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Seismology

Paper of the Month

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.

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.

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

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.

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