SANTIAGO - A strong, 6.9 magnitude earthquake struck off the coast of Chile on Tuesday night, but there were no immediate reports of damage, the Chilean emergency office said.The US Geological Survey said the quake, initially reported as magnitude 6.8 and then 6.6, was at a very shallow depth of 10 km. under the seabed. Its epicenter was 93 km. northwest of Coquimbo and 466 km. northwest of Santiago.The earthquake did not trigger a tsunami, the Pacific Tsunami Warning Center said.Chile's ONEMI emergency office said there were no preliminary reports of damage, while Chile's navy said the quake was not strong enough to generate a tsunami.