Well i can answer both your questions in one answer.
Ok go under the bike path and go in the secret entrance(not the one you can see)
Ok go to the hiker on the right and stand right above him
Then walk EXACTLY 12 steps to the left
Then head straight up and you will find yourself in the secret entrance where you can find TM26 Earthquake. in that cave you can also test your bike skills and catch a level 17-21 Gible, a strong Dragon/Ground type that evolves into a Garchomp.
Also you can get TM 26 earthquake at the battle frontier for 48 battle points
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Earthquake (TM26) is your reward after you beat Giovanni
Garchomp actually doesn't learn earthquake by increasing lvl....but it learns through tm26(which is earthquake).
TM 26 (Earthquake) is a reward for beating the 8th Gym Leader in Viridian City
The Pokemon Aggron is the final evolution of the Pokemon Lairon, which evolves at level 42. It is not able to learn the move Earthquake through leveling, but it can be taught Earthquake using TM26 at any time.
TM26 (Earthquake) can be obtained inside Relic Castle after beating the Pokémon League (Elite Four). It's on the first floor, in the room that leads to the exit to the northwest of the desert, near the stairs.