If someone puts dynamite across the San Andreas fault and blows it up would that big earthquake happen?

No.

There isn’t enough energy in a stick of dynamite.

Even a several kiloton nuclear weapon denoted on the surface probably wouldn’t be enough. Too much of the energy would be lost to the air.

It might be if it was buried several hundred to serval thousand feet below the surface where the soil and rock would actually absorb the shockwave.

For a surface denotation you would need a weapon of several megatons.

Keep in mind that any of these would only result in a local event. I doubt a several megaton warhead buried thousands of feet would result in much more than a local seismic event. It just requires so much energy.