Hi Carol. Picture a wave, ripple or a pulse crossing a fault. It affects a greater portion of the fault at the time it crosses then it would if it were moving up or down the fault. It doesn't add stress to the fault, but sets up a condition that allows the fault to move. Keep in mind that this only occurs when that segment of the fault is close to breaking anyway. It can also occur with the wave, ripple or pulse moving up or down the fault if the fault is very close to breaking. It's kind of like a boat getting hit broadside with a wave as opposed to getting hit head on by a wave. Hope this explains it. Take Care…Don in creepy town