SCEC Award Number 21105 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
SCEC Priorities 1c, 3c, 1d SCEC Groups FARM, SDOT, Seismology
Report Due Date 03/15/2022 Date Report Submitted 05/28/2022
Project Abstract
 Observations suggest that mature faults such as SAF are “weak,” i.e. operate at low average shear stress compared to that expected from Byerlee’s law. We explore two classes of “weak” fault models through modeling: (I) chronically weak faults due to persistent fluid overpressure and (II) quasi-statically strong but dynamically weak faults due to shear-heating effects. For models with realistic static stress drops of 1-10 MPa, models (I) result in crack-like and moderately pulse-like ruptures while models (II) result in relatively sharp self-healing pulses, with significantly different radiated energy per seismic moment (proportional to apparent stress): comparable to inferences from megathrust events for models I, and much higher scaled radiated energy for models II, up to an order of magnitude, consistent with regional estimates for large continental earthquakes. These results suggest potentially different physical conditions and rupture style for large megathrust vs. continental earthquakes. Another possibility is that radiated energy is underestimated by teleseismic methods. We find that geometric pulses, due to finite seismogenic-zone depth, behave similarly to crack-like ruptures. We also find a systematically decreasing number of small events in simulations with more enhanced dynamic weakening; hence paucity of small events on mature faults may be related to enhanced dynamic weakening. We are investigating this behavior in models with fault heterogeneity. Due to rapid co-seismic weakening and healing, models II with self-healing pulses can maintain 10-20 MPa higher absolute stresses for the same shear heating constraints than chronically weak models I with crack-like ruptures.
Intellectual Merit Our study aims to determine which models of low-stresses faults are consistent with basic observa-tions, 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. One of our key findings is that continental and megathrust mature faults have potentially different properties, since regional estimates of radiated energy per moment for continental-fault events are consistent with our models of self-healing pulses on quasi-statically strong but dynamically weak faults while (much lower) teleseismic estimates for megathrust faults are consistent with our models of crack-like ruptures on chronically weak faults. It is also possible that radiated energy estimates are unreliable and need to be reevaluated and potentially improved.
Another key finding is that simulated faults with increasingly efficient dynamic weakening have in-creasingly more scale-dependent average shear prestress, with lower average prestress before larger events. Such a property of the stress field should favor larger events and this is indeed what occurs: fault models with increasingly efficient dynamic weakening result in fewer small events and hence smaller b-values of the earthquake frequency-magnitude distributions. Our findings suggest that the paucity of microseismicity observed on some mature fault segments, such as the Cholame and Carrizo segments of the San Andreas Fault may indicate that they undergo substantial dynamic weakening during earthquakes ruptures. Rapid weakening and healing during the resulting self-healing pulse-like rupture propagation allows substantial motion to occur locally at low dynamic resistance (10-20 MPa or less), consistent with low heat production, while larger fault areas away from the slipping zone can maintain higher stress levels perhaps more consistent with the geodynamic estimates of average fault stress (30 MPa or more) required to maintain the surface topography.
Our goal to produce models of low-stressed SAF segments consistent with basic observations will help the development of realistic earthquake simulators with predictive power. The proposed modeling significantly 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 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 pos-sible extreme events, based on physical models and integrations of laboratory, field and seismolog-ical studies; and (c) contribute to the development of realistic scaling laws for large events. The find-ing that the paucity of small events is potentially indicative of enhanced dynamic weakening and propensity for larger events has implications for early warning. Three graduate students have gained valuable research experience by participating in the project and interacting with the SCEC community.
Exemplary Figure Figure 2: Higher absolute stress levels in comparison with shear heating stress for self-healing pulse-like ruptures on quasi-statically strong, dynamically weak faults (B) vs. crack-like rup-tures on persistently weak faults (A). The spatially-averaged shear stress (black line) for the chronically weak fault producing crack-like ruptures (A) is always within one static stress drop from the shear heating stress that reflects mainly dynamic shear resistance during slip. The av-erage shear stress for self-healing pulses is 2-4 static stress drops higher than the shear heating stress, due to rapid dynamic weakening and co-seismic re-strengthening that result in significant stress undershoot. This difference can potentially merge the gap between the less than 20 MPa absolute stresses inferred from heat measurements, which closely corresponds to the dynamic level of frictional resistance, with higher average fault stress (30 MPa or more) potentially re-quired to maintain the surface topography (Fay and Humphreys, 2006; Lamb, 2006). Adapted from Lambert and Lapusta (2022).