To Quiet Down, To Calm Down, To Subside, To Die Down
intransitive verb, godan verb
You already learned that 鎮める means "to appease," but this one is what happens after they've been appeased. They start to quiet down.
しずまる
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You haven't learned this reading yet, so here's a mnemonic to help you: When you're in the library the librarian is always telling you to quiet down. She's (しず) so mean! She never lets you talk in the library. Who's mean to you? She's mean to you!!
とりあえず、今は父の怒りが鎮まるのを待ちましょう。
For the time being, let’s wait until dad’s anger has calmed down.