Being one of the people left behind as California, Las Vegas, Washington, D.C., etc. all succumb. The LA footage, particularly, may remind some of its viewers of the former twin towers of the World Trade Center in New York, New York collapsing with people inside of them.
The LA subway train falling into one of the chasms: imagine being on that train.
Being on the streets of LA while the earthquake is going on. A giant crack appears along the ground and the dirt beneath your feet starts tilting, causing cars to come hurtling towards you at apparently terminal velocity. In the trailers that show that clip, one of the people knocked into the chasm by a falling car screams.
While the plane takes off in L.A, you can see as they fly past that there are people fleeing and trying to avoid a horrific death, their distant screams combining with the sight to be a truly haunting sight.
And at last, seeing the entirety of Los Angeles (and highly possibly most of the West Coast) slide into the ocean.
Yellowstone's eruption. It doesn't go off as in Supervolcano, with a strong plinian fissure eruption at first and with more vents opening up until the entire caldera is pockmarked with similar plinian eruptions. Here, the entire caldera swells and bursts like a blister, going off like a nuke. When the protagonist looks behind him, he sees the entire horizon on fire. Even Charlie looks terrified for a few moments. And then come the lava bombs and pyroclastic flows, the former consisting on not only freshly erupted material but also bits of the caldera thrown up in the blast, complete with rocks studded with trees. The flows pretty much incinerate everything they engulf and pack so much power that when it engulfs and destroys a town near the runway, it uproots a radio tower and smashes it into scrap with ease.
Basically being anywhere on Earth at any point in time. If the earthquakes and volcanoes don't get you, you can bet the tsunamis will. Even if you're in a plane, all it does is to just prolong your fate a bit longer. Not being on Earth could possibly be even worse because there would likely be no way to contact Earth.
If you're still left alive after the caldera goes off, you get front-row seats of a tidal wave going to drown you!
Falling to your death. Too many times to count.
The people in the terminal's Mass "Oh, Crap!" when they see the pyroclastic flow from Yellowstone barreling right towards them, as they gaze off, wondering what it is before realising to their terror and quickly huddling and backing away from the windows en masse as a fireman in the group screams at them to get away from the windows. Considering the fate of the poor bastard in the ATC, it likely didn't do anything to save them.
In the scene where the plane in Las Vegas takes off, the guy on the ATC lets out a chilling, painful scream as he is fried alive inside the control tower when the ash cloud hits it.
Imagine making a narrow escape from the disasters that hit your area, arriving at the rendezvous point for your rescue aircraft just in the nick of time with your family, and being flown to the Himalayas... only to make the horrifying discovery that your assigned Ark was one of the ones that were damaged. They eventually did manage to squeeze everyone onto the remaining Arks, but can you imagine the absolute terror of being in that situation? As if that is not bad enough, imagine finally being told to go to the boarding bridge, ending up on the edge, and then going over as the crowd just keeps on shoving while the bridge lowers into view.
Charlie's whole speech about what would happen prior to the destruction of Earth (global economic turmoil, war, genocide, etc.) is particularly unnerving.
Tony's phone call to his son. He hears the dying screams of his granddaughter before the line goes dead.
Imagine being a person on board the cruise ship as it capsizes. If you somehow survive, you now face the nightmare of being trapped in an upside down boat that will inevitably sink. And because the mainland has been utterly devastated, there's no chance at a rescue whatsoever. No one is coming to save you. Almost makes death by volcano/earthquake/tidal wave seem like a relief because at least it'd be over quickly.
Hawaii in flames (which now serves as the page image). There is absolutely nowhere to land because everything is burning.
The roaring noise when the quakes hit Los Angeles and Washington DC sound like an angry monster is attacking.
The line: "All global ground communication has ceased." is this. Just in general. But especially in a time where electronic communication goes on all the time.
The scene where the tsunami arrives in India after the Earth's crust shifts. It's big enough to push clouds out of its way. Just imagine something like that coming toward you, knowing with absolute certainty that there's zero chance of survival.
The death of poor Gordon being crushed by the Ark's gears. Doubles as a tearjerker consider he saved his entire family twice and for being a nice guy and was so close to freedom only to die horribly.
Ridiculous symbolism aside, the scene of the Vatican's destruction is doubtlessly terrifying. This is The Vatican, the holiest city in the Catholic religion, and it's obliterated all while the gathered masses beg God for His salvation. And just before St. Peter's Basilica collapses due to a violent earthquake, the camera cuts to a group of bishops praying in the Sistine Chapel before it shifts to the painting "The Creation of Adam" on the ceiling. Remember, in the Abrahamic religions, Adam was the first human created by God. When the earthquake starts, a crack travels up the ceiling and goes between the space between Adam and God's fingers before the entire ceiling gives way starting from the crack between their hands. It's actually quite heartbreaking because the crack basically means God has left humanity.
The way it happens as well. As the Basilica collapses, the dome crashes to the ground and rolls forward, crushing thousands as they run screaming for their lives.