SM
Seismology

Laura Parisi

Laura is a post-doctoral fellow at King Abdullah University of Science and Technology. Her scientific interests lie in the field of the passive-source seismic imaging across scales. She is enthusiastic to be an ECS-rep to give her contribution to the SM Division especially to promote gender equality in Seismology, increase the number of ECSs actively involved in the Division’s initiatives and improve the GA experience for young seismologists.

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.

 

References

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.

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 — Seismic anisotropy

Paper of the Month — Seismic anisotropy

“SEISMIC ANISOTROPY AND MANTLE DEFORMATION: WHAT HAVE WE LEARNED FROM SHEAR WAVE SPLITTING?” (M. K. SAVAGE, 1999) commented by Dr. Jessica Johnson

 

Jessica Johnson from the University of East Anglia (UK) is our guest author of the PoM blog series of this month! She has chosen to comment on the paper “Seismic Anisotropy and mantle deformation: what have we learned from shear wave splitting?” (M. K. Savage, 1999). Firstly, let me introduce Jessica to discover why this paper is so important for her and then lets enjoy together her PoM!

At the University of Leeds, under the supervision of Prof. Neuberg, her MSci dissertation investigated the trigger mechanism of LP events at Soufriere Hills volcano, Montserrat. Jessica’s PhD thesis was titled “Discriminating between spatial and temporal variations in seismic anisotropy at active volcanoes”, and was carried out under the supervision of Prof. Savage and Dr. Townend at Victoria University of Wellington. She completed a two-year research fellowship at the Hawaiian Volcano Observatory (HVO), mainly working on shear wave splitting analysis at Kilauea and developing FEMs to explain unique patterns of ground deformation. Her second post-doctoral position was at the University of Bristol on a Marie Curie Incoming International Fellowship. Since 2015, she has been a lecturer in Geophysics at the University of East Anglia, where herresearch continues to focus around volcano geophysics.


“When deciding which paper to write about for this ‘Paper of the Month’, I flip-flopped between a classical paper and an important recent one. A lot of my research centres around seismic anisotropy (the variation of seismic wavespeed with direction) so I wanted to do the subject justice. However, the topic, and in particular the existence of temporal changes in seismic anisotropy, is hotly debated.

The first significant observation of large-scale seismic anisotropy was in 1964, when Harry Hess found that seismic refraction measurements in oceans showed that the P wave velocity of the upper mantle (Pn) was consistently higher for profiles recorded perpendicular to an oceanic spreading centre than for profiles recorded parallel to the spreading centre. The measurement of seismic anisotropy has since been found to be a proxy for determining the direction of maximum horizontal compressive stress (SHmax) in the crust; applied stress can cause microcracks to preferentially open parallel to the maximum compressive stress, creating an anisotropic medium with the fast direction parallel to SHmax. Measurements of seismic anisotropy have been used to detect fabric and stress in ice flows and in the Earth’s crust, flow in the upper mantle, topography of the core-mantle boundary and differential rotation of the inner core.

Even with this rich history of research behind it, and countless papers using and advancing the use of seismic anisotropy to understand the Earth at different levels (a google scholar search showed that over 100 papers have been published with seismic anisotropy or shear wave splitting in the title in 2016 alone), there is still much that is unknown about the phenomenon. As such, I have chosen what I consider a classical and extremely important paper by Professor Martha Savage: “Seismic anisotropy and mantle deformation: What have we learned from shear wave splitting?” It is a review paper, being published in Reviews of Geophysics, but it highlights some of the ongoing questions, which even 17 years on have not been completely answered. It is this aspect of the paper that I find so inspiring. This paper does not pretend to know all of the answers but it is an honest account of the state-of-the-art, which encourages the continued interrogation of the way we understand the Earth. I first read this paper when preparing for my PhD, and have referred to it frequently since. It is usually the first paper that I point new students towards as it not only gives a concise overview, but it is refreshingly still relevant. While Savage concentrates this paper on mantle deformation, most of the ongoing questions are relevant for seismic anisotropy studies on all scales.

Shear wave splitting in an anisotropic crust. Anisotropy is caused by preferentially aligned cracks due to a maximum horizontal compressive stress (SHmax). A vertically propogating shear wave that is arbitrarily polarised gets split into a fast wave with polarisation (φ) parallel to crack alignment, and a slow wave, which is polarised at 90° to φ. The waves are seperated with delay time δt.

