Paper of the month — The origin of volcano-tectonic earthquake swarms by Roman and Cashman (2006)

Paper of the month — The origin of volcano-tectonic earthquake swarms by Roman and Cashman (2006)

We are pleased to propose you a new Paper of the Month written by Dr. Derek Keir on volcano seismology.

Derek’s PhD thesis was on the “Seismicity of the Ethiopian rift” and conducted at Royal Holloway University of London under the supervision of Prof. Cindy Ebinger and Prof. Graham Stuart of the University of Leeds. Towards the end his PhD studies, the Dabbahu rifting episode started (September 2005) and formed much of the focus of his research for a decade. During 2006 and 2007 he worked as a teaching fellow at Royal Holloway, and then went on to a three year NERC fellowship at the University of Leeds during 2008-2010. He there worked with Prof. Tim Wright’s InSAR group to integrate seismic and geodetic constraints on dike intrusion. Since 2011, he has been a lecturer, and then from 2015 associate professor at the University of Southampton. Since 2016 he also holds the position of associate professor at the University of Florence. He works on a range of tectonic and volcanology problems, mainly in extensional settings.

I have decided to write about the 2006 Geology paper titled “The origin of volcano – tectonic earthquake swarms” by Roman and Cashman since it provides an exceptionally eloquent summary of how earthquake locations and focal mechanisms can be used to interpret magma dynamics, and why different volcanoes or volcanic settings show varying seismic characteristics. The paper was initially very useful for me personally since it was published near the start of the 2005-2010 Dabbahu rifting episode (e.g. Wright et al., 2005; Keir et al., 2009) and provided me, at the time very much a volcano novice, with a clear and concise picture of how to interpret the high-frequency seismic signals so commonly associated with magma motion. I have since recommended it as reading to a large proportion of my PhD and masters level students.

The use of earthquakes is an important tool in volcanology and volcano monitoring (Sparks et al., 2012), since the motion of magma in the Earth’s crust causes localised stress changes that can induce failure on new or pre-existing fractures near the intrusion. The majority of the earthquakes are called volcano-tectonic (VT) earthquakes because the individual waveforms have an appearance, with clear P- and S-wave onsets and high frequency content, the same or similar to regular tectonic earthquakes occurring on faults with slip not induced by magma motion (Roman and Cashman, 2006).

The location of these VT earthquakes can potentially provide clues to where magma is moving, and the earthquake focal mechanisms can provide clues to the type of fault slip, from which the orientation of the stress field can be inferred.

Despite the relatively simple idea that magma can stress the rock enough to cause an earthquake, variable and complex patterns of earthquakes in space and time can be observed around the magma bodies inside real volcanoes. The fundamental reasons for the variable distribution in space and time of VT earthquakes, and also the different types of focal mechanisms observed at different volcanoes were previously very difficult to discern in the published literature.

This paper starts off by providing the best summary I have read to date on three fundamental models of VT earthquakes by integrating the location of earthquakes relative to the associated intrusion with the orientation of the stress field and resultant focal mechanism. The paper describes that VT seismicity is commonly caused by stresses induced near the tip of a propagating intrusion, with the focal mechanisms consistent with the regional tectonic stress orientation. In this model the earthquake activity moves through time and tracks the position of the leading edge of the new intrusion.

The more important of the 2 alternative models is that for an inflating intrusion. In this model the earthquakes can be distributed all around the magma body, with no time migration. The compression created by the magma inflation against the wall rock can act against regional tectonic stresses to locally rotate the principal stresses, which can be inferred from a 90 degree rotation in earthquake focal mechanisms. The description of the various models is supported by a fantastic figure that incorporates all these elements, and is even directly useable in all three different types of regional stress fields by simply rotating the diagram.

The paper then draws in information from various examples of seismicity during volcanic eruptions in order to interpret fundamental controls and driving mechanisms of earthquakes, with links to magma rheology and dynamics. The major outcome is that the examples of a lack of hypocenter migration but with stress field rotation occurs before the eruption of magmas that undergo extensive crystallization during ascent and are commonly of intermediate composition. They suggest various mechanisms of increased normal stress at the intrusion wall that ultimately causes a stress field rotation including shear dilatency of magma and vesiculation of bubbles in the melt. In contrast, examples of migrating hypocenters with no stress field rotation are commonly associated with basaltic magma where stress changes associated with intrusion dilation are low compared to regional tectonic stresses. In such settings, amplification of regional stresses at the leading edge of relatively rapidly propagating intrusions causes the migrating earthquake pattern.

Since the publication of this paper the volcanology community has seen a rapid increase in the numbers of multidisciplinary studies at volcanoes that include ever more dense deployments of monitoring equipment and inclusion of satellite derived measurements of gas release and deformation (e.g. Sigmundsson et al., 2015). As predicted towards the end of the Roman and Cashman paper these new studies are providing ever better constraints on the forces associated with magmatic processes and how these interact with regional stresses in order to fully understand how magma interacts with rock. Despite these developments this paper still remains an extremely insightful piece of research that should be the starting point for all volcanologists wishing to use earthquakes to understand how magma moves.



Keir, D., Hamling, I.J., Ayele, A., Calais, E., Ebinger, C., Wright, T.J., Jacques, E., Mohamed, K., Hammond, J.O.S., Belachew, M., Baker, E., Rowland, J.V., Lewi, E. and Bennati, L, 2009, Evidence for focused magmatic accretion at segment centers from lateral dike injection captured beneath the Red Sea rift of Afar, Geology, 37, 59-62.

