MEXICO CITY, A powerful 8.2-magnitude earthquake rocked Mexico late Thursday, killing at least 15 people and triggering a tsunami alert in what the president called the quake-prone country’s biggest one in a century. The quake hit offshore in the Pacific at 11:49 pm , from the coastal town of Tonala, in far southern Chiapas state, Mexico’s seismologic service said. “It was a major earthquake in scale and magnitude, the strongest in the past 100 years,” said President Enrique Pena Nieto in an address from the National Disaster Prevention Center’s headquarters, where he was supervising the emergency response. The US Geological Survey put the magnitude slightly lower, at 8.1. That is the same as a devastating 1985 earthquake that killed more than 10,000 people in Mexico City — the country’s most destructive ever.