist after the ’85 quake.

Dr. Cuauhtemoc Abarca Chavez had been jogging when the ’85 quake hit and single-handedly pulled several people from the rubble.

With federal funding, he later formed a group that educated the entire sprawling neighborhood on earthquake safety.

Only last Saturday, Gutierrez said, she received her first safety visit from a city official, who delivered the same information.

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Nine-year-old Rafael Angel Maldonado had a sanguine view as he left the neighborhood’s nearby elementary school.

He said that his school has held four earthquake drills each year in every grade since he entered. He knew precisely where to go and what to do when the school was evacuated early Thursday, along with every other public school in the city.

“I’m more afraid of getting shot to death than I am of earthquakes,” he said.