Roman, D.C., and Cashman, K.V., 2006, The origin of volcano-tectonic earthquake swarms, Geology, 34, 457-460, doi: 10.1130/G22269.1.

Sigmundsson, F., and 37 others, 2015, Segmented lateral dyke growth in a rifting event at Bardabunga volcanic system, Iceland, Nature, 517, 191-195.

Sparks, R.S.J., Biggs, J. Neuberg, J.W., 2012, Monitoring volcanoes, Science, 335, 1310-1311.

Wright, T.J., Ebinger, C., Biggs, J., Ayele, A., Yirgu, G., Keir, D., Stork, A., 2006, Magma-maintained rift segmentation at continental rupture in the 2005 Afar dyking episode, Nature, 442, 291-294.

Paper of the Month – Bubbles and seismic waves

Modified figure based on “Tiny Bubbles” by frankieleon 

Our paper of the month is  Bubbles attenuate elastic waves at seismic frequencies: First experimental evidence” (N. Tisato et al., 2015) commented by Luca De Siena.

Luca De Siena is Lecturer in Geophysics at the School of Geoscience, University of Aberdeen (UK). He received his PhD from the University of Bologna (Italy) with a scholarship from the INGV-Osservatorio Vesuviano for his work on seismic attenuation imaging of Mount Vesuvius and Campi Flegrei volcanoes. During his postdoc at the Institut für Geophysik, Westfälische Wilhelms Universität (Münster, Germany), Luca worked on the development of novel imaging techniques using stochastic wave propagation, whose application has led to novel attenuation and scattering models of Deception Island (Antarctica), Tenerife (Spain), and Mount St. Helens (US) volcanoes. His research interests include the development and application of attenuation and scattering tomography at lithospheric and mantle scales, and in sub-basalt/reservoir settings.

Luca will present us a paper by Tisato et al. that finally provides experimental evidence on the effects of fluids and gasses on seismic attenuation. The results nicely connect seismology with rock physics, and are important for any seismologist interested in using amplitude information to track fluids in settings, like volcanoes and reservoirs, where they represent a clear hazard/resource. The paper gives insight into processes that open a new seismology-rock physics research path, and better connects our Division with Geochemistry and Volcanology.

“Seismic attenuation is an outstanding tool to image the physical and thermal properties of the lithosphere, particularly in volcanic areas. But any seismologist studying and imaging attenuation in 3D is aware of a long-standing issue with researchers in different disciplines, such as petrology and volcanology: they want magma, and they will see it in our model. Since attenuation is so sensitive to hot structures and physical changes they will just pick an anomaly and model a sill.

Probably, also the seismologist wants that anomaly to be magma, in order to publish the highest-impact journals and be highly cited. For the average reader and the editor of these journals, there is in fact an ocean (of interest) between the “Seismic attenuation imaging of Yellowstone magma sill” and the “Seismic attenuation imaging of a high-attenuation domain under Yellowstone caldera that could be a magma sill/fluid reservoir/hot rock topping melting, please pick one”. The truth is we still have a long way to be able to characterize that domain in terms of magma/fluids/heterogeneity just by looking at seismic attenuation.

In their paper, Tisato et al. take a step towards this direction by concentrating on bubbles: in a laboratory, they prove that these microscopic objects are able to attenuate seismic waves at frequencies we use in the field. In addition, the best way to model this attenuation for imaging purposes is wave-induced-gas-exsolution-dissolution (WIGED), which I knew was an effective model to reproduce high seismic attenuation in magmas. Finally, a way to prove that magma fills all my low-Q areas? Not so much.

Bubbles are in fact crucial ingredients to model attenuation in fluids, and their relative percentage reduces and distorts seismic amplitudes in ways I have seen in seismic volcanic waveforms. I first read the paper with amazement at what our colleagues in rock-physics can actually pull out today. They can reproduce the physical processes I have been using throughout my career for imaging the Earth in their laboratories. The demonstration that the WIGED model is most effective to describe attenuation provides us with an ideal analytical input to image the Earth with attenuation, linking to petrological quantities related to the physical and chemical state of the Earth. The study thus provides us with an opening to multi-scale laboratory- field imaging techniques using attenuation.

The main strength of the work other researchers and industry will see is its application to fluid/gas monitoring. The use of seismic tomography based on WIGED is potentially a novel 4D technique better apt to monitor hazardous volcanic and reservoir structures. To me, the paper is the demonstration that seismology can aim to characterize the Earth and its complex processes at scales so far unexplored, once correct theoretical models and experimental evidences are provided, providing more reliable constraints to other disciplines.

The questions that came to my mind after reading it: Is the scale and level of heterogeneity in the laboratory the same I use in forward modeling? What if, with their results, I would be able to apply attenuation imaging to sample scale? And maybe use poroelasticity to link Q with porosity and permeability? A paper that lets you with so many ideas and is so good at connecting seismology with other disciplines is rare, and certainly worth reading.”

Reference: Tisato, N., Quintal, B., Chapman, S., Podladchikov, Y., & Burg, J. P. (2015). Bubbles attenuate elastic waves at seismic frequencies: First experimental evidence. Geophysical Research Letters, 42(10), 3880-3887.

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

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?


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.