SCEC Award Number 12227 View PDF
Proposal Category Collaborative 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 Ares Rosakis California Institute of Technology
Other Participants Prof. Ares Rosakis (no additional funds requested)
Postdoctoral fellow Dr. Vito Rubino
SCEC Priorities 3e, 4b, 4d SCEC Groups FARM, GMP, Seismology
Report Due Date 03/15/2013 Date Report Submitted N/A
Project Abstract
 Supershear rupture transition on uniform fault requires high shear prestress, as indicated by analytical and experimental studies. 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 possible to resolve when favorable heterogeneities in fault properties are considered. In simulations, the stress field of the main sub-shear rupture dynamically triggers a secondary crack at the location of favorable heterogeneity. The secondary crack then transitions to supershear speed under a wide range of conditions, eventually merging with the main crack. Our goal is to design and conduct laboratory experiments that demonstrate supershear transition by dynamic triggering of a favorable patch. Two possibilities are being considered: a lower-friction patch, achieved by manipulating the surface preparation procedure, and a pre-existing crack, created by shaving a small amount of the bulk material above and below the patch. 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. Our results indicate that it is indeed possible to find suitable experimental configurations. Experiments will be conducted next.
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. Our results indicate that it is indeed possible to find suitable experimental configurations. Experiments will be conducted next.
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 has gained valuable research experience by participating in the project and interacting with the SCEC community. He was also participated in several outreach activities oriented towards sixth to ninth graders of the Los Angeles area.
Exemplary Figure Figure 1. Investigating supershear transition on faults with low prestress. (a) Schematics of a model in which a main crack would approach a region of favorable heterogeneity (red), potentially triggering a supershear secondary rupture. The favorable heterogeneity can be a preexisting subcritical crack, a patch with higher prestress, or a patch with lower static friction strength. (b) Laboratory confirmation of this supershear transition mechanism: schematics of test specimen geometry and loading configuration. Bottom left: Simulated rupture time along the interface reported by Liu and Lapusta (2008). Under the stress field of the advancing main crack, the secondary subcritical crack begins to spread and eventually propagates at supershear speeds. The entire process occurs at shear prestress levels substantially smaller than those required for transition by the Burridge-Andrews mechanism (about twice smaller in this example) Bottom right: Rupture speed of the secondary crack reported by Liu and Lapusta (2008). The rupture speed first approaches the Rayleigh-wave speed and then apruptly jumps to supershear speeds. We have determined the experimental parameters that should enable us to observe this phenomenon. The experiments will be conducted next. Rubino, Lapusta, and Rosakis, manuscript in preparation, 2013.
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