Not unusual - this is a very active region of the San Andreas, there are swarms every few months.
hate to quote yahoo. finally on drudge.
“Our system is choking on so many earthquakes.” (USGS seismologist)
http://ca.news.yahoo.com/swarm-quakes-rattle-california-arizona-usgs-221806620.html
The so-called “seismic swarm” is a relatively rare occurrence.
“We haven’t seen one of these since the 1970s, and there was another one back in the 1930s,” said USGS seismologist Lucy Jones.
“Our system is choking on so many earthquakes.”
From the story on Drudge.