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Robert G Green

Robert G Green

Post Doctoral Research Associate - Seismology

Geophysics, Geodynamics and Tectonics

Bullard
Email:
Office Phone: +44 (0) 1223 337183

Research Interests

Follow me on Academia.edu

Cambridge Volcano Seismology at the Royal Society Summer Science Exhibition - @ExplosiveEarth 

My research uses data collected from a local passive seismic network in the volcanic zones of north and central Iceland. This network is run by the Cambridge University Volcano-Seismology group (led by Professor Bob White) in collaboration with the University of Iceland, and is supported by grants and equipment loans from the Natural Environment Research Council. The bulk of the array of seismometers is focussed around the central volcano Askja, though we also have excellent coverage to monitor earthquakes around the volcanoes beneath the Vatnajökull ice cap.

Current Research Areas:

Velocity structure of the Icelandic crust

I have produced seismic velocity models for the crust across Iceland, covering the entire Vatnajökull icecap region and Northern Volcanic Zone with excellent resolution. I use surface wave dispersion from ambient noise cross-correlation and array techniques with teleseismic surface waves to extract group and phase velocities across Iceland. Tomographic maps show excellent correlation between tomographic maps and the volcanic rift zones. Further work will involve jointly inverting this surface wave dispersion with crustal receiver functions and body wave travel times to improve our model of the Icelandic crustal velocity structure.

Seismicity of the Bárðarbunga-Holuhraun 2014 dyke intrusion:

In late August 2014, as initial earthquakes and ground deformations signalled unrest at Bárðarbunga volcano in central Iceland, we were able to respond immediately to deploy seismometers to monitor the volcanic unrest as it unfolded. An intrusion of magma propagated horizontally through the subsurface at a depth of ∼2-8km for a distance of 45 km before it erupted at Holuhraun on 29th August. Small earthquakes occurring towards the tip of the magma as the rock fractured ahead of it allow us to track the migration of the magma intrusion through the subsurface.

As the magma propagated we installed more seismometers ahead of its projected path, and so have been able to record the earthquakes of this volcanic eruption in unprecedented detail. The eruption of 29th August was short lived, but the fissure eruption began again on 31st August and did not end until 28th February. An estimated 1.1 cubic km of lava was erupted over six months (http://en.vedur.is/earthquakes-and-volcanism/articles/nr/2947) covering an area of over 85km2. During this period thousands of small earthquakes were recorded at the northern end of the dyke where it fed the fissure eruption at the surface.

 

This seismic station was installed just hours before the magma erupted at the surface.

 

We are currently using this data to investigate the process by which this magmatic dyke fractured its way through country rock and how it came to the surface. 

We have also used the dataset to investigate triggering of earthquakes in the surrounding volcanoes.

Earthquake Triggering during Dyke propagation:

As the magmatic dyke propagated, the deformation stressed the surrounding crust, and this effect was particularly felt at some of nearby volcanos. Using the seismicity to map the location of the dyke in the subsurface, and GPS measurements to model how much it opened we can calculate the stress changes felt at these nearby volcanic areas. As the stress increased, the number of earthquakes in these locations increased, and then as soon as the stressing rate became negative the earthquakes in these regions were completely suppressed. This effect (known as a “stress shadow effect”) has been typically hard to demonstrate in the past but is remarkably clear here.

Green, Greenfield, White NatGeosci (2015) 

http://dx.doi.org/10.1038/ngeo2491 or email for a copy

 

Earthquakes at Askja and Bookshelf Faulting near Herðubreið:

In the region of the central volcano Askja, over 5000 micro-earthquakes (<M3) are detected and located annually. Persistent seismicity occurs beneath a hydrothermal field in the caldera, and episodic earthquake activity is seen at depths of greater than 15km below the volcano. Most of the micro-seismicity however occurs in swarms or bursts of activity, beneath the table mountain of Herðubreið, which I have mapped as excellently constrained strike-slip faults. The bookshelf faults and their associated rotations represent the accommodation of the expected right-lateral transform shear between two overlapping spreading centres.

Green, White, Greenfield NatGeosci (2014)

http://dx.doi.org/10.1038/ngeo2012 or email rgcg3@cam.ac.uk for a copy.

bookshelf webpage 

 

Fieldwork in Iceland – Maintaining and developing the seismic network

Deploying seismometers in Askja caldera:

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Our research in Iceland would not be possible without our collaboration with Bryndis Brandsdottir at the University of Iceland, and the unmatched field skills of Sveinbjörn Steinthorsson. My PhD studentship is funded by a NERC and a CASE award from BP. I am supervised by Professor Bob White and Professor Keith Priestly. The National Environment Research Council has funded much of our research in Iceland and many of the instruments are loaned from SEIS-UK.

Iceland - Cambridge researchers:

Bob White, Tim Greenfield, Rob Green, Thorbjorg Agustsdottir, Jenny Woods, David Pugh, Juerg Schuler, Jenny Jenkins, Bryndis Brandsdottir, Sveinbjörn Steinthorsson.

Our youtube channel can be found at: 

https://www.youtube.com/channel/UCkku2ejeDWRFTbhgTz7o84g

Research Supervision

I have been supervised by Professor Bob White and Professor Keith Priestly.

Key Publications