Finally, the paper on seismic moment tensors and b-values calculated for acoustic emissions has been published in Geophysical Research Letters. Here is the link and reference to the paper:
Kwiatek, G., Goebel, T., and G. Dresen (2014). Seismic moment tensor and b value variations over successive seismic cycles in laboratory stick-slip experiments. Geophys. Res. Lett. 41, DOI: 10.1002/2014GL060159. [ Article page ]