The hypothesis is examined that concentrations of regional stress in or near mafic instrusions in the felsic continental plate may cause nearby seismicity. If the intrusions are stiffer than the plate, stress concentrations of only 20 or 30 percent above regional values are likely, but intrusions which have been weakened (as by serpentinization) may concentrate stresses more than 200 of seismicity may then occur in the continental plate near the intrusive.