see also:

Official chinese version of 21 t-TNT-Explosion in Tianjin is a LIE! (crater-lake is 95 x 104 m)

scientific online Crater-Calculator (in new tab):

New calcultions with nuclear device:

Medium Atomic Demolition Munition ;

Medium Atomic Demolition Munition (MADM) was a tactical nuclear weapon developed by the United States during the Cold War. They were designed to be used as nuclear land mines and for other tactical purposes, with a relatively low explosive yield from a W45 warhead, between 1 and 15 kilotons. Each MADM weighed around 400 lb (181 kg) total. They were produced between 1965 and 1986

other calculations: 

(aboveground explosion: 21 t-TNT >> 15.6 m crater,  100m crater needs 16600 t-TNT

or 4000 t-TNT underground >> 100m crater: (possbly nuclear)