SCEC Award Number 13169 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Long-term behavior of faults with heterogeneous strength
Investigator(s)
Name Organization
Nadia Lapusta California Institute of Technology
Other Participants PhD student Junle Jiang
SCEC Priorities 3c, 3e, 6b SCEC Groups FARM, Seismology, SDOT
Report Due Date 03/15/2014 Date Report Submitted N/A
Project Abstract
 Dynamic rupture simulations suggest that fault heterogeneity can strongly influence dynamic rupture and earthquake patterns. Its effects are typically studied in simulations of isolated dynamic events. To study the long-term effects of heterogeneity, we simulate earthquake sequences and slow slip in fault models with laboratory-derived friction laws, including enhanced co-seismic weakening due to shear heating. Such simulations reveal how prior slip affects earthquakes and how large earthquake ruptures interact with fault heterogeneity. We find that large earthquake events can penetrate into deeper creeping regions, if enhanced co-seismic weakening is activated. Such deeper penetration results in much diminished concentration of seismicity at depth in the interseismic period; the seismicity would be expected otherwise to concentrate at the bottom of the seismogenic zone. This simulated behavior is consistent with observations on some major fault segments with large historical events. In simulations, fault segments hosting such deeper-penetrating earthquakes are characterized by deeper coseismic slip, larger spatial extent of ground motion, and time-dependent locking depths. One important implication is that using interseismic geodetic observations alone may underestimate the rupture extent of such earthquakes.
Intellectual Merit The main goal of this work is to study how dynamic rupture behavior and earthquake patterns evolve in the presence of fault heterogeneity over long-term fault slip, using laboratory-derived friction laws including enhanced coseismic weakening due to shear heating. To study the long-term effects of heterogeneity, we simulate earthquake sequences and slow slip in fault models with laboratory-derived friction laws, including enhanced co-seismic weakening due to shear heating. Such simulations reveal how prior slip affects earthquakes and how large earthquake ruptures interact with fault heterogeneity. We focus on exploring the possibility that large earthquakes can penetrate below the traditionally defined seismogenic zone, enabled by enhanced dynamic weakening at high slip rates. Our simulations reveal that such penetration is indeed possible for realistic physical properties and allow us to determine various observables for such earthquakes, including temporal and spatial locations of microseismicity, post- and inter-seismic geodetic signals, and coseismic near-field ground motion. One novel concept from this study is that the absence of microseismicity streaks at the bottom of the traditionally defined seismogenic zone may point to deeper penetration of the past large event in the area. Our study also shows that the locking depth in such cases is strongly time-dependent and decreases with time. Therefore, using current interseismic coupling may underestimate the potential coseismic rupture extent.
Broader Impacts Large-scale simulations carried out by SCEC teams have the potential to provide novel and critical information for the assessment of seismic hazard in Southern California. The results of this project, when further developed, would (a) provide better understanding of the long-term behavior of faults, including nucleation conditions and seismicity at rheological boundaries; (b) provide better assessment of seismic hazard and evaluation of possible extreme events, based on physical models and integrations of geodetic and seismological observations; and (c) contribute to the development of realistic scaling laws for large events. A student and a postdoctoral fellow have gained valuable research experience by participating in the project and interacting with the SCEC community.
Exemplary Figure Figures 1 and 2.
Caption:
Earthquakes can penetrate into the deeper creeping fault extension, enabled by enhanced dynamic weakening at high slip rates. