natural_disaster_ruler_start_effect

natural_disaster_ruler_start_effect = {
	save_scope_as = ruler
	scope:situation.situation_center_province ?= {
		if = {
			limit = { scope:ruler = county.holder.top_liege }
			save_scope_as = disaster_province
		}
	}

	if = {
		limit = { NOT = { exists = scope:disaster_province } }
		scope:situation_sub_region = {
			random_situation_sub_region_county = {
				limit = { scope:ruler = holder.top_liege }
				title_province = {
					save_scope_as = disaster_province
				}
			}
		}
	}
	tgp_activate_catalyst_against_hegemon_effect = {
		HEGEMON = scope:ruler.top_liege
		CATALYST = catalyst_hegemon_natural_disaster
	}
	scope:situation = {
		switch = {
			trigger = situation_type
			natural_disaster_earthquake = {
				scope:ruler = { natural_disaster_start_great_project_effect = { TYPE = earthquake } }
			}
			natural_disaster_flood = {
				scope:ruler = { natural_disaster_start_great_project_effect = { TYPE = flood } }
			}
		}
	}
	scope:great_project ?= {
		set_variable = {
			name = situation
			value = scope:situation
		}
		scope:situation = { # Nested to prevent errors from non-existence
			add_to_variable_list = {
				name = great_projects
				target = scope:great_project
			}
		}
	}
}

As written in game/common/scripted_effects/10_dlc_tgp_natural_disaster_scripted_effects.txt.