SCEC Award Number 12226 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Auto-acoustic Compaction in Steady Shear Flows: A New Velocity-Weakening Mechanism?
Investigator(s)
Name Organization
Emily Brodsky University of California, Santa Cruz
Other Participants
SCEC Priorities 3a, 3b, 3d SCEC Groups FARM, Geology
Report Due Date 03/15/2013 Date Report Submitted N/A
Project Abstract
 Faults are filled with gouge which can flow as a granular material. Granular flows have complex rheologies that depend strongly on both velocity and boundary conditions. At low shear rates, granular flows usually are quasi-static with rheology identical to standard solid rock friction. At high shear rates, granular flows behave as kinetic gas with the rheology dominated by inertial collisions. In between the extremes is a largely unexplored region. Establishing the constitutive law and physical controls on granular rheology through a wide range of velocities is crucial for understanding the coseismic strength of faults.

We had previously explored the transition to rapid shear under constant pressure boundary conditions. We found that the ordinarily expected shear dilatation is suppressed at intermediate velocities (0.1 - 10 cm/s) for angular particles as a result of the acoustic vibrations produced during the flow. The noisier angular grains produce larger acoustic vibrations as they jostle past each other. The compaction produced by this internally generated noise is comparable to compaction produced by externally imposed vibrations of the same amplitude. This feedback between acoustic noise generated inside the fault zone and fault zone rheology is a newly identified process that can affect coseismic rheology, but it had only been observed under constant stress conditions. With SCEC funding, we started to evaluate the importance of this process in earthquake slip by extending the previous study to constant volume boundary conditions. Under constant volume conditions, we observed a velocity weakening effect similar to that observed earlier in constant stress conditions.
Intellectual Merit Establishing the constitutive law of faults during earthquake slip is a major goal of SCEC. The newly discovered process of auto-acoustic compaction may be an important component of the constitutive law.
Broader Impacts Undergraduate intern Kevin Bernando from Bristol Community College had his first research experience with SCEC support. Bernando, a first-generation American and mature student, had not previously seriously considered a research career. The internship opened his career possibilities significantly as he draws close to completing his undergraduate degree.
Exemplary Figure Figure 2. Auto-acoustic weakening demonstrated directly with constant volume experiments. Green line shows the measured normal stress which exhibits the weakening effect as anticipated based on the constant stress experiments. The red line is the observed acoustic amplitude generated by the granular flow. Significant weakening occurs when the acoustic amplitude becomes large.
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