Pete & Russ,

Yes, M 6.0 is not "the big one", however with earthquakes there are several other factors that can make even a moderately strong quake very dangerous. The depth at which the quake occurs is important, as well as the local geology.

This M 6.0 quake under the Napa area was at a depth of about 11.3km (7.0mi). By way of comparison, the 2011 M 6.3 under Christchurch New Zealand was at a depth of only about 5km (3 mi). Apparently Christchurch is also underlain by relatively soft sediments. There were 185 deaths in Christchurch, at least 220 major trauma cases, and about 6000 people were treated for various less severe injuries.

In many situations, even a "relatively small by our standards" quake can be quite deadly.