Lancaster earthquakes - how dense are they?

Or, after another entry on the Lancaster earthquake, you might be asking how dense is Rob?

from http://neic.usgs.gov/neis/bulletin/neic_bcah_d.html

The gray color suggests that there is about 0.1 magnitude 5 earthquakes in that region per year, or only 1 every 10 years (which actually seems a bit high to me). But in the Lancaster area, colored white, the frequency is even less. This actually isn't a very interesting diagram here because of the paucity of large earthquakes. We'll have to look at this type of diagram later for a larger earthquake, presumably in some other part of the world near a plate boundary.