  We monitor dynamic rupture propagation during laboratory stick-slip
experiments performed on saw-cut Westerly granite under upper crustal
conditions (10-90 MPa). Spectral analysis of high-frequency acoustic waveforms
provided evidence that energy radiation is enhanced with stress conditions and
rupture velocity. Using acoustic recordings bandpass filtered to 400-800 kHz
(7-14 mm wavelength) and highpass filtered above 800 kHz, we back projected
high-frequency energy generated during rupture propagation. Our results show
that the high-frequency radiation originates behind the rupture front during
propagation and propagates at a speed close to that obtained by our rupture
velocity inversion. From scaling arguments, we suggest that the origin of
high-frequency radiation lies in the fast dynamic stress-drop in the breakdown
zone together with off-fault co-seismic damage propagating behind the rupture
tip. The application of the back-projection method at the laboratory scale
provides new ways to locally investigate physical mechanisms that control
high-frequency radiation.
