Without somebody going to Shanksville and doing sonic tests we do not know if the explosion she "felt" was transmitted via the Earth's crust, or as a shockwave through the atmosphere.
Just my personal opinion, but she does specifically mention a sound in her accounts of what happened. If it were the transmission for the earth that set her off:
A) Why didn't she make mention of the thunderous boom she would have heard AFTER reacting
B) Why would she talk about initially reacting to a sound AND a feeling?
To me, it seems more likely she felt the impact as a slight shaking, then heard the explosion, then reacted.
Now all of this is still assuming her 5 minute response time...
And here's where I'd like to throw another speculative spanner in the works.
She says she was watching the WTC and Pentagon attacks on TV at the time. UA93 crashed 1003. At 0959 The South Tower collapsed. At the time that UA93 crashed, we can imagine Val was engrossed in the horror of the WTC having actually totally collapsed right before her eyes. To the best of her knowledge thousands upon thousands of lives had just been snuffed out like candles in the wind.
And yet, when a plane flew over, she just instantly lost interest in the television, rushed outside, and snapped a photo?
She didn't linger for a few moments? Her actions weren't delayed just a little?
Basically, we have found the following evidence:
Following the instant 5s response hypothesis, we find there is still ample in the equation to allow a 600m mushroom cloud to form as, and where Val snapped it. Indeed, we have evidence to suggest the cloud could comfortably be much larger.
In ADDITION to this, there are a number of LIKELY factors that could have contributed, in varying degrees, to the photo having been taken much later, and the mushroom cloud having drifted closer to Val (thus allowing for a much smaller cloud).
These two considerations, combined, indicate there is absolutely no plausible grounds for considering Val's photo is anything other than what it appears to be - the only existing documentation of the immediate aftermath of the destruction of UA93 and all aboard.
May they rest in peace.
-Andrew