The magnitude 4.4 earthquake that struck near Westwood is the most significant shake in Southern California since a 5.5 earthquake hit Chino Hills in 2008, a U.S. Geological Survey seismologist told reporters at a news conference Monday morning. Robert Graves said there have been at least six aftershocks since the 6:25 a.m. earthquake. The largest so far has been a magnitude 2.7 earthquake that struck five miles northwest of Westwood.