IN the wake of the powerful earthquake that jolted Mexico's Pacific Coast Oct. 9, Mexicans are again edgy about the stability of their footing.
They have reason for concern: This quake, registering a magnitude of 7.5 to 7.6, is the second in less than a month to hit the region. It was centered in the western states of Colima and Jalisco. At least 66 were killed.
Built on a former lake bed, Mexico City is peculiarly vulnerable to shaking from as far away as 300 miles, such as from the Oct. 9 quake. On Sept. 14, a 7.2 temblor in the southern state of Guerrero also rocked the capital.
The two quakes have put to the test safety measures the country has taken since the colossal 1985 one that rocked Mexico City. Stricter codes now exist for new construction as well as reinforcement guidelines for older buildings. A warning system has been installed in Mexico City to help people evacuate quickly. During the Guerrero temblor the sirens worked, but this time they did not. The system does not yet include quakes emanating from Colima and Jalisco.
Centered just off the coast of the port of Manzanillo between the popular resorts of Puerto Vallarta and Acapulco, the Oct. 9 earthquake sent strong tremors hundreds of miles inland. It was felt as far north as Texas.
In Mexico City, mindful of the 1985 quake that killed 10,000, panicked capital residents fled to the streets when the city began to rumble shortly after 9:36 a.m.
They watched homes and skyscrapers quiver as shock waves undulated through the soft ground beneath their feet. But no deaths were reported in the city of 22 million.
''We have made a lot of progress since 1985,'' said Carlos Valdes Gonzalez, chief of the National Seismological Service in Mexico City. ''It's important to recognize that we live in a country of heavy seismicity, and there's no reason to believe it's going to stop. People have begun to take notice of earthquakes in their daily lives. Not everybody - it's difficult to force people to understand earthquakes. But many have.''
Experts warn, however, that the new disaster-prevention consciousness in Mexico City may not extend to the countryside.
While Mexican authorities have upheld new building rules in the capital, ''they have not enforced that as much in rural areas,'' said John Anderson, a seismologist at the University of Nevada, Reno. ''In many of those areas there may be no building codes whatsoever.''
Whether by faulty building standards or brute force of nature, hundreds of homes, businesses, and public buildings collapsed or suffered severe damage in Colima and Jalisco. Hotels were hit especially hard.
The Hotel Costa Real collapsed in Manzanillo, killing at least 13 and trapping more than 30 others in rubble ringed by palm trees. In addition, a state attorney general's office collapsed in Manzanillo, killing eight. At least seven others died in other towns where a school and a church were among the destroyed buildings.
Many complained after the 1985 quake that government authorities were slow to respond. That did not appear to be the case this time, as Mexican military units were rapidly deployed in the rescue effort. In a statement to the nation Oct. 9 before he left for a visit to the United States, President Ernesto Zedillo Ponce de Leon promised swift federal aid and said the damage did not appear to be ''of an unmanageable magnitude.''
Meanwhile, a Jalisco government spokesman, Andres Klasser, said: ''You know we're always expecting an event of this magnitude, but we never know when it will hit. To some degree we were unprepared. But we have known how to respond.''