Shan's comment seems reasonable...that out of all the quakes, few are triggered by P (or S) waves. But, I don't know the numbers, and some areas like Yellowstone or Long Valley may be more susceptible. However, a very large quake is liable to to trigger some other quakes by P or S waves. For example, if the M7.4(?) Landers quake could trigger other quakes both by P and S wave and also by static stress, then you would expect a similar quake to trigger others. So, perhaps few quakes are triggered by P or S waves, but most M7+ quakes in continental crust will trigger at least a couple other (small?) quakes.
Qualifier: I am just using logic; I don't know the actual numbers.
Chris