> Who's to say there aren't 1000 or 10k year cycles of earthquakes?

The Stratigraphy department of the International Union of Geological Sciences would be the obvious first port of call.

Follow that up with an off book chat with digital archivists for commercial oil and gas exploration geologists.

Or influence by the moon just as it pulls on oceans. Lot of liquid magma down there.

Or maybe some once in a rare incident involving coronal mass ejections somehow affecting the core of the planet. Could even be a delayed effect.