Groundquake

A groundquake, also called a seismic quake, was a geological event in which parts of a planet's surface violently shook, often damaging buildings and causing damage. They could occur seasonally, as was the case on the planet Dalna.

During the Clone Wars, the Kindalo species caused a series of groundquakes on their homeworld Aleen in order to try to close a breach which had opened up between their subterranean world and the surface, allowing in air that was toxic to them. The quakes devastated the surface, and greatly affected the Aleena species that lived there, causing the Galactic Republic to send aid to the world. Among the aid workers were the droids R2-D2 and C-3PO, who, after learning about the cause of the quakes, managed to reseal the breach and so end the quakes.

Groundquakes occurred regularly on the planet Gorse when its moon Cynda approached during its elliptical orbit. During a quake, parts of the planet's surface violently shook and rocked, often causing damage to buildings. The quakes occurred to release the energy that built up through the gravitational pull of Cynda and the Gorse system's star on Gorse. Droid workers would occasionally fall into the acid baths used by Moonglow Polychemical on Gorse during groundquakes. When Lal Grallik—the manager of Moonglow—was murdered in one of the baths by Count Denetrius Vidian, it was claimed she had accidentally fallen in during a quake. When Vidian later planned to destroy Cynda, the science officers on board the Imperial-class Star Destroyer Ultimatum predicted that the loss of the moon would increase groundquakes on Gorse.

During a vision on Coruscant, Palpatine thought he felt the ground shake but remembered that it was not real, as it was physically impossible for a quake of that magnitude to occur as Coruscant was an artificially regulated planet.

Groundquakes first appeared in the Star Wars Legends novel Lando Calrissian and the Mindharp of Sharu, which was written by L. Neil Smith and released in 1983. They were confirmed to be canon by their appearance in Mercy Mission, an episode of the Star Wars: The Clone Wars television series, which aired in 2011 as part of the show's fourth season.