Oh, there he goes off to his room to write that hit song "Alone in my principles."
Question #91812 posted on 11/08/2018 7:12 a.m.
Q:

Dear 100 Hour Board,

What happens if all 15 general authorities die at once? Has the Church made provisions in the event of this catastrophe? How will the next prophet be Chosen? Or would God have to come personally and choose another prophet?

-Designated Survivor(?)

A:

Dear afraid not,

All the keys are only held by those 15 (source), so if they died we'd need someone like Peter to come back and restore those keys. I imagine it'd be like when Brigham Young was proclaimed to be the prophet, in that there would be a large miracle to witness to the Church that that person was to be the next prophet.

Thankfully, it's highly unlikely this would ever happen—if you've noticed, we rarely have all 15 gathered together. During each session of General Conference there's always a few apostles missing. So those would be the ones to carry on the keys if that was ever necessary.

-guppy of doom

posted on 11/12/2018 11:47 a.m.
All 15 of them were at the Saturday afternoon session of the last General Conference - the camera pans past all of them during the sustaining of church officers. They generally only miss sessions of general conference if they are physically unable to attend.

Also, they all meet together weekly in the temple, unless someone is sick or on a multi-week trip, which certainly happens on a regular basis. Their meetings are on Thursdays, I believe, because a lot of their travel is from Friday to Monday.

In all, I'd say they're all in the same place much more often than, say, the President of the US and the various successors for that office (VP, Speaker of the House, Senate Pro Tempore, and the entire Cabinet)
posted on 11/12/2018 11:47 a.m.
I bet the other two corrections mention this, but the way i understand DC107:26 is the 70 would preside and have authority to reconstitute the Twelve and first presidency,.