Note this is a stellar black hole merger of several tens of solar masses.
Imagine the disturbance of a galactic core black hole mergers of millions of stellar masses. These are probably much rarer, but do occur when galaxies merge.
If this happened in the centre of the Milky Way, we're about 25k light years away.
Let's say 2 1 million solar masses black holes merged there... and they also gave off about 3/60 of their mass as radiation, that's about 100'000 solar masses being radiated 25k light years away.
Using my calculation in the other post, we're talking 10^52 Joules. Across a distance of 25'000 light years, or about 10^20 metres, that's then decreased by 10^40 (inverse square) so we're left with about 10^12 Joules...
Which is good news! If that happened in the Milky Way, we would probably survive it - though we'd definitely notice some strange atmospheric effects...
