SCEC Award Number 13176 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 Professor Ares Rosakis
Postdoctoral fellow Vito Rubino
SCEC Priorities 3e, 4b, 4d SCEC Groups FARM, GMP, Seismology
Report Due Date 03/15/2014 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 overall levels of shear prestress. The apparent incompatibility between supershear propagation and low level of shear prestress is resolved in the presence of favorable heterogeneity. In our simulations, the stress field of the main rupture dynamically triggers a secondary shear crack at the location of heterogeneity under much lower prestress that would be needed on homogeneous faults. This secondary shear crack transitions to supershear speed under a wide range of conditions. We have been working on demonstrating experimentally such supershear transition by dynamic triggering of a secondary rupture at a favorable patch. Two possibilities are being considered: a pre-existing crack created by shaving a small amount of the bulk material above and below the patch, to either reduce or completely remove the contact over the patch, and a lower-friction patch, achieved by manipulating the surface preparation procedure. We have conducted laboratory experiments with zero-strength patches produced by shaving enough material so that the fault is open over the patch. Such patches lead to supershear transition in some experiments but not others. Sometimes, we observe rupture arrest over the patch. Our current work is directed towards ensuring repeatability of experiments done under nominally the same conditions and considering other kinds of weaker patches, with lower but non-zero normal stress and lower friction coefficients.
Intellectual Merit The main goal of this project is to address the following question: Can supershear earthquakes propagate 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 performed theoretical simulations to design a suitable experimental setup, determined the experimental parameters that would enable us to experimentally observe supershear transition under low fault prestress, and significantly advanced our ability to obtain a detailed full-field characterization of the displacement and strain during dynamic triggering using the digital image correlation technique. We have also conducted preliminary laboratory experiments that show promise but require improvements in repeatability.
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 2. Numerical simulations of the experimental setup that illustrate rupture transitioning to a supershear speed at a weaker patch. Slip rate vs. distance from the rupture initiation site are plotted at selected times. (a) Reference case of a homogeneous interface: the shear prestress is too low for supershear transition, and sub-Rayleigh rupture is obtained. (b) The interface contains a patch of lower normal stress; supershear transition occurs at the patch despite low prestress.