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posted by martyb on Wednesday December 05 2018, @12:55AM   Printer-friendly
from the I'm-all-shook-up dept.

Just before 9.30am on Sunday 11 November, a series of unusual seismic pulses rippled around the world almost undetected.

The waves rang for over 20 minutes, emanating about 15 miles off the shores of Mayotte - a tiny island in the Indian Ocean between Madagascar and Africa.

From here, they reverberated across Africa, setting off geological sensors in Zambia, Kenya, and Ethiopia.

They crossed the Atlantic, and were picked up in Chile, New Zealand, Canada, and even Hawaii nearly 11,000 miles away, the National Geographic reports.

Despite their huge range, the waves were apparently not felt by anybody. However, one person monitoring the US Geological Survey's live stream of seismogram displays did notice the unusual waveform and posted it to Twitter, sparking the interest of other geologists and earthquake enthusiasts.

[...] The bizarre waveform is what scientists call "monochromatic". Earthquakes normally produce waves of so many different frequencies, the wave readings appear more jumbled.

But the mystery waveform from Mayotte was a crisp zigzag, which repeated after steady 17-second intervals.

"They're too nice. They're too perfect to be nature," joked the University of Glasgow's Helen Robinson, who is study[ing] for a PhD in applied volcanology.

https://www.independent.co.uk/news/world/earthquake-seismic-waves-mayotte-madagascar-volcanic-activity-science-a8659236.html


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  • (Score: 2) by c0lo on Wednesday December 05 2018, @06:08AM (12 children)

    by c0lo (156) Subscriber Badge on Wednesday December 05 2018, @06:08AM (#769990) Journal

    Glaciers aren't planet-global despite your assertion to the contrary.

    The deformation big enough glaciers (and the Greenland and Antarctica are fucking big) impose on the crust is global though.
    Things like isostasy [wikipedia.org] and extra bulge of water (which previously used to stay closer to the Poles) now pressing down on the Equator guarantee it, in spite of your wilful ignorance or dismissal of them.

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  • (Score: 1) by khallow on Wednesday December 05 2018, @02:22PM (11 children)

    by khallow (3766) Subscriber Badge on Wednesday December 05 2018, @02:22PM (#770072) Journal
    And this global deformation, which let us note is not very big by the time it gets to Madagascar, manifests at a single place why?
    • (Score: 2) by c0lo on Wednesday December 05 2018, @04:36PM (10 children)

      by c0lo (156) Subscriber Badge on Wednesday December 05 2018, @04:36PM (#770138) Journal

      And this global deformation... manifests at a single place why?

      Let us also note that nobody said it manifests in a single place.
      It creates effects in multiple places, some which can be noticed, others that nobody cared to look for. In the latter case, it doesn't mean they don't exist.

      which let us note is not very big by the time it gets to Madagascar

      Not very big in terms of what?
      E.g. neither very big is the distance the tectonic plates suddenly move relative to each other, but the resulting earthquake can release a huge amount of energy.

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      • (Score: 1) by khallow on Wednesday December 05 2018, @04:53PM (9 children)

        by khallow (3766) Subscriber Badge on Wednesday December 05 2018, @04:53PM (#770144) Journal

        Not very big in terms of what? E.g. neither very big is the distance the tectonic plates suddenly move relative to each other, but the resulting earthquake can release a huge amount of energy.

        That's a good example. A big quake can displace rock by meters. The global deformation (outside of the ice sheets) is no more than a few centimeters of water (divide that by 2.5 roughly to get deformation of bedrock). So effect is at least two orders of magnitude smaller than a large earthquake.

        • (Score: 3, Interesting) by c0lo on Wednesday December 05 2018, @05:28PM (8 children)

          by c0lo (156) Subscriber Badge on Wednesday December 05 2018, @05:28PM (#770164) Journal

          A big quake can displace rock by meters.

          "Can" but it's not required to.

          It can be a displacement of only centimetres, all that matters is the energy released. Very hard rocks will accumulate more energy before the sudden fracture. A very quick displacement will produce the same "big quake" effect over smaller displacement values.

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          • (Score: 1) by khallow on Wednesday December 05 2018, @06:17PM (7 children)

            by khallow (3766) Subscriber Badge on Wednesday December 05 2018, @06:17PM (#770197) Journal

            It can be a displacement of only centimetres,

            Ok, show me an example of that.

            Very hard rocks will accumulate more energy before the sudden fracture.

            Not a couple orders of magnitude difference.

