What is the default Earthquakes game rule in CK3?

Default: Earthquakes will occur at a normal rate. (patch 1.20.1.2)

Does changing the Earthquakes rule disable achievements?

No: none of its 3 settings carries the flag blocks_achievements in patch 1.20.1.2.

Game rule · Difficulty

Earthquakes

Default
Rare
Disabled

Earthquakes is a game rule in Crusader Kings III (Difficulty) with 3 settings: Default (the default), Rare and Disabled. None of its settings disables achievements. Patch 1.20.1.2, from game/common/game_rules/00_game_rules.txt.

This section as JSON: /raw/game-rules.json

Quick answers (2)

What is the default Earthquakes game rule in CK3?: Default: Earthquakes will occur at a normal rate. (patch 1.20.1.2)
Does changing the Earthquakes rule disable achievements?: No: none of its 3 settings carries the flag blocks_achievements in patch 1.20.1.2.

3 settings

Default
Default
Earthquakes will occur at a normal rate.
natural_disaster_earthquakes_regular
Rare
Earthquakes will occur less frequently.
The script checks it once, in 1 place
On actions (the game's own hooks) 1
- natural_disaster_earthquake_maintenance_pulse
Every has_game_rule = natural_disaster_earthquakes_few in game/common and game/events, by the top-level object it sits in.
natural_disaster_earthquakes_few
Disabled
Earthquakes will never occur.
The script checks it once, in 1 place
On actions (the game's own hooks) 1
- natural_disaster_earthquake_maintenance_pulse
Every has_game_rule = natural_disaster_earthquakes_disabled in game/common and game/events, by the top-level object it sits in.
natural_disaster_earthquakes_disabled

Source: game/common/game_rules/00_game_rules.txt line 1620 (key natural_disaster_earthquakes), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.