SCEC Award Number 23130 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 students Taeho Kim, Mary Agajanian
SCEC Priorities 1c, 3c, 1d SCEC Groups FARM, Seismology, SAFS
Report Due Date 03/15/2024 Date Report Submitted 11/16/2024
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 simulations: (I) chron-ically 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 sharper self-healing pulses, with significantly different radiated energy per seismic moment: comparable to inferences from megathrust events for models I, and much higher for models II consistent with regional estimates for large continental earthquakes. These results sug-gest potentially different physical conditions and rupture style for large megathrust vs. continental earthquakes. We also find a systematically decreasing number of small events in simulations with more enhanced dynamic weaken-ing; hence paucity of small events on mature faults may be related to enhanced dynamic weakening. 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. We find significant differences in fault behavior between different types of heterogeneity and quasi-dynamic vs. fully dynamic simulations. We show that favorable heterogeneity can induce supershear ruptures on faults with average shear stresses lower than what would be needed on uniformly stressed faults, which helps explain how mature faults can be low-stressed and host supershear 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 Carri-zo segments of the San Andreas Fault may indicate that they undergo substantial dynamic weaken-ing 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 esti-mates of average fault stress (30 MPa or more) required to maintain the surface topography. Our results show that the average fault shear stresses inferred from heat and topography data can be dif-ferent for physical reasons. We find that different types of heterogeneity can potentially be distin-guished by their combined effects on the range of slip behaviors observed; for example, strong het-erogeneity in normal stress can lead to similar initiation of smaller and larger events, while strong heterogeneity in characteristic evolution distances (another typical heterogeneity assumed) leads to significantly different initiation of smaller and larger events. One potential contradiction in observa-tions of mature strike-slip faults is that they are low-stress but tend to host supershear earthquakes; our simulations show that this contradiction can potentially be reconciled by favorable heterogeneity such as stress concentrations on rheological transitions that should exist on the bottom of all locked fault segments.
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. The finding that the paucity of small events is potentially indicative of enhanced dynamic weakening and propensity for larger events has implications for early warning. Models with enhanced dynamic weakening and normal stress heterogeneity support the paradigm that large events are small events that run away, with no special preparatory processes before large events. Two graduate students have gained valuable research experience by participating in the project and interacting with the SCEC community.
Exemplary Figure Figure 5: Supershear transition of dynamic rupture on a strike-slip segment with a seismogenic ve-locity-weakening (VW) region with a weaker patch surrounded by a stably creeping velocity-strengthening region: snapshots of (a) slip rate and (b) shear stress in the VW region. The transition occurs due to a combined effect of stress concentrations that exist near the VS/VS boundaries (red color in panels b) and the weaker patch (visible as a square in panels a, snapshots 4-6). The transi-tion occurs under lower average levels of stress than would be needed on a uniformly stressed fault.
Linked Publications

Add missing publication or edit citation shown. Enter the SCEC project ID to link publication.