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Earthquakes felt by Eeyore

Earthquakes felt by Eeyore

“It’s snowing still,” said Eeyore gloomily.
“So it is.”
“And freezing.”
“Is it?”
“Yes,” said Eeyore. “However,” he said, brightening up a little, “we haven’t had an earthquake lately.”
A.A. Milne, The House at Pooh Corner (1928)

The above is a quote from one of two classic Winnie-the-Pooh books by A. A. Milne. Upon reading this, you might experience one or more of the following sensations:

  • You are repulsed by the fact that this seismology blog entry starts with something as juvenile as Winnie-the-Pooh (In this case: bear with me, it will make sense)
  • You think nothing too much of it and continue reading
  • You are charmed by the ‘Always look on the bright side’-message from the gloomy Eeyore
  • You are utterly confused as to where the hell this is going

Personally, I experienced the ‘charmed’ option. However, it was quickly followed by the following thought process:

So cool that there is a mention of an earthquake in a classic book! … Milne’s Winnie-the-Pooh books take place in the fictional Hundred Acre Wood … This is generally accepted to be a fictionalisation of the Five Hundred Acre Wood in Ashdown Forest in East Sussex in the South of England … Wait … “we haven’t had an earthquake lately” … Since when are there earthquakes in Sussex such that they can be felt by humans (if not stuffed animals) multiple times during their lifetime?!

So, that, my dear readers, is what brought me to my latest google-escapade: finding out more about historic earthquakes in East Sussex. Wouldn’t you love to be me with all these exciting hobbies? I thought so.

Let’s first define what we are exactly trying to figure out: Were there any earthquakes (felt) in Sussex in the time frame 1925-1928? We pick this time frame as Milne first moved to East Sussex in 1925 to a country home, Cotchford Farm in Hartfield. Hence, any earthquakes felt there by Milne, his son (Christopher Robin, I kid you not) or his son’s stuffed animals (i.e., Eeyore) must be felt after 1925. In 1928 the book was published, so earthquakes must be felt before then.

Within this time frame, there are only 4 earthquakes listed in the UK Historical Earthquake Database. 2 of which are in Scotland with a local magnitude of ~4 and hence cannot be felt in East Sussex. Another earthquake is in the North Sea between Scotland and Norway, so again, no chance of Winnie-the-Pooh feeling this one. The 1926 earthquake in Ludlow (West Midlands, epicenter ~235 km from Cotchford Farm) with local magnitude 4.8 is already quite a bit closer to East Sussex. Several people from Somerset to Lincoln reportedly felt an earthquake intensity of 5 on the Medvedev-Sponheuer-Karnik scale (= felt indoors by most, outdoors by few, Medvedev et al., 1964). However, there is no mention of people in East Sussex feeling this particular earthquake.

And so, we have exhausted our first line of inquiry.

But worry not, dear readers! We can also look at the list of Significant British Earthquakes that the British Geological Survey provides us with!

This list gives us 4 significant earthquakes that were felt in the UK between 1925 and 1928. One of them is the aforementioned 1926 Ludlow earthquake, and another one happened in 1925 in Brittany, which was felt as far east as the Weymouth area in Dorset (which is much farther west than East Sussex). That leaves us with 2 other interesting earthquakes: the Channel Islands earthquakes that occurred in 1926 (ML5.5) and 1927 (ML5.4) which both had the same epicenter ~246 km from Cotchford Farm. The 1926 earthquake was reportedly felt as far east as Hove (~35 km from Cotchford Farm), and the 1927 earthquake was felt at low intensities as far east as Worthing (~45 km from Cotchford Farm) and even in London (lending credibility that it could also be felt at Cotchford Farm).

In case you forgot where Sussex was. Intensity distribution Channel Islands earthquake on February 17, 1927 (figure obviously modified from Ambraseys et al., 1985). Click to enlarge.

But why stick to possibilities, maybes, and mights, when we can actually do the math (or rather calculation) to figure out if Eeyore and his friends felt any of these earthquakes? Musson (2005) proposed an equation to calculate the intensity attenuation in the United Kingdom on the European Macroseismic Scale (Grünthal, 1998) for earthquakes recorded in the local magnitude. And today is our lucky day, because the earthquakes listed in the catalogs of the British Geological Survey are indeed listed in local magnitude*. Hooray!

Using the equation:

I = 3.31 + 1.28ML – 1.22lnR

where R is the hypocentral distance, we calculate the intensities at Cotchford Farm for the Ludlow earthquake and the two Channel earthquakes. We find that the Ludlow earthquake had an intensity of 2.8, the 1926 Channel Islands earthquake had an intensity of 3.6 and the 1927 earthquake had an intensity of 3.5. On the European Macroseismic Scale (Grünthal, 1998), intensities of 2 are considered ‘Scarcely felt – only at isolated instances of individuals at rest and in a specially receptive position indoors’, an intensity of 3 is ‘Weak – felt indoors by few; people at rest feel a swaying or light trembling’, and an intensity of 4 is ‘Largely observed – felt indoors by many, felt outdoors by very few’. As stuffed animals are usually inside and at rest (who really knows?) it is likely that Eeyore and his friends felt these earthquakes! This means that Eeyore is referring to these two Channel Islands earthquakes and perhaps also the Ludlow earthquake in The House at Pooh Corner.

