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

Other known variants

x2
Sand SwirlDoes 20 damage to each Defending Pokémon. The Defending Pokémon can't retreat until the end of your opponent's next turn.


EarthquakeDoes 10 damage to each of your Benched Pokémon. (Don't apply Weakness or 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.