Shear wave splitting in an anisotropic crust. Anisotropy is caused by preferentially aligned cracks due to a maximum horizontal compressive stress (SHmax). A vertically propogating shear wave that is arbitrarily polarised gets split into a fast wave with polarisation (φ) parallel to crack alignment, and a slow wave, which is polarised at 90° to φ. The waves are seperated with delay time δt.

In essence, the theme of this paper is the interpretation and inferences made from the measurement of shear wave splitting. Shear wave splitting occurs when a shear wave travels through a seismically anisotropic medium, splitting into two orthogonal quasi-shear waves orientated according to the fast and slow directions of anisotropy. Assuming that the seismic anisotropy has been measured accurately, its existence could be due to temperature and pressure, partial melt, stress, strain history, composition and/or orientation of the material. Savage explores the evidence for each type of anisotropic mechanism in different tectonic regimes and relates the evidence to the models. The paper walks through the analytical steps of deciphering the anisotropic signal. Even here, the paper points out that assumptions or inferences must be made such as the location along the wavepath that the anisotropy occurs, the homogeneity (or heterogeneity) of the anisotropy, or the anisotropic symmetry system.

In this 1999 paper, Savage suggests that the measurement of shear wave splitting is reasonably routine, and she concentrates mainly on the achievements and challenges associated with its interpretation. Today there are numerous studies that use freely available software, following traditional methods, to measure seismic anisotropy. Some of these recent papers have a “black box” feel about them in that the authors are assuming the method is so well tried and tested that it does not need to be addressed. However, Savage also alludes to the ever increasing capability in computing technology and the fact that understanding will likely change in the future.

As with many disciplines, it seems that the more we know, the more we realise that we don’t know. Researchers (myself among them) have found it necessary to go back to the measurements themselves and ask fundamental questions such as what exactly is being measured? What artefacts exist in the measurements? What factors interfere with the measurements? Is there observer bias in the measurements? Why is there so much scatter in the measurements?

Tomographic methods, high-density arrays, sophisticated modelling and decades of seismic data have helped the community come some way toward answering the Big Questions posed by Savage such as “Where is the anisotropy really occurring?”, “What causes the observed variations of splitting parameters?” and “Is anisotropy telling us about mantle flow or lithospheric deformation, or both (or neither)?”. All of these questions are currently being addressed within the community. Indeed, it is the continuing existence of these questions that causes so much of the controversy around the use of seismic anisotropy.

The measurement of seismic anisotropy has the potential to be an extremely powerful tool in understanding the Earth at all scales. Of particular interest to some is the capacity to use seismic anisotropy to independently measure and monitor in situ stress variations in the crust, both spatially and temporally. This ability would have implications for the monitoring of active volcanoes and earthquake-prone regions, assisting in risk mitigation efforts. In addition, stress monitoring in the crust would be useful in various engineering and energy sectors.

This important review paper should be the starting point for any scholar wishing to embark on a seismic anisotropy journey. Savage not only explains the phenomenon clearly and highlights important achievements, but applies the scientific method within the review paper to emphasise the caveats and future challenges. There is also a helpful mini-tutorial in the appendix to get you started!”

Reference

Savage, M. K. (1999). Seismic anisotropy and mantle deformation: What have we learned from shear wave splitting? Reviews of Geophysics, 37(1), 65–106. article. http://doi.org/10.1029/98RG02075


Is Savage (1999) one of your favorite classic paper as well? Do you want to add anything to Jessica’s comment? Use the space below to add your comment!
Are you an experienced seismologists and you want to be our next PoM author? Contact us at sm-ecs @ egu.eu

What was going on this week in the world of the ECSeismologists?

What was going on this week in the world of the ECSeismologists?

The 2nd TIDES training school!

About 60 ECSeismologists (42% women, 58% men; 67% PhD students, 33% others) gathered in Sesimbra (Portugal) from 19th to 23th Sep 2016 to be trained on seismic noise, from the physics of noise generation to seismic interferometry. Two of us had the opportunity to attend the school and we are happy to share our experiences in the blog!

