“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.
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
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!
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What are the implications of the findings from the paper for current research in geophysics, as discussed by Jessica Johnson?