SCEC Award Number 19086 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 Taeho Kim (Graduate Student)
SCEC Priorities 5a, 1e, 2e SCEC Groups FARM, CS, SDOT
Report Due Date 04/30/2020 Date Report Submitted 05/03/2020
Project Abstract
 For physics-based predictive modeling of destructive large dynamic events, it is important to consider sequences of earthquakes and aseismic slip (SEAS), because prior slip events, including aseismic slip, may determine where earthquakes would nucleate, as well as modify stress and other initial conditions before dynamic rupture. Furthermore, such simulations provide a framework for determining physical properties consistent with a range of observations including interseismic surface motions, microseismici-ty, past events, and thermal constraints, and hence inform us about the current state of a fault segment or system and potential future rupture scenarios. Simulating long-term slip histories is quite challenging because of the variety of temporal and spatial scales involved. We have been developing a boundary-integral methodology (BIM) for simulations of SEAS; in part, we have expanded our BIM methodology to include along-fault diffusion and an approximation for off-fault plasticity, and compared the accuracy and efficiency of different variable time-stepping schemes. We have also participated in the code com-parison initiated 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 the issue of 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, including the probability of jumps. 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 incorporating 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.
Broader Impacts The results of this project would (a) provide better understanding of appropriate ways of modeling 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. Two graduate stu-dents and an undergraduate research student have gained valuable research experience by participating in the project and interacting with the SCEC community.
Exemplary Figure Figure 3: Models with the same cumulative frequency-magnitude statistics (top) can result in quite different fre-quency of jumps across a barrier in simulated 4000-year history of earthquakes and aseismic slip (bottom). (A & C) Fully dynamic and (B & D) quasi-dynamic simulations with oversized cells of 1 km and different assumptions for physical conditions result in events with comparable average stress drops and slip. Slip contours are plotted every 0.5 s; ruptures that jump across the VS barrier are colored blue. All four simulations have comparable popu-lation statistics with a b-value of about 0.33, while the rate of two segment ruptures varies from 0 to 100%.