WASHINGTON - A magnitude 6.1 aftershock shook central Chile on Wednesday, four days after a massive 8.8 earthquake killed more than 800 people, the US Geological Survey reported.

It said the quake, shallow at a depth of 21.4 miles (34 km), was centered in the ocean, just 24 miles (39 km) west of Valparaiso, and struck at 10:59 pm (0159 GMT on Thursday).

Chile has been wracked by some 150 strong aftershocks since Saturday.