SCEC Award Number 20079 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 Graduate student Valère Lambert
SCEC Priorities 1c, 3c, 1d SCEC Groups FARM, SDOT, Seismology
Report Due Date 03/15/2021 Date Report Submitted 04/22/2021
Project Abstract
 Observations suggest that mature faults such as the San Andreas Fault (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. In the parameter regimes that result in 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. The two types of models significantly differ in terms of radiated energy per seismic moment (which is proportional to apparent stress): models I result in scaled radiated energy comparable to inferences from megathrust events, while models II result in much higher scaled radiated en-ergy, up to an order of magnitude, consistent with limited regional estimates for large continental earthquakes. The-se results suggest potentially different physical conditions and different predominant rupture style for large meg-athrust vs. continental earthquakes. Another possibility, which we are working to explore, is that radiated energy is underestimated by teleseismic methods. We find that geometric pulses, due to finite seismogenic-zone depth, be-have similarly to crack-like ruptures. We also find a systematically lower average prestress before larger ruprtures in models with enhanced dynamic weakening, which translate into 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.
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. One of our key findings is that continental and megathrust mature faults have potentially different properties, since regional esti-mates 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 esti-mates 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 increasing-ly 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 Andre-as Fault may indicate that they undergo substantial dynamic weakening during earthquakes ruptures.
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 signifi-cantly 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 recur-rence.”
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. The finding that the pau-city of small events is potentially indicative of enhanced dynamic weakening and propensity for larger events has implications for early warning. 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 3: Fault models with more efficient dynamic weakening result in less earthquake sequence complexity, pro-ducing fewer smaller events (left column) and hence smaller b-values of the earthquake frequency-magnitude dis-tributions (right column). 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 (e.g., Hauksson et al._2012; Jiang and Lapusta,_2016) may indicate that they undergo substantial dynamic weakening during earthquakes ruptures. (A-D) Frequency-magnitude and (E-H) cumulative frequency-magnitude statistics for simulations with increasing efficiency of enhanced dynamic weakening (models TP 1-4 from Figure 2). From Lambert et al. (2021b).