SCEC Award Number 17154 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 PhD student Stephen Perry, postdoctoral scholar YenYu Lin
SCEC Priorities 1c, 3c, 1d SCEC Groups Seismology, CS, FARM
Report Due Date 06/15/2018 Date Report Submitted 06/24/2018
Project Abstract
 A number of 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 in comparison with what would be expected from Byerlee’s law and numerous laboratory experiments on static or low-slip-rate friction. We have been working to (i) explore different models for “weak” mature faults through dynamic modeling, (ii) determine which friction and other fault proper-ties in such models are compatible with observations such as magnitude- and depth-independent seismic stress drops of 1-10 MPa, radiation ratios of 0.3-1.0, etc; and (iii) establish whether there are observable differences be-tween the acceptable models. We have found that fault models with rate-and-state friction and additional co-seismic weakening in the form of thermal pressurization of pore fluids are consistent with a number of observa-tions including magnitude-independent stress drops, breakdown energy increasing with the earthquake moment, and radiation ratios of about 0.5. However, such models (a) produce mostly crack-like ruptures for the rage of pa-rameters considered and (b) result in reasonable stress drops and sub-melting temperature increases for relatively small interseismic effective stresses of 50-100 MPa, which would require chronic fluid overpressure on faults be-low 3 km or so. We are in the process of examining models with more pronounced pore pressurization and addi-tional stronger co-seismic weakening such as flash heating that have been shown to produce pulse-like ruptures. To enable comparison of our simulations with observations, we have developed a number of ways of quantifying the observables from our simulations.
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, includ-ing 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 ex-treme 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. A graduate student and a postdoctoral fellow have gained valuable research experience by participating in the project and inter-acting with the SCEC community.
Exemplary Figure Figure 1. Fault models with rate-and-state friction and additional co-seismic weakening in the form of thermal pressurization of pore fluids are consistent with a number of observations including magnitude-independent stress drops, breakdown energy increasing with the earthquake moment, and radiation rati-os of 0.3-1.0 (Perry et al., 2018). (a) Accumulated slip profiles for a portion of the sequence of events produced by a rate-and-state fault model with thermal pressurization in a 12 km velocity-weakening re-gion. (b) Breakdown energies from our simulations compared to those inferred for natural events by Rice (2006). Our models are able to match the trend of the observed events quite well. (c) Stress drops for events in the simulation with thermal pressurization and a 24 km long velocity-weakening region. Complete rupture events have filled-in symbols. d) Seismically estimated radiation ratios (η) and actual radiation ratios ηA vs. average slip (left), in comparison to those inferred from large earthquakes (right) from Venkataraman and Kanamori (2004).