It is said that at the binary black hole collision LIGO detected recently, the energy equivalent of 3 solar masses has been released.
Since no matter can escape a black hole, the only source I can imagine is kinetic energy, so the rest mass of the new formed black hole should then be at least the combined rest mass of the two former black holes plus maybe some extra relativistic mass from the motion they had before they merged, which might not be completely converted to gravitational waves.
If the velocity of the former black holes was like suspected c/2, their relativistic masses would be 15% higher then their rest masses. The rest mass of the black holes are assumed to be 30 solar masses each. That would rawly fit the numbers.
Also the earth and the moon do not seem to lose any mass, but they also produce tiny gravitational waves.
Is my assumption that no rest mass, only relativistic mass, so velocity is converted to gravitational waves correct, or am I overlooking something? If so, what is the mechanism to convert material from inside the former event horizons to energy radiating away as waves?