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The demiplane spell creates a door (and a demiplane) that leads to an extradimensional room of a fixed size:

[...] When opened, the door leads to a demiplane that appears to be an empty room 30 feet in each dimension, made of wood or stone. [...]

It seems to me that, at least by my understanding, that is the physical size limit of this pocket plane. It isn't meant to be extended, or else the spell will have mentioned what happens when you dig through the wood or stone lining the demiplane.

However, the earthquake spell says:

You create a seismic disturbance at a point on the ground that you can see within range. [...]

Fissures. Fissures open throughout the spell’s area at the start of your next turn after you cast the spell. A total of 1d6 such fissures open in locations chosen by the DM. Each is 1d10 x 10 feet deep, 10 feet wide, and extends from one edge of the spell’s area to the opposite side (emphasis mine). A creature standing on a spot where a fissure opens must succeed on a Dexterity saving throw or fall in. A creature that successfully saves moves with the fissure’s edge as it opens.

What would happen to a demiplane created by the demiplane spell if fissures opened on its floor due to the earthquake spell? Would the demiplane itself be destroyed? Or would it create a deeper floor in the demiplane? Is this a case where there isn't enough RAW to say with certainty one way or the other, and that it rests solidly in DM fiat territory?

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It's up to the DM

As you've noted, the room is a clearly defined space, but earthquake does things outside of that space. There aren't specific rules for this type of interaction, so how it plays out will be up to the DM.

As usual, in these cases I always lean towards what makes sense or would be interesting from a story-perspective. The mechanics are less important than the story when you encounter these edge cases, so go with what tells a good story.

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