A powerful magnitude-8.3 earthquake shook Chile's capital Wednesday night, causing buildings to sway and people to take refuge in the streets. Several strong aftershocks hit within minutes as tsunami alarms sounded in the port of Valparaiso and authorities issued a tsunami alert for the country's entire coast.

The U.S. Geological Survey initially reported the quake at a preliminary magnitude of 7.9 but quickly revised the reading to 8.3.

U.S. officials said the quake struck just offshore in the Pacific at 7:54 p.m. (6:54 p.m. EDT, 1154 GMT) and was centered about 141 miles (228 kilometers) north-northwest of Santiago. It said the quake was 4.8 miles (5 kilometers) below the surface.

Chile's emergency office warned that big waves caused by the quake could hit the coast by 11 p.m.