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

Guard PressDuring your opponent's next turn, this Pokémon takes 20 less damage from attacks (after applying Weakness and Resistance).



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.