Today, the underlying mechanisms that generate earthquakes at greater depth, i.e. deeper than 60 km, are still unknown. These earthquakes cannot be explained in the same way as shallow earthquakes, because pressure and temperature conditions are different. Due to better seismic imaging methods it is possible to localize even smaller and deeper earthquakes.
By looking at seismic images of subduction zones some scientists found a correlation between the occurrence of earthquakes within the downgoing slab and the stability of minerals and suggested that earthquakes occur at depths where mineral reactions happen and stop at a distinct depth where no further metamorphic reactions happen.
The aim of these deformation experiments is to demonstrate that the hypothesis that metamorphic reactions during eclogitization from blueschist to eclogite may be a viable mechanism to explain the generation of intermediate depth earthquakes in subduction zones.
The deformation experiments were performed using a D-DIA apparatus at the Advanced Photon Source in Chicago (for more information about the APS Chicago please visit https://www1.aps.anl.gov/). This press is able to reach very high pressures and also temperatures which is indispensable if you are interested in processes which occur at deeper depth in Earth’s lower crust or upper mantle. Another advantage of this apparatus is that it is situated on a synchrotron beamline. The X-rays from the synchrotron enable us to:
1) Monitor the deformation, hence the shortening of the sample using radiography.
2) Visualize if there are mineral reactions occurring during deformation. This is possible due to powder diffraction of the sample (for a detailed description of powder diffraction please visit http://en.wikipedia.org/wiki/Powder_diffraction). Each set of rings corresponds to a distinct mineral. This means that the disappearance/appearance of rings correlates to the breakdown/ growth of minerals.
These methods are very helpful for our investigations because both images, the radiography of the sample and the diffractogram, can be taken in situ. So we can record if there is a change in deformation behavior and/ or change in mineral assemblage in the sample and if they are coupled during the whole experiment!
The video of the diffratograms shows that there is a new ring appearing, which means a new mineral growing at the end of the experiment:
The micro-CT video shows how the sample looks like after the deformation experiment:
You can see that there are cracks crosscutting the sample. We suppose that these cracks formed due to the growth of the new mineral. Details about micro-CT can be found here: http://en.wikipedia.org/wiki/X-ray_microtomography
A detailed microstructural analyses of this sample will give us more information.