What would happen to a person standing on the edge of a faultline when it moves?

Submitted by Endorkend t3_10wxhnh in askscience

Currently there are pictures floating around Reddit of how the earthquakes in Turkey resulted in a fault line visually having shifted several meters.

What would happen to a person standing right on the edge of this?

Most images show one side moved (like the traintracks one showing the tracks didn't shift on one side, yet having moved several meters on the other side).

So, extended:

  • What would happen to a person standing on the moving side.
  • What would happen to a person standing on the side that didn't move.

Considering even a whales call can cause physical damage to a person if they get to close, I'm suspecting the release of energy alone isn't particularly healthy, no matter which side you're standing.

Additionally, are there any known cases of a person or telemetry device experiencing this?

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CrustalTrudger t1_j7plgdi wrote

The first thing to cover is that both sides moved, the appearance that one side moved is just a perspective / reference frame thing. The easiest way to consider this is through the concept of elastic rebound, basically the idea that the upper part of the Earth's crust behaves like an elastic material. There's a diagram on the wiki page, but others are better, like this one. Referring to that, the underlying idea of elastic rebound is that if you start from an undeformed state (i.e. line A-B-C-D-E-F-G), as the two sides of a fault move, areas in the "far field" (e.g., spots A' and G') record the full motion, but as you approach the "locked" fault, there is increasingly less interseismic (i.e., between earthquake) deformation until you get to the fault (point D) where there is no deformation. This is equivalent to flexing an elastic beam that you hold parallel to yourself and you pull one side toward you and one side away from you, the center of the beam will not move. Eventually, the stored elastic strain overcomes the friction of the fault and the elastic deformation is "recovered" and points near the fault move to "catch up" with the far field deformation, by varying degrees depending on their proximity to the fault (e.g., point B' doesn't move too much, point C' moves more, and point D bifurcates into points O and P).

As to what would happen to a person on one side or the other or straddling the rupture, for sure you'd fall down. Beyond that, and barring that nothing fell on you, you didn't fall into a fissure that opened up along something like a mole track, you didn't end up sinking into a liquefaction feature, or were damaged by the eruption of something like a sand boil, I'm not sure you'd necessarily be injured. I'm not aware of any indication that seismic waves have ever induced air pressure waves to the point where they'd be physically damaging to a person for example.

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Endorkend OP t1_j7pqys6 wrote

So the shockwaves will almost purely go through earth then?

I've seen footage of volcanoes blowing where there is a visual airborne shockwave.

I guess it's because fault line movements are inside the earth and involve movement of obscene distances it gets spread better, but I thought that with the stupendous amount of energy released it would still have a decent effect above land too.

Thanks!

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CrustalTrudger t1_j7pye07 wrote

The interaction of seismic waves and the surface of the Earth can produce measurable pressure waves in the atmosphere (e.g., Donn & Posmentier, 1964), but generally nothing that's going to be damaging. This of course depends on what the fluid in contact with the solid earth surface is though as a tsunami effectively represents a displacement wave from surface deformation (from actual vertical change in the ocean bottom as opposed to seismic waves specifically), but here, it's not the pressure wave that's dangerous per se, but the resulting inundation when this approaches shore.

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Sub0ptimalPrime t1_j7qaru9 wrote

It's also important to note that the energy epicenter is below ground. So the release of energy (the equivalent of the volcanic "explosion" you speak of) is actually below ground at the point of greatest elastic rebound (or friction overcome). That "explosion" has to then travel through thousands of feet of rock (depending on how deep the epicenter is, which is controlled by what kind of plate margin it is), so it is greatly dissipated.

Edit: has to travel through *miles of rock ("thousands of feet of rock" wasn't wrong, but doesn't put the reader in the right frame of magnitude)

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GaudExMachina t1_j7qx8ut wrote

To piggyback, I checked around and seismologists suggest this was at a depth of 18 km below the surface. Not as deep as some, so more surface effects.

Also a horizontal strike slip, so lateral movement between the two plates, so there won't be much vertical component. 7.8 is a massive earthquake, but by no means as insanely powerful as some of the 9+ that have hit Chile within recorded history. I recall reading that one of those in the 1950s had an offset of 30 meters along its rupture zone (deep in the earth), but I'd need to go find a source on that.

For an exceptionally rough estimate, OP could try envisioning the ground suddenly shifting laterally 15 meters while they stood upon it, then scale that back by a factor of more than 10 as this one was considerably less powerful.

Devastating for a building that rises multiple stories, while having a narrow base and made out of inflexible materials. But for a person on the surface of the earth, it would knock them flat.

Edit: Good illustration posted in pics
https://www.reddit.com/r/pics/comments/10wsr6n/anatolian_plate_moved_335_meters_after_the/

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PlainTrain t1_j7t2w9x wrote

Does the shake time correlate to the time it takes for the earth to move? I.e. if the quake lasts a minute, does that represent the time for the slip to take place in, or is the slip more instantaneous and the shake time measures the propagation of the rupture? Or something else?

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CrustalTrudger t1_j7tymzs wrote

Good question! So the duration of the ground motion in a specific place is not usually directly related to what we would call the "source time function" (STF), i.e., a description of how long the earthquake rupture on the fault took to occur.

Let's first consider the STF, this is typically considered in terms of moment rate per time (e.g., Figure 2 in Vallee, 2013) where the total seismic moment released during an earthquake (which directly relates to the mangitude, i.e., the moment magnitude) is effectively the area under a STF curve. From figure in the linked paper we can see that the same magnitude earthquake can have different patterns of moment release (i.e., Figure 2 a-c are all the same magnitude events and thus released the same total moment, but with either ruptures that occurred more slowly or quickly so moment rate varies between them). There are a variety of details of an earthquake where the STF is important, but as we'll see, duration of ground shaking at a location is not usually one of them.

If we shift our attention to the duration of ground motion, we can consider a range of empirical equations that have developed to try to estimate duration of shaking, specifically Table 1 from Yaghmaei-Sabegh et al., 2014, we can see that total moment (in the form of moment magnitude) appears in all of these equations, but none of them directly consider anything about the STF or speed of the rupture in a formal sense. Instead, you'll see that in addition to the magnitude, there are few other general earthquake properties (e.g., depth of the hypocenter), but then a lot of things specific to the "site" you're considering, both in the sense of things in relation to the specific earthquake (e.g., distance from the rupture) but also more generally (e.g., soil type, etc.). This reflects that broadly, while there are obvious controls from the available seismic energy (which will be dictated primarily by the total moment, i.e., magnitude, and the sites distance from the source), there are also a lot of site effects which can impact duration of shaking (and other important details, like peak ground acceleration, dominant period of the shaking, etc). In detail, the type of rocks and their geometry can play a large role in the specifics of shaking in a particular place. E.g., seismic waves in sedimentary basins tend to "reverberate" and thus the duration of shaking can be significantly longer than outside the basin and as they reverberate, they can have both constructive and destructive interference with each other and in many cases can amplify shaking at particular frequencies (which is very important to understand if you're trying to engineer a building to survive an earthquake).

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