• Question: What kind of waves are present during an earthquake? ...

    Asked by anon-252499 on 30 Apr 2020.
    • Photo: Malgorzata Drwila

      Malgorzata Drwila answered on 30 Apr 2020:

      Finally a question for me! I was working as a seismologist for some time, analyzing natural and human-induced earthquakes.

      The earthquakes produce seismic waves and there are few types of them.

      The fastest waves are acoustic waves, called also primary waves (because they arrive to seismometers as the first ones). There are the same type of waves that people make while speaking or that musical instruments make, but for earthquakes acoustic waves have much lower frequencies.

      Then there are secondary waves, they arrive as the second ones. they are also called shear waves, because they wiggle in the direction perpendicular to their propagation. By measuring the time difference between arrival of primary wave and secondary wave it is possible to calculate how far the earthquake happened.

      The last ones to arrive are the surface seismic waves, and those are the most dangerous for buildings. They are usually the strongest and they cause the biggest damage. They are formed out of primary and secondary waves reaching the surface.

      Also earthquakes can cause tsunami waves in the ocean, but those are not seismic waves.

    • Photo: Susan Cartwright

      Susan Cartwright answered on 30 Apr 2020:

      I think Malgorzata’s got this one covered! The only thing I would add is that comparing the different types of waves from distant earthquakes is what allows us to understand the interior structure of the Earth. For example, secondary (S) waves will not pass through liquid, and this causes a “shadow” region on the other said of the world from the earthquake where S waves are not seen, because to get there they would have to travel through the Earth’s liquid iron outer core, and they can’t. This is what tells us that part of he Earth’s core IS liquid. Because the density of the rock affects the speed at which the waves travel, we can also use the arrival times of different waves to map out how the density of the Earth varies with depth.

    • Photo: Paula Koelemeijer

      Paula Koelemeijer answered on 30 Apr 2020:

      Something to add about these different types of waves: P-waves, S-waves and surface waves (one type which are called Love waves!) are all travelling waves – which means that they travel from the source (earthquake) to the receiver (seismic station).

      However, when we have very large magnitude earthquake, we also get a different type of wave – standing waves! This is because the waves have so much energy that they can circle the Earth multiple times and build up a standing wave pattern (through something called constructive interference). These standing waves (called Earth’s free oscillations) can last for weeks or months after large earthquakes. Rather than measuring the arrivals of these waves, we measure their frequency of oscillations, effectively the resonance frequencies of the Earth!

      Similarly to musical instruments, where the sound depends on the thickness of length of a string, these resonance frequencies depend on the material inside the Earth. We can therefore learn about the interior of the Earth by measuring these resonance frequencies! One of the most widely models of the Earth (called PREM) was made primarily from these types of observations.

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