HR: 08:45h
AN: S11E-04    [Abstracts]
TI: Laboratory Experiments and Theoretical Studies of Rupture Modes and Supershear Transition
AU: * Lu, X
EM: xiaol@caltech.edu
AF: California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, United States
AU: Lapusta, N
EM: lapusta@its.caltech.edu
AF: California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, United States
AU: Rosakis, A
EM: rosakis@aero.caltech.edu
AF: California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, United States
AB: Theoretical studies have shown that the issue of rupture modes has important implications for fault constitutive laws, stress conditions on faults, energy partition and heat generation during earthquakes, scaling laws, and spatio-temporal complexity of fault slip. Early theoretical models often treated earthquakes as crack-like ruptures, but seismic inversions indicate that earthquake ruptures may propagate in a self-healing pulse-like mode. A number of explanations for the existence of slip pulses have been proposed, including strong weakening of the interface with sliding rate, interaction of rupture with local heterogeneities, and normal stress variation due to a bimaterial effect. We observe pulse-like and crack-like rupture modes in the experimental configuration of a Homalite plate with inclined interface prestressed both in compression and in shear, similarly to faults in the Earth's crust. Dynamic rupture is initiated by exploding a 0.1 mm nickel wire. Digital high-speed cameras are used to record photoelastic images. Two interferometry-based velocimeters are used to determine the history of relative sliding velocity at one location along the interface. Our results indicate that pulse-like ruptures can exist on such interfaces in the absence of a bimaterial effect or local heterogeneities. For a set of experiments with increasing ratio of shear to normal prestress, which is achieved by increasing the inclination angle of the interface, we observe a change in rupture modes from pulse-like to crack-like. This systematic variation is consistent with the theoretical study of velocity-weakening interfaces by Zheng and Rice (1998). We also establish experimentally, for the first time, that both pulse-like and crack-like rupture modes can transition to supershear speeds. After the supershear transition, both modes have speeds within the open interval \sqrt{2} Cs to Cp, where Cs and Cp are the S- and P-wave speeds of Homalite, respectively. However, the rupture speed of pulse-like ruptures is lower. These supershear speeds are consistent with the analytical predictions of the velocity-weakening model of Samudrala et al. (2002). The agreement between our experimental observations and models of velocity-weakening faults suggests that velocity-weakening friction plays an important role in dynamic behavior of ruptures and implies that expressing dynamic weakening of friction solely in terms of slip may not be a sufficiently general description. We will also present our current efforts to further analyze the experiments, including the potential effects of rupture initiation procedure. Our preliminary experimental measurements of vertical motion of points close to the interface indicate that there is no opening of the interface during sliding at locations where we determine the rupture mode, although more analysis is need to determine whether there is any significant normal stress variation. We are in the process of including normal stress variations into our existing numerical code to investigate this issue, as well as to study which friction law is most consistent with the experimental observations. We are also working on quantifying the parameters of the explosion and determining the friction properties of Homalite (collaboration with N. Beeler and B. Kilgore (USGS), C. Marone (Penn State), and G. Ravichandran (Caltech)).
DE: 7209 Earthquake dynamics (1242)
DE: 8020 Mechanics, theory, and modeling
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8163 Rheology and friction of fault zones (8034)
SC: Seismology [S]
MN: 2007 Fall Meeting