Gamma-ray shielding against bursters

[selden]

Some tiny fraction of extrasolar gamma rays do penetrate the Earth's atmosphere all the way to the surface. Extrasolar GRBs were first detected by military satellites looking for the gamma rays emitted by nuclear bomb testing. If some fraction can escape through the atmosphere, the same proportion can also come down through that same depth of atmosphere.

If a GRB happens close enough, that tiny fraction is going to be very large in absolute terms.