Holidaymaker Maria Ivleva would have been killed instantly by the huge explosion just centimetres beneath her.
Reports in Russia say the device - which killed 224 people on October 31 - was likely to have been planted under seat 31A, where Miss Ivleva was sitting.
The teenager from St Petersburg had been on holiday in Sharm El Sheikh with her 44-year-old mother Marina, who was sitting in seat 31B.
There is no suggestion that any of the Russian tourists were suicide bombers.
Instead, one theory put forward is that the explosive device could have been planted under the seat in the space where the lifejacket is supposed to be.
The bomb, which Isis (Islamic State) has claimed responsibility for, destroyed six rows of seats and left a huge hole in the tail of the plane which sent it tumbling out of the sky at a rate of 6000ft a minute.
Russian news channel LifeNews reported: "Investigators and secret service experts have managed to locate the seat under which the terrorists planted the bomb which destroyed Airbus A321. The bomb - which was equal to one kilo of TNT - exploded and its force went forwards and hit the rows up to 27. The wave went slightly backwards too and hit row 32. The experts who tested the parts and bodies noted that the people who occupied the seats near the bomb almost certainly died at once."
Investigators said the bomb may also have been planted under seat 30A, where 77-year-old Russian pensioner Nadezhda Bashakova was sitting.
The Kremlin has offered a £33million ($76 million) reward for information about the incident on October 31, which was Russia's worst air disaster.