Game concept
Earthquake
The game also links it as Earthquakes.
From the game's encyclopedia
An Earthquake is a type of Natural Disaster which causes grievous and sudden destruction in a small number of Counties.
Earthquakes can only occur in regions which are more prone to seismic activity.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Earthquake is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "An Earthquake is a type of Natural Disaster which causes grievous and sudden destruction in a small number of Counties." It is part of Disaster. The game also links it as Earthquakes. Patch 1.20.1.2, from game/common/game_concepts/tgp_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/tgp_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.