If I have a Tera Pokemon on my Bench when I use Landorus' "Earthquake" attack, do they take 10 damage or not?
No, the Tera Trait protects them from damage by your own attacks as well.
Earthquake






x2

Water Gun


EarthquakeThis attack does 20 damage to each of your Benched Pokémon. (Don't apply Weakness and Resistance for Benched Pokémon.)
If I have a Tera Pokemon on my Bench when I use Landorus' "Earthquake" attack, do they take 10 damage or not?
No, the Tera Trait protects them from damage by your own attacks as well.