So now, whenever you are reading Winnie-the-Pooh (either to yourself or to children) keep in mind that Winnie-the-Pooh, Eeyore, Piglet, Owl, Rabbit, Kanga, Roo, Tigger and of course Christopher Robin himself, might have felt these 1926 Ludlow and 1926, 1927 Channel Islands earthquakes while playing in the Hundred Acre Wood; making Eeyore justifiably relieved in 1928 that they haven’t had an earthquake lately.**


* Don’t worry if you would ever like to do something similar as I do here with your earthquakes and they are not in local magnitude! Musson (2005) provides an equation to convert local magnitude and moment magnitude.
**Of course, you could argue about the long-term memory capacity of stuffed animals, but let’s not get into that now, shall we?


References:

Ambraseys, N. “Intensity‐attenuation and magnitude‐intensity relationships for northwest European earthquakes.” Earthquake engineering & structural dynamics 13.6 (1985): 733-778.
Grünthal, G. (ed.), 1998, “European Macroseismic Scale 1998”, Cahiers du Centre Europèen de Gèodynamique et de Seismologie,15, Conseil de l’Europe, Luxembourg.
Medvedev, Sergeĭ V., Wilhelm Sponheuer, and V. Karnik. “Seismic intensity scale version MSK 1964.” United nation educational, scientific and cultural organization, Paris (1965): 7.
Musson, R. M. W. “Intensity attenuation in the UK.” Journal of Seismology 9.1 (2005): 73-86.

Harsher than reviewer 2?

Harsher than reviewer 2?

Have you ever wanted a reviewer who really tells it how it is? You should consider submitting a paper to the truly special publication ‘Frontiers for young minds’.

Frontiers for young minds  is a journal for students between ages 8 and 15 that are curious and passionate about science. However, what’s truly special about this journal is that it is also reviewed by students of the same age, assisted by a science mentor. The journal aims to communicate cutting edge science to young readers in a way that they find both understandable and interesting. Therefore, kids and teenage “young reviewers” are called upon to make sure that complex terms are explained or weeded out, basics are introduced at the beginning, and also that the article is an exciting read.

In going through this process, the young reviewers are supposed to learn about science and the process of peer review, while the scientists who wrote the article receive feedback about their science communication skills and how much their science appeals to an open-minded lay public. At the same time, the journal is building up a collection of texts that can be used by science teachers and interested lay persons – and that are hopefully more exciting and up-to-date than many schoolbooks can ever aim to be. Many reviews are actually performed by school classes who work on it together as a project.

The first articles were published in 2014; the journal is open-access and financially supported by the Jacobs foundation. This enables the journal to make submissions free for authors. Articles are subdivided into thematic groups. “Core concept” articles lay the foundation for young readers to understand the more current contributions, or “new discoveries”, based on recently published papers.

The journal for kids is the junior branch of a “grown up” series of open access journals called Frontiers. Frontiers is itself a young publication series, having started out in 2007. While several frontiers journals such as Frontiers in Neuroscience are widely known and highly ranked among the open access journals of their respective fields, Frontiers in Earth Science, which started out in 2013, has published only about 250 articles so far, and has yet to be assigned an impact factor. Thus, it is not surprising that most articles featured in Frontiers for young minds come from the fields of neuroscience and other medical research fields. Still, the section ‘Understanding the Earth and its resources’ features articles relating to geoscience, in particular environmental science. Who knows who will write and review the first contribution in seismology?

While the idea behind the journal is great – imagine how excited you would have been as a kid if the editor of National Geographic wrote to you to ask your opinion on the latest article about Polar Bears? – it obviously also provides a convenient platform for Frontiers to raise their visibility with a new generation of authors and/or their scientist parents. An open question for me is how well the young reviewers are made aware that peer review is not only a process that should embellish the language of articles and make them more readable, but is most importantly an instrument of critical and sometimes fierce scientific debate. It does not become quite clear either whether the young editors are granted the power to flatly reject a submission if they do not like it!

What is certain, though, is that school kids make the perfect reviewers. A blog associated to the publication lets us read some of the young reviewers’ comments on submitted manuscripts. While some politely draw attention to the fact that basic experimental procedures are undocumented –

“It would be helpful if they told us how they took the measurement of brains without actually having to remove the brain.”

others find more direct words about the quality of the manuscript:

“This seems important, but the way it is written is so boring I can’t even get to the end.”

Wouldn’t you have liked to write that under one review or the other…

We are excited to see who will be the first seismologist to brave the harsh review of a classroom full of nine-year-olds! You can have a look at the author guidelines here. Good luck! And let us know if you get published.


Edited by ECS representatives Laura Ermert and Matthew Agius.

MOVEMBER! Cancer awareness and seismology.

MOVEMBER! Cancer awareness and seismology.

