Bob....it is likely that the faults are steep to vertical strike-slip faults that extend from the surface down to the mid crust...so earthquakes could occur anywhere on them vertically....and the small area may mean just a small patch is active (or 2 small patches). I would question whether the depths are accurate......with automatic locations...and way out in the desert...
The above is liable to be accurate, but I don't know anything about the particular faults ...
Chris