What does it mean when an author is said to be anonymous?

Anonymous just means the author doesn't want their name/identity to be revealed. Or, it isn't clear who the original author was. In the case of journalism, anonymous could be credited to protect the identity of the source.

did you check the study guide?

mount st. helens in washington erupts, causing a massive avalanche and killing 57 people on this day in 1980. ash from the volcanic eruption fell as far away as minnesota. seismic activity at mount st. helens, which is 96 miles south of seattle, began on march 16.