SCEC Award Number 21106 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Optimizing and further developing simulations of sequences of earthquakes and aseismic slip
Investigator(s)
Name Organization
Nadia Lapusta California Institute of Technology
Other Participants Valère Lambert
Taeho Kim
Luca Dal Zilio
SCEC Priorities 5a, 1e, 2e SCEC Groups FARM, SDOT, CS
Report Due Date 03/15/2022 Date Report Submitted 05/28/2022
Project Abstract
 Physics-based predictive modeling of destructive large dynamic events relies on simulations of sequences of earthquakes and aseismic slip (SEAS), because prior slip events, including aseismic slip, determine where earthquakes would nucleate and modify stress and other initial conditions before dynamic rupture. We have expanded our boundary-integral methodology (BIM) for SEAS simulations to include along-fault diffusion and an approximation for off-fault plasticity. We have also participated in the code comparison led by Drs. Erickson and Jiang and developed a benchmark for future code comparison on interaction between two seismogenic (velocity-weakening) fault segments separated by a velocity-strengthening barrier, a problem of high relevance to rupture jumps between segments and UCERF. Our findings show that quasi-dynamic treatment of wave-mediated stress changes and using oversized cells significantly affects the long-term behavior of the model. Moreover, the long-term simulations of long enough faults produce different earthquake sequences and different jump rates even when well resolved by all known metrics. We also find that matching frequency-magnitude statistics and static stress drops does not imply the same probability of ruptures jumping across the unfavorable barrier.
Intellectual Merit Our study aims to verify, optimize and develop codes that simulate earthquakes sequences and aseismic slip, including the problems of earthquake jumps between segments. Our goal is two-fold: (i) improve the boundary-integral approach for simulations on planar faults by incorporat-ing a range of physical ingredients, including representations of fluid flow, fault roughness, and off-fault inelasticity and (ii) use the simulations to understand the impact of different modeling assumptions on the modeling outcomes and connections between different observables. Our findings that (i) quasi-dynamic treatment of wave-mediated stress changes and using oversized cells significantly affects the long-term behavior of the model, including the probability of jumps and (ii) matching frequency-magnitude statistics and static stress drops does not imply the same probability of ruptures jumping across the unfavorable barrier have direct implications for large-scale simulations of earthquakes sequences and fault interactions and the resulting estimates of seismic hazard. These findings imply that care must be taken in evaluating the stability of the outcomes, such as rupture jump probabilities, to the assumptions of the model regarding inertial effects and resolution. In fact, the significant sensitivity of the rate of multi-segment ruptures to small changes in our numerical models implies that this hazard parameter may also be sensitive to physical perturbations on natural faults and motivates finding more reliable metrics for de-scribing long-term fault behavior and assessing seismic hazard, tasks for which physics-based modeling is well-suited.
Broader Impacts The results of this project (a) provide better understanding of factors affecting the long-term behav-ior of faults; and (b) contribute to the development of realistic scaling laws for large events. Two graduate students have gained valuable research experience by participating in the project and interacting with the SCEC community.
Exemplary Figure Figure 4. Models with comparable frequency-magnitude statistics and static stress drops but different rates of two-segment ruptures. (A-F) Cumulative frequency-magnitude histograms (top) and history of cumulative slip (bottom) over 4000 years in (A-C) fully dynamic and (D-F) quasi-dynamic SEAS simulations. Contours for seismic slip are plotted every 0.5 s, with rup-tures that jump across the VS barrier colored blue. All six simulations produce comparable average static stress drops and comparable population statistics with a b-value around 0.33. However, the rate of two-segment ruptures varies from 0 to 1. Adapted from Lambert and Lapusta, 2021.