MEXICO CITY — Nearly two-thirds of the buildings that collapsed in Mexico City’s monstrous earthquake last month were built using a construction method that is now forbidden in seismic hotspots in the United States, Chile and New Zealand, according to new data compiled by a team of structural engineers at Stanford University. The suspect building technique called flat slab — in which floors are supported only by concrete columns — caused 61 percent of the building collapses last month in a magnitude 7.1 quake, which killed 369 people and blanketed tree-lined avenues in rubble.