This question already has an answer here:
Since it was reported recently that the supermassive black holes at the centres of galaxies may in fact be as many as 20,000 smaller black holes I wondered if, if these black holes collide with one another, why they wouldn't be producing gravitational waves all the time. But then I also wondered that, if whilst accepting material, supermassive black holes must drastically increase the density of that material before it collides with the SMBH why this wouldn't also cause gravitational waves. Since GW's can (presumably) be caused by anything, since colliding neutron stars also emit these waves, why wouldn't any other source emit them also?
Also, why could they not be caused by seismic activity? Even if there's a minute probability of such events occurring surely it's possible that, given all the potential sources of a matching signal, that it would happen eventually? What are people's thoughts on this matter?