“If it was seismic activity beneath Grand Teton which caused the fissure...”
That would already have been known. I’m guessing it is a rock slab that has broken off of an existing cliff and has the risk of cascading broken rocks down. They’ll send ugys up on ropes with bars and maybe jackhammers to bring it down in a controlled manner.
I think this is the same talus field my kids and I climbed on years ago (before they put up the “no trespassing” signs). I still am amazed when I think of how my three and six year olds scrambled around on those boulders. Yeah - I suppose one of the boulders might have shifted and squished them - but they didn’t.
Alpine guide team in training session stumbled on rock fissure that widened one day to the next. They determined area unsafe, invoked restrictions on traversing the area.