MEXICO CITY — A powerful earthquake shook central and southern Mexico on Friday. The U.S. Geological Survey calculated its magnitude at 7.5 and said it was centered near the Pacific resort of Acapulco, where many Mexicans are vacationing for the Easter holiday.The quake shook Mexico City for at least 30 seconds, with buildings swaying as people fled high rises and took to the streets. Because of the Easter holiday, that city was less crowded than usual.Mexico City is vulnerable even to distant earthquakes because much of it sits atop the muddy sediments of drained lake beds that quiver as quake waves hit.The magnitude-8.1 quake in 1985 that killed at least 6,000 people and destroyed many buildings in Mexico City was centered 250 miles (400 kilometers) away on the Pacific Coast.