A powerful earthquake rocked eastern Turkey and the Soviet Union south of the western Caucasus Mountains Wednesday, killing at least five people and collapsing buildings, Turkish and Soviet authorities said. The quake was centered in the Soviet Union near Leninakan, between the Georgian capital of Tblisi and the Armenian capital of Yerevan, the Tass news agency said. Each capital has a population of about 1 million. There were no immediate reports on Soviet casualties.