MEXICO CITY -- (AP) -- Flags were lowered to half-staff, memorial Masses were
 celebrated in empty lots where buildings once stood, and ambulances sounded
 their sirens Tuesday to commemorate the 15th anniversary of the massive 1985
 earthquake in Mexico City that killed more than 9,500 people.

 Federal and local officials gathered in the main plaza of North America's largest
 city and held a moment of silence to remember the victims of the 8.1-magnitude
 quake, which
 Mexican TV stations broadcast images of Mexico City buildings still considered
 dangerous 15 years later.
 also destroyed 400 buildings and weakened 5,700 others.

 The government came in for harsh criticism because of its slowness in responding
 to the disaster, which struck in the early morning hours of Sept. 19, 1985. In
 addition, many of the deaths occurred in poorly constructed government offices,
 apartment buildings or hospitals.

 Interior Secretary Diodoro Carrasco said the tragedy motivated the government to
 be better prepared for the next big quake, which scientists have warned is
 inevitable.

 ``The population has to be alert and this is the goal of these new information
 systems for natural disasters,'' Carrasco said at the ceremony, also attended by
 firefighters and rescue teams.

 In the wake of the 1985 quake, Mexican authorities implemented a seismic alert
 system consisting of a series of sensors placed along a seismic gap in the
 Pacific Coast state of Guerrero. The system, part of the National Disaster
 Prevention Center, could give Mexico City residents as much as 50 seconds
 warning prior to a major earthquake, allowing them to seek safer areas -- such as
 open spaces outside.

 The alert system is set off only by quakes of magnitude 6.5 or higher, and works
 only if an earthquake originates between the Pacific Coast resort towns of
 Acapulco and Zihuatanejo. That was not the epicenter of the 1985 quake.

 The area was chosen because of logistical reasons and the estimated probability
 of future quakes.

 That stretch of the Pacific Coast registered three 7.5-magnitude earthquakes
 between 1899 and 1907, but no major earthquakes have occurred since 1911,
 leading scientists to believe another major earthquake might be due soon.

 On Tuesday, Mexican television stations broadcast images of Mexico City
 buildings that are still considered dangerous and uninhabitable 15 years later, but
 which have been neither repaired nor demolished.