SCEC Award Number 22067 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 Taeho Kim, graduate student
SCEC Priorities 1c, 3c, 1d SCEC Groups FARM, SDOT, SAFS
Report Due Date 03/15/2023 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 modeling: (I) chroni-cally 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 meg-athrust 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 phys-ical conditions and rupture style for large megathrust vs. continental earthquakes. Another possibility is that radiat-ed energy is underestimated by teleseismic methods. 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. 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 chroni-cally weak models I with crack-like ruptures. We are investigating this behavior in models with fault heterogeneity and finding significant differences in behavior between different types of heterogeneity and quasi-dynamic vs. fully dynamic simulations.
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 results show that the average fault shear stresses inferred from heat and topography data can be different for physical reasons. We find that different types of heterogeneity can potentially be distinguished by their combined effects on the range of slip behaviors observed; for example, strong heterogeneity 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.
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. 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 2. (A-D) Simulated sequences of earthquakes and aseismic slip (SEAS) on chronically weak faults with fault heterogeneity. (E) For sufficiently heterogeneous normal stress, events of all sizes have the same initial moment rate evolution, with large events essentially being small events that run away. (F) In contrast, for heterogeneous characteristic slip distance, larger events have more substantial initial moment rate evolution that can be distinguished from smaller events. Hence these two heterogeneity distributions can be distinguished from observations. (G) Simu-lations with quasi-dynamic approximation (left) result in more complex earthquake sequences, with larger b-values, than the fully dynamic simulations (right).
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