Only those who have actually been in mortal peril can know how they would react to it it. I know exactly how I would, for I did once face imminent death, or thought I did. This was some years ago, and I will first digress and recount a more recent experience.
I was at the delicatessen counter of the supermarket the other day, admiring a Parma ham and looking to see if it was approaching its Display Until date and if they had halved the price as they do sometimes. Suddenly, all the lights went out.
It was very very dark, and for a few moments there was complete silence except for a little bit of whimpering from understandably frightened children. Then, as their mothers re-assured them, the great British Blitz/Dunkirk spirit burst forth like a flashing sword from its sheath, and there were jokey cries of "Don't panic!", "We're all doomed!", "Anybody got a shilling?" and so on.
The lights came on after about five minutes, but if the blackout had been longer I've no doubt that the older shoppers would have kept up our courage by belting out some of the wartime favourites (there was one called When the Lights Go On Again, All Over the World..), and the pious ones would have launched into Abide With Me. It was a heartwarming occasion, though I suppose there may have been some who took advantage of the darkness by stuffing their pockets with packets of bacon or even indulging in acts of an improper nature.
The incident which tested me with much greater rigour took place some years ago when I was staying on the seventeenth floor or thereabouts of the Hotel Okura in Tokyo. When the water glass on my table started to jiggle about and there was a quiet rumble I smiled knowingly, well aware that such tremors are frequent in Japan. But the jiggling got much greater and the noise louder, and when the guests in the adjoining rooms started running into the corridor and screaming in Japanese it occurred to me that they must have known something really serious was happening. Perhaps this was The Big One which everyone said was bound to come one day. Believing that I was about to provide some corner of Minato-ku which would be forever England, and that there was nothing whatsoever that I could do about it, I lay down on my bed in the foetal position, put a pillow over my head and began to curse my luck out loud, blasphemously and obscenely.
Natural as this reaction might have been, I felt a little sheepish when the jiggling stopped, but happily there was no-one to witness my behaviour. This was not at all a proud occasion, but at least I have known ever since how I react to mortal peril.
A good ten minutes after all was quiet, a soothing voice came from the loudspeakers: "Do not be ararmed. This hoteru is earthquake-proof". Even if they'd told me that before, I doubt if I would have been completely re-assured.