• Question: What is a seismic wave?

    Asked by anon-252841 to Ry, Paula on 4 May 2020.
    • Photo: Ry Cutter

      Ry Cutter answered on 4 May 2020:

      Seismic waves, are waves that travel through the Earth. I’m not an expert on this area, Paula should be able to give a much better answer! Earthquakes are a big problem for gravitational wave detectors though. They have to be sensitive enough to detect changes of a distance of less than a proton! They do this by suspending mirrors in a very complicated way. If there’s an earthquake, the mirrors shake entire centimetres, this doesn’t sound like a lot, but it’s 10000000000000 times too much movement! This means seismology is really important so we can predict when Earthquakes are coming so we can minimise the damage to our measurements!
      –
      We can also see similar seismic waves in the sun and other stars! We call these studies, helioseismology (our Sun), and astroseismology (other stars). These waves let us see properties inside of the star, like how dense the star is and how the material is being churned inside of it! This is all using the same principals of seismolgy!
      –
      We’ve also been able to find tectonic plates in other planets. I believe we’ve even measured marsquakes! Seismology is a very useful and important science!
      –
      Brilliant Question,
      Ry

    • Photo: Paula Koelemeijer

      Paula Koelemeijer answered on 4 May 2020:

      Seismic waves are generally seen as elastic waves, so that after they have passed through a material, there is no deformation (the medium returns back to its earlier state). There are a lot of different seismic waves though, we firstly class these between travelling waves (like a sound wave while you talk) and standing waves (like the waves on the string of a musical instrument).

      Travelling waves are the waves that people generally think about – there are body waves – P and S waves (the primary and secondary waves) that travel throughout the Earth (including the core). These are first arrivals and can be used as early warning for the larger waves that come later. These surface waves only travel along the surface of the Earth and therefore do not loose their energy as quickly and keep having larger amplitudes. Because of this, they cause most damage in earthquakes! All these waves travel with velocities of a few km/s, so that it takes about 20 min to reach the other side of the Earth!

      Standing waves only occur after really large earthquakes (magnitude 7.5 or larger), because the travelling waves continue to travel the Earth and build up constructive interference patterns! Effectively, the Earth is resonating with long periods of minutes! The entire Earth deforms during these oscillations, but the movements are only small.

      Similar to the helioseismology Ry talked about, we can use seismic waves to image the interior of the Earth, and there is a lot of overlap between these different fields. We have to work together to get our instruments to other planets, so that we can observe quakes on the Moon and Mars – called moonquakes and marsquakes! The Marsquakes are only a recent thing, and hopefully will tell us more about the interior of Mars!

      Let me know if you want to know more or some links with information!

Comments