A strong earthquake hit Mexico early Friday, knocking out power in parts of Mexico City and sending frightened residents into the streets of Acapulco. There were no immediate reports of damage or injury. The quake had a preliminary magnitude of 6 and was centered 40 miles northwest of Acapulco, the U.S. Geological Survey said. It hit shortly before 1 a.m. local time, setting off alarms and sending people out into the streets in their pajamas. Tourists in the Pacific Coast resort city of Acapulco rushed out of their hotels and huddled together, fearing aftershocks. The earthquake was underground, the USGS reported, and there was no danger of a tsunami.