A powerful 6.4 magnitude earthquake hit San Juan Province in Argentina late Monday night, according to early reports from the U.S. Geological Survey.   The quake, which was felt as far away as Santiago, Chile, sent items flying off store shelves, damaged buildings and caused cracks in a highway in San Juan, based on videos posted on twitter.  There were also reports of power outages, but no immediate confirmation of casualties.   San Juan Governor Sergio Unac on Tuesday urged people to stay calm while they assess the impact from the quake, which was followed by a series of less powerful aftershocks.     The U.S. Tsunami Warning System said the earthquake in west central Argentina did not pose a tsunami threat and no warning was posted.     Initial reports indicate the quake, struck at a shallow depth of 10 kilometers beneath the epicenter near Pocito, Argentina. 
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