Wikipedia:Reference desk/Archives/Science/2022 October 7

= October 7 =

Ocean retreat ahead of megatsunami
Before a normal tsunami wave reaches land, there is a characteristic retreat of the ocean from the shore. Since megatsunamis have a different cause, does this retreat still occur? 184.67.135.194 (talk) 19:02, 7 October 2022 (UTC)


 * That is often what happens, but some tsunamis arrive with the wave first rather than the trough, it depends on the nature of the initial displacement of the seafloor. In the 2004 Indian Ocean earthquake and tsunami, the movement of the seafloor was to the west, meaning that a trough was first to arrive on Sumatra and Thailand, but that was not the case in India, Sri Lanka and East Africa where the wave arrived with no drawback. This is a normal geometry for both megathrust earthquakes and underwater landslides, which typically slump away from a coast. In both cases the trough arrives first at the local coast, whereas for most transoceanic tsunamis the wave comes first, but you only get significant tsunamis that cross oceans for the largest earthquakes, so they are overall less likely to cause significant damage. For a megatsunami, again it depends on the cause. If it's a sector collapse of a volcanic island, that's just a very large landslide so you would expect both positive and negative tsunamis. A volcanic explosion like Krakatoa would, I assume, be positive in all directions, so no drawbacks. Mikenorton (talk) 19:29, 8 October 2022 (UTC)


 * As our article on tsunamis tells, a wave has a series of crests and troughs and either of them may lead. So there may be a retreat of the ocean when the tsunami arrives at the shore, but not always. I assume that when a tsunami is caused at a fault where one side moves up and the other moves down, the tsunami on the climbing side has a leading crest and the tsunami on the falling side a leading trough, at least initially.
 * A megatsunami caused by a landslide falling into the sea must always start with a leading crest, or so it appears to me.
 * Can a tsunami that originally had a leading crest get a leading trough? Physics allows it, marginally. If the wave velocity depends on wavelength, the group velocity is different from the phase velocity, so that the phase of the wave at the start of the group changes and a leading crest can change into a leading trough. For gravity waves in water, the velocity increases with wavelength, but asymptotically approaches a constant when the wavelength is much longer than the depth of the water. For tsunamis, the wavelength is normally much longer than the depth of the water, so any such phase shift at the leading edge of the tsunami will be slow. The phase velocity is faster than the group velocity, so you need at least π phase shift before there is a leading trough. Given the depth of the ocean and the period of the tsunami, it should be possible to calculate how long it takes before there's a leading trough, but it's too late here to do that calculation now. I think megatsunamis could have pretty short periods, given the small area where they could be caused. PiusImpavidus (talk) 19:57, 8 October 2022 (UTC)
 * You're right about the small source area compared to something like the 600 km long rupture in the 2004 Indian Ocean event and it appears that a flank landslide on La Palma would generate a tsunami that was almost entirely a leading positive, see Figure 5 e.g. here. For comparison, modelling of the tsunami for the 2004 Indian Ocean earthquake, File:2004 Indonesia Tsunami Complete.gif, shows the leading positive going west and the negative going east. Note also in that animation the formation of reflected waves from the east side of India, so irregular coastlines can generate a more complex response. Mikenorton (talk) 15:44, 9 October 2022 (UTC)
 * Thank you @Mikenorton and @PiusImpavidus for your lucid replies and resources. This is very good to know for people in earthquake/tsunami zones! 184.67.135.194 (talk) 17:40, 11 October 2022 (UTC)

Thoughts and understanding and researching are organic?
Are thoughts and understanding and researching considered organic according to up-to-date science? I am not sure, but as far as I know, memory is a results of neurons connections. Are thoughts and understanding the same? Is there a part in our mind that is not organic? ThePupil (talk) 20:02, 7 October 2022 (UTC)
 * See hard problem of consciousness. --Trovatore (talk) 20:08, 7 October 2022 (UTC)
 * Speculations have been around for at least 30 years (to my knowledge) that thought and/or consciousness may arise from quantum phenomena in the microtubule structures within neurons. See Orchestrated objective reduction, and some of the 'See also' links in that article. {The poster formerly known as 87.81.230.195} 90.193.128.129 (talk) 01:31, 8 October 2022 (UTC)
 * In any way, there is plenty of evidence that Dualism is a position that is scientifically unsupportable. In particular, damage to the brain (wether by trauma or disease) is systematically connected with loss of function. --Stephan Schulz (talk) 00:02, 10 October 2022 (UTC)
 * But phenomenal consciousness is not scientifically measurable in the first place. --Trovatore (talk) 15:52, 10 October 2022 (UTC)
 * Is any part of the mind damaged in case of organic brain trauma? Or there is something that is not demmeged? (for example the ability to dream).ThePupil (talk) 06:42, 10 October 2022 (UTC)
 * Well, what is "the mind"? But many observable functions are damaged by some kinds of organic damage. WW1, for all its horror, was an important stepping stone, because of the large number of soldiers receiving head wounds and (because of much improved care) actually surviving. Things like losing speech or vision processing or the formation of long- or short-term memories can all be tracked to damage in specific parts of the brain. --Stephan Schulz (talk) 15:51, 10 October 2022 (UTC)