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.