MEXICO CITY (Reuters) - The school collapse this week that killed at least 19 children during an earthquake has prompted many Mexicans to question whether building codes developed after a devastating 1985 temblor are too easily flouted. Stringent codes enacted after a massive temblor killed thousands three decades ago minimized damage this week across the metropolis of 20 million people, even if experts say it is nearly impossible to design structures to withstand any quake.  At least 293 people died in Tuesday s quake, measuring 7.1 on the Richter scale, but the toll could have been far worse. Still, the collapse of some structures built under the new codes, including the Enrique Rebsamen school that became a symbol of this week s tragedy, has sparked concerns over implementation of the rules.  There are still gaps that mean new buildings don t always fully comply,  said Roberto Meli Piralla, a structural engineer and expert on seismic preparedness at the National Autonomous University of Mexico. It is too early to know if code violations, neglect or corruption were factors in the collapse of the school or about 50 other buildings in Mexico City. Yet engineers said that challenges remain in enforcing the codes uniformly and ensuring that builders, inspectors and property owners follow approved plans. Inspectors can be criminally liable if a building they OK proves structurally unsound. The mayor of the district where the school is located said her office has been unable to find documentation to assess its construction and structural history. She blamed problems, including corruption, on a predecessor administration.  When we came into office, there were many files missing,  said Claudia Sheinbaum, the mayor of the Tlalpan district. She said she had no proof that corruption had anything to do with the collapse.  Before the quake, municipal officials were already  discussing revisions to strengthen code oversight, said Piralla, who is part of an engineering group that made recommendations to the city in recent weeks.  One suggestion, according to another engineer involved in the discussions, was to have an independent, third-party review of designs before inspectors sign off on them.         The Secretariat for Housing and Urban Development, the municipal office that oversees the code, could not be reached for comment. Even if compliance were universal, engineers said, there is no way to fully prepare because earthquakes vary widely. Tuesday s temblor generated a different frequency of  shockwave from the 1985 quake and toppled mid-sized buildings of roughly six to eight stories, compared with towers of about 12 to 15 floors in 1985.  It s not possible to say what problems could have been foreseen,  said Antonio Gallardo, a member of the board of architects in Mexico City, a council that upholds professional standards across the capital. Most of the damage on Tuesday happened in central and southern districts where soils are less solid than in northern neighborhoods, and urban development, even after improvements in recent decades, is still patchwork. Although the toppled buildings are still being investigated, some were built after the 1987 code, which has periodically been revised since. At the school, which is in southern Mexico City, the older of two structures remained standing while a newer building collapsed. It was built early last decade and opened in 2004, according to several people familiar with its history. The school is a private facility founded in the 1980s by the family that runs it, according to two adult former students. The principal, a daughter of the founders, lived in an apartment that was part of the collapsed structure. She survived the quake, current and former students said. Neither the principal nor other family members could be reached for comment. But local authorities, neighbors and  parents are demanding answers.  Who could have approved this school?  asked Raul D az, a doctor whose 7-year-old son, Eduardo, died at the school.  The building was poorly built.  If prior quakes are any guide, investigations and a full accounting will take years. But experts said it would be impossible to build structures that could withstand all quakes.  There is always going to be a degree of uncertainty,  said Mary Comerio, a professor of architecture and researcher on disaster recovery at the University of California at Berkeley. She cited the 2011 earthquake that leveled even costly, modern structures in Christchurch, New Zealand. The extent of a phenomenon known as liquefaction, in which solid soil behaves like a liquid, surprised scientists. Notwithstanding architectural advances, many other variables come into play including human error, ground conditions, the location, depth and force of an earthquake.  In 2006, Comerio authored a paper in the journal Science called  Can Buildings Be Made Earthquake-Safe?   The answer is largely no,  she said. 