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What if a single rock the size of the rockfall that triggered the world's tallest tsunami were to free-fall 3,000 feet directly onto land and not into water?

On the night of July 9, 1958, an earthquake along the Fairweather Fault in the Alaska Panhandle loosened about 40 million cubic yards (30.6 million cubic meters) of rock high above the northeastern shore of Lituya Bay. This mass of rock plunged from an altitude of approximately 3000 feet (914 meters) down into the waters of Gilbert Inlet (see map below). The impact force of the rockfall generated a local tsunami that crashed against the southwest shoreline of Gilbert Inlet.

The wave hit with such power that it swept completely over the spur of land that separates Gilbert Inlet from the main body of Lituya Bay. The wave then continued down the entire length of Lituya Bay, over La Chaussee Spit and into the Gulf of Alaska. The force of the wave removed all trees and vegetation from elevations as high as 1720 feet (524 meters) above sea level. Millions of trees were uprooted and swept away by the wave. This is the highest wave that has ever been known.

Quote taken from: https://geology.com/records/biggest-tsunami.shtml

Another reference: https://earthquake.alaska.edu/60-years-ago-1958-earthquake-and-lituya-bay-megatsunami

What if this rockfall had happened over land, but as a single huge 40 million cubic yard rock falling straight down?

For example...

  • Would the rock have created a seismic event of its own (if so, how large)?
  • Would the rock have created a crater?
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    $\begingroup$ The Sarez Earthquake seems to have involved more material falling from a greater height. $\endgroup$ – simplicio Apr 26 at 19:28
  • $\begingroup$ Fair point. I realized after posting that the rockfall in question was likely not a single rock and didn't fall straight down. I'm curious what would happen if a single giant rock free-fell a huge distance onto the ground. How much of an impact (pun kind of intended) would it make? I edited my question to reflect this intent. $\endgroup$ – bob Apr 26 at 20:14
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The energy of the rock at the time of hitting the earth is mgh.

No rock we know of is going to be able to survive this collision with out breaking into pieces.

Non the less it will be a big impact and depending on the geology of the location it hits a variety of reactions scenarios can happen.

If the soil is aggregate of silt and sand and gravel, it would part into several shear rupture sections which look like slices of shell pattern surfaces starting from the bottom surface of the rock and turning up exiting the earth surface a few hundred yards outside of the impact zone and probably even eject some material out like a bomb crater. This scenario will have shakes that could be recorded miles away.

The calculation of how much of the momentum of rock will be shared with the shear material and accelerating them will be involved but not impossible.

If the geology of the impact area is of very low bearing like mostly silt and loose clays, the rock my lose most of its kinetic energy by just sinking into the dirt mostly with a giant humph with a cloud of dust rising.

If the geology is hard or rocky with the 'optimal' amount of mass and resilience it could create a substantial earthquake by resonating with the impact.

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  • $\begingroup$ The first part sounds terrifying--I wonder how far a large (multiple tons) fragment might fly. The last part sounds interesting and reminds me of how in parts of the world that still do mining, a mine collapse can cause a small earthquake. $\endgroup$ – bob Apr 30 at 14:36
  • $\begingroup$ I also wonder how loud of a sound it would make in the different scenarios. I guess it might depend on how it lands? I would assume it would be louder if it had a large flat area that it managed to land on pretty much all at once? $\endgroup$ – bob Apr 30 at 14:37

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