SCEC Award Number 13042 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title An Experimental and Theoretical Study of the Dynamic Formation of Fault Zone Rocks and Their Influence on the Propagation of Earthquake Ruptures
Name Organization
Charles Sammis University of Southern California
Other Participants Aris Rosakis
Harsha Bhat
SCEC Priorities 3, 4, 6 SCEC Groups FARM, Seismology, Geology
Report Due Date 03/15/2014 Date Report Submitted N/A
Project Abstract
Large earthquakes propagate on faults, which are generally meters wide zones of crushed rock. The primary objective of this ongoing project is to understand how this fault zone is created and modified by successive earthquakes, and how it affects the propagation and seismic coupling of an individual event. Toward this goal, we have developed a fully dynamic micromechanical damage mechanics, which we have used to simulate earthquake ruptures. We have demonstrated strong asymmetries in damage generation and propagation direction associated with fault zone structure. We are currently carrying out a set of laboratory experiments to validate the theory. In order to generate off-fault damage in the laboratory, we have had to use “candy glass”, the extremely fragile material used in movie stunts. Preliminary studies using small explosions have shown that we can generate damage at the correct spatial scale to test our model. Although we have yet to generate a laboratory earthquake in this material (because it is so fragile) we have measured its elastic moduli, its critical stress intensity factor, and the size and density of its dominant flaws, all of which are required inputs to the mode. We have also perfected a laser technique to produce broadband velocity seismograms at selected points on the sample surface. We are continuing our efforts to generate spontaneous ruptures in this material.
Intellectual Merit This work is at the cutting edge of theoretical damage mechanics and laboratory fracture mechanics. We have combined the quasi-static micromechanical model developed by Ashby and Sammis (1990) with recent theoretical and experimental advances in dynamic fracture propagation to produce model based damage mechanics that has been shown to correctly predict the strength of marble over 10 orders of magnitude in loading rate (Bhat et al., 2012). The Rosakis high-speed digital photo lab at Caltech was the first to produce high-speed movies of dynamic mode II ruptures in photoelastic plates and to observe the interaction of these ruptures with off-fault fracture damage. As soon as we solve our current experimental problems, we will be the first to observe the generation of off-fault damage in the process zone of a dynamic mode II rupture.
Broader Impacts The post doctoral fellow trained on this project, Harsha Bhat, has begun a career on the faculty at IPGP Paris. This work has impact beyond earthquake mechanics. Other problems in the Earth Sciences that involve the generation of fracture damage at high loading rates include the seismic coupling in underground explosions (chemical and nuclear) and the modeling of meteorite impact.
Exemplary Figure Figure 1. Snapshot of a bilateral rupture propagating on the boundary between damaged and undamaged rock. Note the generation of dynamic damage in the tensile lobe of the right rupture tip and its slower propagation. Rupture tips are denoted by the inverted triangles.