Unlike some other TMs in Pokémon Scarlet and Violet, TM 149, Earthquake, is not an item that can be found lying on the ground or hidden behind a bush. A powerful Ground-Type Physical Attack, Earthquake hits all Pokémon on the field for 100 Power, making it a valuable attack to clear out large groups of enemies. In addition, TM 149 allows the player to teach Earthquake to a Pokémon that might not be able to normally learn the move or learns it later in their level-up line.
The key difference about TMs in Pokémon Scarlet and Violet compared to previous installments is that TMs are now a recipe. A move like Waterfall, Earthquake, or Fly in Pokémon Scarlet and Violet needs to be crafted using the TM recipe and a TM Machine before it can be taught to a Pokémon. So if a player wants their Tauros to use Earthquake, they need to go through a few steps.
How To Find and Craft TM 149, Earthquake
According to Game8, Earthquake is rewarded to the player by the Cascarrafa (North) Battle League Representative after the player has beaten five trainers in the Asado Desert. This is very straightforward, as the representative is outside Cascarafa's northern Pokémon Center, on the border with the Asado Desert. Furthermore, the trainers in the desert are easy to spot. So if the player is looking to catch Dunsparce in Pokémon Scarlet and Violet, they can knock the five-trainer requirement out in the process.
Once the representative gives the player the Earthquake TM, they need to craft a single-use version. It's a bit pricey, requiring 12000 League Points, 3 Diglett Dirt, 5 Phanpy Nails, and 3 Barboach Slimes. However, the Dirt, Nails, and Slimes are all drop rewards from their titular Pokémon evolution lines, so they require a bit of grinding. The player can sell some extra materials if they need more League Points, but it should be fine if they've been battling Team Star and doing Pokémon Scarlet and Violet's Tera Raids.
Once the materials are acquired, the TM can be crafted at any Pokémon Center's TM Machine. This is a single-use item, so it is recommended that the player gives it to a Ground-type Pokémon with a high attack stat, such as the Pokémon Scarlet and Violet's exclusive Paradox Pokémon.
Source: Game8.co