But first, what is TIDES? Ok, apart from the thing the moon does to Earth? TIDES stands for TIme DEpendent Seismology, an EU COST action that aims to structure the EU seismological community for the development of data-intensive, time-dependent techniques for monitoring Earth active processes (e.g., earthquakes, volcanic eruptions, landslides, glacial earthquakes) as well as oil/gas reservoirs. TIDES is envisioned to connect European laboratories in Academia and Industry with complementary skills and will organise a series of workshops and advanced schools to train the next generation of scientists. 

The school in Sesimbra was the second training opportunity organised by the TIDES COST action (the first one was held in Jun 2015 in Bertinoro, Italy). At this year’s school, ECSs  were taught on ambient seismic noise generation mechanisms, correlations of scattered wave fields, and their applications to imaging Earth structure. For the occasion, a large group of  experts (Erhard Wielandt, Fabrice Ardhuin, Eléonore Stutzmann, Wayne Crawford, Michel Campillo, Céline Hadziioannou, Andreas Fichtner, Andrew Curtis, Roel Snieder, Christoph Sens-Schönfelder, Lapo Boschi, Florent Brenguier, Chris Bean and Thomas Lecocq) filled the schedule with engaging lectures.

Roel Sneider talking about measuring and interpreting time-lapse changes in recorded waveforms.

Roel Sneider talking about measuring and interpreting time-lapse changes in recorded waveforms.

 Most of the lectures were a combination of theoretical background and current research problems. The opportunity to deepen our knowledge on the subject through their teaching was of course invaluable. But also precious, from an ECS perspective, was the opportunity to listen to scientific discussions between the trainers! Particularly appreciated by the ECSs were the lectures in which the scariest equations of the seismic noise were explained step by step, those in which advice on “how to do” or “how to be a good noise seismologists” were dispensed and those where unanswered, open questions were introduced and discussed.

All ECSeismologists had the opportunity to actively contribute their share of knowledge and excitement to the school by presenting their research with short talks and posters. The training was completed by two hands-on practicals given by the PhD students Laura Ermert and Korbinian Sager.

ECS-rep Laura Ermert talking about noise processing and noise source imaging during on of the two practicals.

ECS-rep Laura Ermert talking about noise processing and noise source imaging during on of the two practicals.

The COST Action TIDES is organised in five Working Groups (“Forward problems, HPC applications”, “Seismic tomography, full waveform inversion, uncertainties”, “Applications in the natural environment and industry”, “Workflow integration of data and computing resources”, Seismic interferometry and ambient noise”). During the training school Yann Capdeville, Ana Ferreira, Chris Bean, Heiner Igel and Martin Schimmel led the meetings of the working groups where everyone (ECSs included!) was invited to share new research and collaboration ideas and existing useful information for people working in the different fields.

Believe it or not: The program also contained some live Fado (traditional Portuguese music) and a documentary on the highest wave ever surfed! Another evening passed quickly while Roel Snieder shared his insights – and asked for ours – on the art of being a scientist.

Susana Custodio joined the school on Thursday afternoon to talk about the historical and seismological aspects of the Lisbon earthquake, which occurred in 1755. She then guided a field trip to magnificent cliffs and dinosaur footprints belonging to two different periods of Jurassic and Cretaceous.

Dinosaur footprints in Cretaceous rocks seen by participants during the field trip.

Dinosaur footprints in Cretaceous rocks seen by participants during the field trip. [Picture taken by Martha Deen].

The enthusiasm of the organization committee was strong, so that the schedule of the school resulted slightly tight. But as we came here from all over Europe (and even from Australia, Saudi Arabia and Brasil) to learn new and exciting science, this made us feel like we actually reached the goal. Moreover, the beautiful beach and the amazing weather we enjoyed during meals and coffee breaks relieved our fatigues, as well as those of our trainers!

As you could imagine, with a diverse program such as this one, such a training school depends heavily on the engagement and initiative of those preparing it, and we felt very grateful to the people dedicating their time to the organization of this event, in particular Julie Baron (Grant Holder manager), Graça Silveira (local organisation), Andreas Fichtner (scientific organisation), Andrea Morelli (Chair of the COST Action). THANK YOU… and see you next year for the 3rd TIDES training school on Seismic Tomography!

Are you curious about the next training school? You can find more information at http://tides-cost.eu and on the facebook page https://www.facebook.com/cost.tides.20152018/?fref=ts

Your ECS-reps Laura Ermert and Laura Parisi