SCEC Award Number 14133 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Reconciling supershear transition of dynamic ruptures with low fault prestress and implications for the San Andreas Fault
Investigator(s)
Name Organization
Nadia Lapusta California Institute of Technology
Other Participants Vito Rubino, postdoctoral scholar
SCEC Priorities 3e, 4b, 4d SCEC Groups GMP, CS, SoSAFE
Report Due Date 03/15/2015 Date Report Submitted N/A
Project Abstract
 Supershear rupture propagation on a uniform fault requires a high level of shear stress, as indicated by analytical and experimental results. Yet many observations suggest that mature strike-slip faults that host large earthquakes operate at low levels of shear prestress. The apparent incompatibility between supershear propagation and low level of shear prestress is resolved in numerical simulations that show that initially sub-shear cracks can transition to and propagate at supershear speeds under lower prestress levels in the presence of favorable heterogeneity. In the simulations, the stress field of the main rupture dynamically triggers a secondary crack at the location of favorable heterogeneity which transitions to supershear speed under a wide range of conditions. Our goal is to investigate this phenomenon experimentally. In this SCEC project, we have developed a unique experimental technique that will enable us to visualize dynamic triggering and supershear transition at an unprecedented level of detail. The technique combines ultra high-speed photography, to capture sequences of digital images during rupture propagation, with digital image correlation, to compute full-field dynamic displacements, particle velocities, strains and stresses. Using this technique, we have measured the evolving frictional properties of a spontaneous dynamic rupture, dramatically improving our understanding of the frictional parameters relevant to the experimental interfaces. Using the experimentally derived frictional properties, we have determined the parameters of the experimental design that should enable us to observe dynamic triggering and supershear transition under low fault prestress. Next, we will confirm these findings by performing the designed experiments.
Intellectual Merit Can supershear earthquakes occur under overall low level of applied shear prestress? For a rupture to transition to supershear speed on a uniform fault, a high level of shear stress is required, as indicated by theoretical and numerical studies as well as laboratory experiments. Yet observations and simulations indicate that well-developed, mature strike-slip faults that host large earthquakes operate at low overall levels of shear prestress. It is important to understand whether low-stressed faults can generate supershear ruptures since supershear rupture can cause much larger shaking far from the fault than sub-Rayleigh ruptures. This is of particular relevance to the Southern San Andreas Fault which is locked and loaded for the next large earthquake. Our goal is to design and conduct laboratory experiments that demonstrate supershear transition by dynamic triggering of a favorable patch, which can occur under much lower overall levels of prestress, as established in numerical models. We have developed a unique experimental technique that will enable us to quantify the process of dynamic triggering in terms of the resulting strains and stresses. This technique combines ultra high-speed photography with digital image correlation to compute full-field dynamic displacements, particle velocities, strains and stresses. This technique has allowed us to measure evolution of friction properties during a spontaneous dynamic rupture. Using the experimentally derived friction properties, we have determined the range of experimental parameters that would enable us to observe supershear transition under low fault prestress according to the prior numerical studies. We are currently working on performing these experiments.
Broader Impacts Understanding the range of potential realistic scenarios on San Andreas and other mature strike- slip faults is crucially important for the estimates of seismic hazard and ground motion. This project aims to demonstrate that supershear earthquakes can occur on faults with low prestress, if suitable patches of heterogeneities are present. The effects of such occurrence on the shaking in Southern California can then be explored in large-scale simulations. A postdoctoral fellow and a student have gained valuable research experience by participating in the project and interacting with the SCEC community. They have also participated in several outreach activities oriented towards sixth to ninth graders of the Los Angeles area.
Exemplary Figure Figure 1.