Does infalling material leaves a detectable imprint on the black hole's event horizon? An external observer will never see anything cross the event horizon, and in most explanations the story ends here.
But I wonder what happens with gravitational effects of the material?
For example super massive black holes at the center of galaxies probably earned most of their mass via accretion disks near their equator. Does that mean we can observe these mascons near their equator?
If it turns out there are no mascons, then what is the physical process that smears out the mass of the infalling material evenly along the horizon? Why doesn't the infalling observer notice this, while falling in?
 A: The following answer assumes that the no-hair conjecture accurately describes physical black holes.
By the no-hair conjecture, any black hole can be uniquely described by specifying three parameters: its total mass, its total angular momentum, and its total electric charge. Therefore, any detail in the event horizon must only depend on these three parameters. More to the point, every collection of infalling matter which influences these parameters in identical ways will leave an identical "imprint".
For example, suppose we had two collections of infalling matter: collection A is a spherical shell of matter that starts at rest, and collection B is a disc of matter that starts at rest. Both collections have the same total mass, and both are electrically neutral. Both also have zero angular momentum. Therefore, both of these configurations will increase only the total mass of the black hole, and their effect on the event horizon is indistinguishable. So no, you can't in general tell if most of the matter fell into the equator of a black hole.
The process that generates this indistinguishability is not currently known (once again, this assumes that the no-hair conjecture is accurate).
