On the origin of thigh-frequency radiation in experimental earthquakes

TL;DR

This study investigates the origin of high-frequency radiation during laboratory earthquakes, revealing it originates behind the rupture front and is linked to dynamic stress-drop and off-fault damage, using back-projection analysis.

Contribution

It introduces a novel application of back-projection to laboratory earthquakes, elucidating the physical mechanisms behind high-frequency radiation.

Findings

High-frequency radiation originates behind the rupture front.

Radiation speed matches rupture velocity.

High-frequency energy increases with stress and rupture velocity.

Abstract

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.