SANTIAGO - A magnitude 6.8 earthquake struck off the coast of Chile on Monday, the US Geological Survey said, but Chilean emergency authorities said they saw no prospect that the tremor would generate a tsunami.The quake struck 400 miles (640 km) west of Puerto Aisen at a depth of 6.2 miles, the USGS said.It said the quake had occurred at 5:49 a.m. Chile time (0943 GMT)Chile's emergency office said on Twitter that the quake did not have the conditions necessary to generate a tsunami. It added the quake was not felt in the country.The USGS pinpointed the quake in the Nazca tectonic plate area.