noun
great disaster; catastrophe; train wreck
Refers to a large-scale, devastating event, often with serious consequences. Can be used literally for natural or man-made disasters, or figuratively for a disastrous situation.
その地震は大惨事を引き起こした。
The earthquake caused a great disaster.
彼の計画は大惨事に終わった。
His plan ended in a catastrophe.
大災害 is often used for natural disasters, whereas 大惨事 can include man-made catastrophes and figurative train wrecks.
Compound of 大 (dai, 'big, great') + 惨事 (sanji, 'disaster, tragedy'). The exact historical derivation is uncertain, but the combination straightforwardly intensifies the meaning of 惨事.