• 災害さいがい — natural disaster
• 火災かさい — fire (disaster)
• 震災しんさい — earthquake disaster
• 水災すいさい — flood disaster
• 防災ぼうさい — disaster prevention
• 人災じんさい — man-made disaster
• 災難さいなん — misfortune, calamity

The kanji 災 combines the radical 火 (fire) and the phonetic component 巛 or 𡿺, historically connected to water flow or misfortune. Together, they symbolize harm caused by uncontrollable forces, especially natural ones like fire and water.

This kanji is used in many compound words for various types of disasters — both natural and man-made.

地震じしんによる 災害さいがいが 広ひろがっている。
The disaster caused by the earthquake is spreading.

火災かさいに 備そなえて 防災訓練ぼうさいくんれんを 行おこないます。
We will conduct a disaster drill in preparation for fires.

“わざわい” is a native kun-yomi used to express misfortune or personal disaster.
“サイ” is the on-yomi used in compound words like 火災 and 災害, frequently seen in news and emergency contexts.