collapse, landslide
坍 centers on earth giving way: a sudden collapse or landslide where ground or a structure falls down.
坍 combines 土 (earth) with 丹, which likely contributes the sound. The character expresses earth collapsing or sliding.
The left side is earth 土, and the right side 丹 looks like a cracked, falling piece of ground. Picture a chunk of red earth breaking off and sliding down: a landslide.
For タン, imagine a tan-colored hillside suddenly collapsing: tan -> タン, and the earth gives way.
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