SCEC Award Number 18085 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Constraining friction properties of mature low-stressed faults such as SAF
Investigator(s)
Name Organization
Nadia Lapusta California Institute of Technology
Other Participants Postdoc Dr. Yen-Yu Lin, PhD student Stephen Perry
SCEC Priorities 1c, 3c, 1d SCEC Groups FARM, SDOT, Seismology
Report Due Date 03/15/2019 Date Report Submitted 04/26/2019
Project Abstract
 Many observations suggest that mature faults such as the San Andreas Fault (SAF) are generally “weak,” i.e. operate at low overall levels of shear stress compared to what would be expected from Byerlee’s law and numerous laboratory experiments on static or low-slip-rate friction. We have been working to explore different models for “weak” mature faults through dynamic modeling, determine which friction and other fault properties in such models are compatible with observations; and establish whether there are observable differences between the acceptable models. To enable comparison of our simulations with observations, we have developed a number of ways of quantifying the observables from our simulations. We find that fault models with rate-and-state friction and moderate additional co-seismic weakening in the form of thermal pressurization of pore fluids, which produce mostly crack-like ruptures, are consistent with a number of observations including magnitude-independent stress drops, breakdown energy increasing with the earthquake moment, and radiation ratios of about 0.5. However, such mod-els result in reasonable stress drops and sub-melting temperature increases for relatively small interseismic effective stresses of 25 MPa or less, which would require chronic fluid overpressure on faults. We have been examining models with more pronounced pore pressurization and additional stronger co-seismic weakening such as flash heating that can produce sharper pulse-like ruptures. The simulated sharper pulse-like ruptures result in much higher radiated efficiencies than the typical seismological inferences, suggesting that either large earthquakes are rarely sharply pulse-like, or the seismically estimated radiated energy from such pulse-like ruptures is significantly underestimated.
Intellectual Merit Our study aims to determine which models of low-stresses faults are consistent with basic observations, including depth-independent stress drops of 1-10 MPa, and hence to put constrains on the absolute lev-els of both shear and effective normal stress at all depths. Our goal to produce models of low-stressed SAF segments consistent with basic observations will help towards developing realistic earthquake simulators with predictive power. The proposed modeling significantly contributes to a number of re-search priorities of FARM, including “Constrain how absolute stress, fault strength and rheology vary with depth on faults,” “Determine how seismic and aseismic deformation processes interact,” and “Use numerical models to investigate which fault properties are compatible with paleoseismic findings, including average recurrence, slip rate, coefficient of variation of earthquake recurrence.”
Broader Impacts The results of this project, when further developed, would (a) provide better understanding of the long-term behavior of faults; (b) provide better assessment of seismic hazard and evaluation of possible extreme events, based on physical models and integrations of laboratory, field and seismological studies; and (c) contribute to the development of realistic scaling laws for large events. Two graduate students and a postdoctoral fellow have gained valuable research experience by participating in the project and interacting with the SCEC community.
Exemplary Figure Figure 1: Models that combine some chronic fault overpressure with moderate enhanced dynamic weakening produce predominantly crack-like ruptures that satisfy a range of generic observations (Perry et al., 2019). (top) Breakdown energies from our simulations compared to those inferred for natural events by Rice (2006). Our models are able to match the observed trend of the observed events quite well. (center) Crack-like events with mild undershoot or overshoot have energy-based properties comparable to seismological observations, as demonstrated here for the values of radiation ratio (aka radiation efficiency); the values for the natural events are from Venkataraman and Kanamori (2004). (bottom) Crack-like ruptures require low effective confining stresses (e.g. < 25 MPa) to avoid melting of the shearing zone, suggesting substantial chronic fluid over-pressurization and/or more efficient thermal pressurization to maintain low enough temperatures to avoid pervasive melt production. The temperature evolution is shown for two models, with the interseismic effective normal stresses of 25 MPa (blue) and 50 MPa (red).