Rotational seismology in short

Rotational seismology in short
Always wanted to know the basics of rotational seismology? Follow us into this short blogpost written by our ECS team member Andreas Brotzer.

Let’s spin it … or rather rotate it. Most of us enjoy a good spin on a carousel or rollercoaster. Well, we are actually doing this permanently since we live on a rotating planet!

Lucky enough, we don’t feel it very much because the rate of rotation is rather constant. But if one looks closely, there are variations indeed. And variations in the rotation rate of the Earth are equivalent to changes in the length of the day. This is  actually rather a geodetic than seismological topic, but it is why large-scale ring laser gyroscopes come into play. These optical, high-sensitive instruments are capable of observing the change in length of day while sitting on Earth. Apart from that, they are also sensing local ground rotations.

And NOW it’s becoming interesting for a rotational seismologist! Seismology is based on elastic wave theory, which not only predicts translational motions, but also the gradient of the wavefield consisting of rotations and strain. So, whenever there are seismic waves arriving wherever you are the ground not only moves sideways or up-/downwards, but also rotates, or rather tilts, dynamically. These movements are very small and hard to observe, which is why they were neglected for a long time. With new technology evolving, such as large ring laser gyroscopes, we are now able to observe these motions and use the additional information. Combining a classic seismometer and a rotational sensor provides observations of 6 degrees-of-freedom as a point measurement. As a result, we can for instance estimate the backazimuth of plane waves, hence the direction they are coming from. One also obtains information about the local phase velocities in the subsurface or can separate the different seismic polarizations (e.g. Love and Rayleigh waves) due to the specific translation-rotational motions they cause. Rotational seismology is a rather new field of research, but it will definitely provide more seismological insights in the future.

Check out these figures for a visual description of the concepts of rotational seismology:


Figure 1: Left side – Schematics of the large-scale ring laser gyroscopes of ROMY (= ROtational Motions in seismologY) (Hand, 2017, Science). Right side – Three axes for translational u and rotational w ground motions.


Figure 2: Polarization analysis using a 6 degree-of-freedom station with rotational and translational recordings to separate e.g. Love and Rayleigh waves.


Figure 2: Transverse acceleration and vertical rotation rate record of the Tohoku-Oki earthquake in Japan in 2011.


Merry Christmas!


This post was part of the seismology ECS advent calendar 2023. Do you want to see more content from the advent calendar? Check out our Instagram @EGU_seismo

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We are a team coming from mixed places with various interests: Dinko is a PostDoc at the University of Zagreb and is the first point of contact for the group. Angel is doing her DPhil at ETH Zurich in Switzerland and is managing our Instagram account. Bruna is doing her DPhill in DIAS, Dublin and takes care of our Twitter account. David is a PostDoc at the University of Lisbon and is spearheading the Campfires. Andreas is pursuing his PhD at Ludwig-Maximilians University in Munich and is helping run the blog. William is a PhD student at the University of Southampton in the UK and is also helping with the blogs. Katinka is a PhD student at ETH Zurich and is also on the blog team. Foivos is our newest member and is a PostDoc at the Istituto Nazionale di Geofisica e Vulcanologia (INGV) in Bologna. Matthew is finishing up his PhD at Oxford University, UK, and is editor of the blogs.

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