            • (Score: 2) by Osamabobama on Wednesday December 05 2018, @11:06PM

              by Osamabobama (5842) on Wednesday December 05 2018, @11:06PM (#770322)

              Here's an example: Slow Slip Events [gns.cri.nz]

              In the New Zealand slow slip events, large areas of land have been observed to move eastward by up to 30mm over days, weeks, or months. Some scientists believe that these movements can shift stress within the Earth's crust and trigger earthquakes, so they are not necessarily benign events.

              So the tectonic plates move a little, slip in some places but not in others, build up stresses, and perhaps cause stronger earthquakes. Maybe that happened in Madagascar, maybe not. It seems a bit early to dismiss it out of hand, though.

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            • (Score: 2) by c0lo on Thursday December 06 2018, @01:44AM (5 children)

              by c0lo (156) Subscriber Badge on Thursday December 06 2018, @01:44AM (#770409) Journal

              It can be a displacement of only centimetres,

              Ok, show me an example of that.

              Deep focus Earthquakes [wikipedia.org] - characterized by almost no surface waves at all and little reverberations - thus the waves travel long distances. They are usually less destructive due to the absence of surface waves, but can be still in "the big ones" category by the amount of energy released.
              By themselves, they can occur with no tectonic fault displacement (but are usually the cause of subsequent shear-stress shallow focus quakes by the extra strain they cause in nearby faults).
              They are mainly caused by chemical (lost of crystallographic water or thermal runaway) and/or phase (under pressure, the material transit into a more denser crystallographic form) transformations.

              (a deep-focus quake can be another possible explanation for the one that's the TFA subject. Still, the almost monochrome frequency is weird).

              Very hard rocks will accumulate more energy before the sudden fracture.

              Not a couple orders of magnitude difference.

              Given the range of temperatures between the upper and lower layer of the crust, definitely there'll be order of magnitude differences.
              The deep focus quakes will involve rocks in almost their plastic phase (fracturing rock aren't the cause), the shallow quakes will involve hard rocks.

              --
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              • (Score: 1) by khallow on Thursday December 06 2018, @03:21AM (4 children)

                by khallow (3766) Subscriber Badge on Thursday December 06 2018, @03:21AM (#770458) Journal
                On that matter, we have this from the National Geographic story:

                Finally, chugging along at the end come slow, long-period surface waves, which are similar to the strange signals that rolled out from Mayotte. For intense earthquakes, these surface waves can zip around the planet multiple times, ringing Earth like a bell, Hicks says.

                However, there was no big earthquake kicking off the recent slow waves. Adding to the weirdness, Mayotte's mystery waves are what scientists call monochromatic. Most earthquakes send out waves with a slew of different frequencies, but Mayotte's signal was a clean zigzag dominated by one type of wave that took a steady 17 seconds to repeat.

                Surface waves are indicative of near surface earthquakes.

                • (Score: 2) by c0lo on Thursday December 06 2018, @03:41AM (3 children)

                  by c0lo (156) Subscriber Badge on Thursday December 06 2018, @03:41AM (#770467) Journal

                  these surface waves can zip around the planet multiple times, ringing Earth like a bell, Hicks says.
                  However, there was no big earthquake kicking off the recent slow waves....

                  vs my:

                  The hypothesis of a 'reconfiguration' of the crust as a whole may explain the large range of this one and the regularity of the wave... this may be a resonating wave ringing the "globular bell"of the Earth's crust.

                  You know? Like a PET bottle one has stepped on and which slowly gets to a configuration closer to the non-deformed shape, occasionally popping when a crease gets smoother.

                  Still a hypothesis (which I don't have time or resources to investigate).

                  --
                  https://www.youtube.com/watch?v=aoFiw2jMy-0 https://soylentnews.org/~MichaelDavidCrawford
                  • (Score: 1) by khallow on Thursday December 06 2018, @07:34PM (2 children)

                    by khallow (3766) Subscriber Badge on Thursday December 06 2018, @07:34PM (#770803) Journal
                    Let us also note that tidal forces create larger distortions as well.
                    • (Score: 2) by c0lo on Friday December 07 2018, @01:45AM (1 child)

                      by c0lo (156) Subscriber Badge on Friday December 07 2018, @01:45AM (#770982) Journal

                      Let us note that those distortions are present with or without post glacial rebound.
                      Expecting the rebound to have no detectable effect on global scale is unreasonable.

                      --
                      https://www.youtube.com/watch?v=aoFiw2jMy-0 https://soylentnews.org/~MichaelDavidCrawford
                      • (Score: 1) by khallow on Friday December 07 2018, @07:00AM

                        by khallow (3766) Subscriber Badge on Friday December 07 2018, @07:00AM (#771059) Journal

                        Let us note that those distortions are present with or without post glacial rebound.

                        Those far greater distortions are present. And this weird thing doesn't happen twice a day.