Roger...you are a seismologist of some sort and I am not, but the weight of what is above is not what is important....it is the change of the weight above. I get deviatoric and differential stress mixed up....but it they have to do with the difference between the maximum and minimum compressive stresses....faults fail with this difference, not the total stress. I thought that stress drops in earthquakes were about 100 bars. If a fault fails every 1000 years...then a 0.2 bar decrease in the vertical stress can cause an earthquake to occur 2 years early for thrust faults..I think this is an over-simplification and John may be able to correct me here..

small stress changes are even more important when we are talking pore fluid pressure within the fault zone....but you would not get an instantaneous pressure pulse to the depth that earthquakes nucleate...

Bottom line...weight of basalt irrelevant..unless there is a quarry and you are removing some of it.
Chris