Two years ago Matthew Agius, the previous ECS-rep of the Seismology Division, wrote a post in this blog about Movember. Movember is a month-long event (in November) during which men grow their moustaches to rise awareness of cancer, especially the ones affecting men.

When at the end of the month Matthew was finally getting rid of his moustaches, he wondered which seismologists had nice fluffy moustaches. Here what he found:

Robert Mallet

Robert Mallet

Robert Mallett: Ireland’s ‘father of seismology

John Milne

John Milne

John Milne: Famous for inventing the horizontal pendulum seismograph.

Meeting of world seismologists at the California Institute of Technology, Seismological Laboratory in 1929.

Meeting of world seismologists at the California Institute of Technology, Seismological Laboratory in 1929.

It seems that having a moustache was the trend a century ago, irrespective of the shape and style; from “Handlebar” to “Horseshoe”, “Imperial” and “Mexican” (check out wikipedia). Look at this Meeting of world seismologists at the California Institute of Technology, Seismological Laboratory in 1929.

Andria Mohorovicic

Andria Mohorovicic

Matthew’s favourite is Mo Bro is none other than Andrija Mohoroviči, known for the Mohorovičić discontinuity and is considered a founder of modern seismology. That’s right, if you want to portray yourself as a modern seismologist he is your moustache model man.

Now it is your turn to get involved: name a moustached seismologist! This can be:

  • a moustached seismologist from the past we didn’t mention above;
  • a seismologist currently active and normally wearing moustaches;
  • a seismologist involved in this current Movember. You can even name yourself. If this is the case, show your pride with a picture on our Facebook page commenting this post!

Post edited by Laura Parisi and Laura Ermert.

Paper of the month — Signal apparition for wavefield separation

Paper of the month — Signal apparition for wavefield separation

Our paper of the month is  “Signal apparition for simultaneous source wavefield separation” (J. Robertsson et al., 2016) commented by Andreas Fichtner.

Andreas Fichtner is Assistant Professor for Computational Seismology at the Swiss Federal Institute of Technology (ETH) in Zurich. He received his PhD from the University of Munich for his work on Full Seismic Waveform Inversion for Structural and Source Parameters. During his postdoc at Utrecht University, Andreas worked on the development of resolution analysis and multi-scale methods for seismic waveform inversion.

His research interests include the development and application of methods for full seismic waveform inversion, resolution analysis in tomography, earthquake source inversion, seismic interferometry, and inverse theory. For his work, Andreas received the Keiiti Aki Award 2011 from the American Geophysical Union and the Early Career Scientist Award from the International Union of Geodesy and Geophysics.

In his paper of the month post, Andreas will present us a recently published paper by Robertsson et al. that describes a new approach to the magical art of source separation – or how to disentangle seismic signals from sources that acted at the same time!  Sounds impossible? Not for an exploration geophysicist!


“One of the most longstanding problems in exploration geophysics is the separation of two wavefields emitted by two different sources. Just imagine, for instance, that two sources are fired, emitting wavefields g(t) and h(t). A receiver records the sum of the wavefields, f(t)=g(t)+h(t). If one could separate g(t) and f(t) from their sum, the time needed for seismic acquisition could be reduced by 50 % because two sources could be fired simultaneously. This is just one of many possible applications of wavefield separation.

While most previous research on wavefield separation focused on temporal encoding of sources, Robertsson and co-workers introduce an entirely new concept that is wonderfully simple and elegant.

They start with the well-known observation that the f-k spectrum for a line of sources recorded at one receiver is restricted to a signal cone bounded by the slowest propagation speed of the medium, e.g. the propagation speed of water in a marine experiment. Thus, most of the f-k domain is empty.

Now you do a little modification to the experiment. Instead of firing all sources along the line in exactly the same way – as is usually done – all odd-numbered sources are fired with some freely chosen modified source signature, such as a filter. Magically, the signal from this modified subset of sources appears in the previously empty part of the f-k domain. From there it can be extracted without any pollution by the even-numbered sources. This ‘becoming visible’ of a wavefield is referred to as ‘signal apparition’ by the authors of the paper.

johansson_modified

While the authors limit their examples to seismic acquisition along a 2D line, many other applications could be envisioned. They include, for instance, the numerical forward simulation of seismic waves from a large number of earthquakes, as needed in waveform tomography.

I chose this paper not only because it offers a solution to a problem that has been studied for a long time, but also because of its beautiful simplicity. The approach works without any assumptions and does not require more than basic Fourier analysis to be fully understood.”


Reference: Robertsson, J. O., Amundsen, L., & Pedersen, Å. S. (2016). Signal apparition for simultaneous source wavefield separation. Geophysical Journal International206(2), 1301-1305.

Do you have questions, suggestions or comments? Please use the space below, or contact us on Facebook or Twitter @EGU_Seismo!

Are you an experienced seismologists and you want to be our next PoM author? Contact us at sm-ecs @ egu.eu

Edited by ECS representatives Laura Ermert, Matthew Agius, Lucia Gualtieri and Laura Parisi.
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