SCEC Award Number 15078 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 Postdoctoral Fellow Vito Rubino; Prof. Ares Rosakis (no support requested for Prof. Rosakis)
SCEC Priorities 3e, 4b, 4d SCEC Groups FARM, SoSAFE
Report Due Date 03/15/2016 Date Report Submitted 03/20/2016
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 overall levels of shear prestress. The apparent incompatibility between supershear propagation and low level of shear prestress may be possible to resolve based on numerical studies of heterogeneous faults in which 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 which transitions to supershear speed under a wide range of conditions. Our goal is to study dynamic triggering and potentially demonstrate experimentally supershear transition by dynamic triggering of a secondary rupture at a favorable patch. In this SCEC project, we have developed a unique experimental technique that enables us to visualize dynamic triggering at an unprecedented level of detail. This 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 frictional properties of a wide range of dynamic ruptures. We have performed analytical and numerical predictions using the experimentally derived frictional properties to design experiments that would enable us to observe supershear transition under low fault prestress. We are currently performing tests featuring patches determined by this approach.
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 study 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 enables 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 spontaneous dynamic ruptures over a broad range of slip rates. 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 in the process of 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 study whether 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 research scientist 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. Dynamic imaging of full-field stresses and friction in laboratory earthquakes obtained with the newly developed ultra high-speed digital image correlation method (Rubino et al., 2016). (a) Schemat-ics of the test specimen geometry and loading configuration. (b) Snapshots of the full-field shear stress for a rupture propagating at a supershear speed (Vr / cs = 1.8). (c) Ratio of shear to normal stress vs. slip at a location along the experimental fault for two experiments with different loading conditions that result in different slip and slip rate histories. The plots resemble a linear slip-weakening friction law but the dependence of dynamic friction fd and weakening distance Dc on slip rate indicate rate and state dependence.
Linked Publications

Add missing publication or edit citation shown. Enter the SCEC project ID to link publication.