Pokémon Brilliant Diamond and Shining Pearl changed several features and mechanics to match updated versions that have been added in games that have come out since players first adventured through Sinnoh in 2006. However, the developers at ILCA also ran certain features back, making them more faithful to how they were in the original gen four games—including altering Technical and Hidden Machines.
Technical Machines (TMs) and Hidden Machines (HMs) are used to teach compatible Pokémon moves they might not learn naturally while leveling up, whether that be purely for the purpose of expanding their moveset in battle or providing the player a way to get past certain obstacles in the overworld.
With HMs being removed entirely in favor of an app on the Pokétch, ILCA also made it so TMs are one-use items again, breaking after they teach a Pokémon the designated move for the first time since Black and White. However, certain NPCs, such as Gym Leaders, will give you more than one TM to use, mitigating the issue somewhat.
This change does mean that sought-after TMs, such as TM26 Earthquake, have become more valuable again since they can’t be used in perpetuity.
Earthquake is the most powerful Ground-type move in the game and one of the strongest moves in general. At base 100 Power and 100 Accuracy, it will cause massive damage to any enemy even if used by a non-Ground-type.
Players looking to teach their Pokémon this move will need to travel deep into Wayward Cave—the smaller section under Cycling Road where you would also normally go to encounter a Gible. Inside, you need to have access to the Strength ability, traverse the bike puzzle all the way to the end, and head South. A single staircase will take you to a dead-end room with Earthquake as the only item inside.
You can get more copies of the Earthquake TM by visiting the Battle Park and exchanging 80 Battle Points for one at the counter on the right-hand side.