Earthquake in Generation 1 is a Physical move(and one with 100 power and great accuracy, making it one of most reliable moves in the game). Therefore it takes your Attack stat(STAB and Type advantage if applicable) and the Defense stat of the target Pokemon when calculating damage. Psychic types typically have low or mediocre HP and Defense stat, hence Physical moves such as Slash, Earthquake, Dig, Drill Peck, Double Edge, Take Down, Karate Chop, Body Slam, Strength and Hyper Beam do enormous damage(if not OHKO). Exeggutor, Hypno and especially Slowbro are the exception due to largely sacrificing Speed for the higher Defense stat. Starmie isn?t a bad Psychic type if you?re looking for a non-traditional Psychic Pokemon since it?s only real deficiency is low HP. The fact it can learn Ice moves to counter Bugs makes it a pretty solid Generation 1 option.