SCEC Award Number 19085 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 Valère Lambert (Graduate Student)
Dr. Yuval Tal (Postdoctoral Researcher)
SCEC Priorities 1c, 3c, 1d SCEC Groups FARM, Seismology, SDOT
Report Due Date 04/30/2020 Date Report Submitted 05/03/2020
Project Abstract
 Observations suggest that mature faults such as the San Andreas Fault (SAF) are generally “weak,” i.e. operate at low overall shear stress compared to what would be expected from Byerlee’s law. We explore different models for “weak” mature faults through dynamic modeling to determine which friction fault properties in such models are compatible with observations and to establish whether there are observable differences between the acceptable models. We have established that crack-like and moderately pulse-like ruptures on chronically weak faults with additional relatively mild co-seismic weakening satisfy both the low-stress, low-heat fault operation and the seismo-logical constraints on stress drop, radiated energy, radiation efficiency, and breakdown energy. We have found that self-healing pulse-like ruptures radiate more seismic energy than crack-like ruptures with the same static stress drop and moment. In fact, the radiated energy per seismic moment can be an order of magnitude higher for the simulated self-healing pulse-like ruptures than the values inferred teleseismically for natural, mostly megathrust, events. These findings suggest that either large earthquakes rarely propagate as self-healing pulses or their radiated energy is sub-stantially underestimated. The higher ratios of radiated energy to moment from our simulated self-healing pulses are potentially consistent with limited regional estimates for large crustal earthquakes, suggesting that the substantial difference in estimated radiated energy could also indicate different physical conditions and different predominant rupture style for large crustal and megathrust earthquakes. Another possibility, which we plan to explore in the future, is that radiated energy is underestimated by teleseismic methods.
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 fault physics as well as the absolute levels of both shear and effective normal stress at depth. Our findings can be inter-preted in two ways. Either crustal and megathrust mature faults are potentially different, since regional estimates of radiated energy per moment for crustal-fault events are consistent with our models of self-healing pulses on quasi-statically strong but dynamically weak faults while (much lower) teleseismic es-timates for megathrust faults are consistent with our models of crack-like ruptures on chronically weak faults. Or inconsistent, by a factor of 5-10, radiated energy estimates from regional and teleseismic ap-proaches imply that radiated energy estimates are unreliable and need to be reevaluated and potentially improved.
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 signif-icantly contributes to a number of research priorities of SCEC, 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 re-currence.”
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. 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: Radiated energy per moment is much higher for self-healing pulses on quasi-statically strong but dynamically weak faults than for crack-like ruptures on chronically weak faults, with potential corre-spondence to crustal and megathrust faults, respectively. (A-B) Slip evolution for a self-healing pulse and crack-like ruptures with similar average static stress drop and slip (contours every 0.25 s). (C-D) The corresponding average evolution of shear stress vs. slip which also illustrates the energy budget, with the total strain energy change (dashed red trapezoid) partitioned into radiated energy (blue shading) and dissipated energy (gray shading). The self-healing pulse experiences substantial stress undershoot. (E-F) The radiated energy to moment ratio for simulated self-healing pulses (yellow to red colors) is much larger than inferred for megathrust earthquakes from teleseismic measurements (pink stars) but consistent with limited regional estimates from large crustal earthquakes (green triangles). The radiated energy to moment ratio for simulated crack-like ruptures (bluish colors) is comparable to teleseismic inferences for megathrust earthquakes. Note that the accuracy of radiated energy estimates needs to be further evaluated, since different methods provide different estimates; dashed lines connect teleseismic and regional estimates for the same earthquake, where available. From Lambert et al. (2020).