Light refreshments will be served at 4:15pm
Using Individual Dynamic Ruptures to Reveal the Nature of Dynamic Friction up to
Very High Sliding Speeds
Ares J. Rosakis
Theodore von Kármán Professor of Aeronautics and Professor of Mechanical Engineering
California Institute of Technology, Pasadena, CA, USA
Friction plays a central role in determining how ruptures propagate along faults in the earth’s crust and release waves that cause destructive shaking threatening our infrastructure and endangering our lives. Yet, the detailed nature of the dynamic frictional laws, which operate on such faults, is one of the biggest scientific uncertainties in earthquake source mechanics. This presentation discusses unprecedented measurements of evolving local, on-fault, friction recorded dynamically on the fault during spontaneous mini-earthquakes, which are created in the laboratory. The measurements are enabled by a new high-speed full-field imaging technique (a dynamic version of Digital image correlation-DIC) and digital, ultra high-speed, photography (up to 10 Million frames/s). This is a new way of inferring friction directly from individual rupture events without having to ever invoke assumptions of uniform sliding along frictional interfaces as is done in all previous frictional studies. The newly developed imaging technique quantitatively captures the full-field evolution of particle velocities, strains, and stresses of both sub-Rayleigh and super-shear ruptures created in the laboratory under conditions mimicking spontaneous rupture nucleation and tectonic loading. The technique combines pattern-matching algorithms with ultra-high-speed photography and highly tailored analysis to obtain full-field time histories in the presence of dynamically sliding discontinuities. Dynamic imaging of particle velocities, sliding speed histories, strains, stresses and transient surface tractions during rupture enables unique observations of key rupture features as well as detailed analysis of dynamic friction in a fully transient setting and at very high sliding speeds (up to 20m/s). Our measurements do not support classical slip weakening as the operant frictional law. Instead they show that friction, in addition to being strongly velocity dependent, has a complex (history dependent) evolution, qualitatively consistent with a rate-and-state frictional law, strongly supplemented with flash heating.