A powerful, 7.8-magnitude earthquake has killed 77 people and injured more than 500 in Ecuador.A state of emergency has been declared in six provinces, after the tremor struck near the country`s coast.The quake, centred near the city of Muisne, shook buildings in the capital Quito more than 160km away (100 miles), BBC reports.There are reports of an overpass being destroyed in the port city of Guayaquil and damage to buildings in many areas.The Pacific Tsunami Warning Centre says hazardous waves are possible on coastlines within 300km.