Dynamic Imaging of Strain and Stress Evolution in Laboratory Earthquakes with the Ultra-High-Speed Digital Image Correlation Technique

Dynamic imaging of strain and stress during rupture enables unprecedented observations of key rupture features as well as decoding the nature of friction. We present the dynamic evolution of strains and stresses in our dynamic rupture experiments. We employ a laboratory earthquake setup to study dynamic ruptures in a highly instrumented setting, where we produce both supershear and sub-Rayleigh events. Earthquakes are mimicked in the laboratory by dynamic rupture propagating along the inclined frictional interface of two quadrilateral Homalite plates prestressed in compression and shear. The diagnostics previously employed in this setup include temporally accurate but spatially sparse laser velocimetry measurements as well as a sequence of full-field photoelastic images. These measurements have been successfully employed to capture important rupture features but they do not give enough information to characterize the full-field strains and stresses. In this study, we obtain the experimental sequences of full-field displacements, velocities, strains and stresses produced under a wide range of slip rates by our newly developed technique of ultra high-speed digital image correlation (DIC). This is the first technique capable of imaging spatial and temporal variations in strains and stresses during spontaneously developing experimental dynamic rupture. This technique combines pattern-matching algorithms with ultra-high-speed photography and highly tailored analysis to obtain full-field time histories. We have verified the accuracy of the measurements by comparing the velocity time-histories at selected locations with the measurements using the well-developed technique of laser velocimetry. The newly developed ultra-high-speed full-field imaging technique can also be used to obtain unprecedented measurements of evolving dynamic friction during dynamic rupture, and we will report on our initial results on the dynamic friction evolution.