Well, in my past education I was taught that we were only ever able, using present methods, to penetrate the Earth a few miles deep. A kimberlite is a bubble of the mantle blown through the crust for reasons that are still poorly understood. The core of liquid iron is more dense than the mantle or crust of the earth.
Earthquake waves are like sound waves.
Quite sounds are not heard over great distances. Loud sounds are heard much further away. Likewise, small earthquakes make sound waves that are detected locally but these waves do not penetrate through the core with enough energy to be detected on the other side of the earth. Only the waves of larger earthquakes have enough energy to penetrate the core to be detected on the other side of the Earth. Sound waves do not have enough energy to permanently affect the core.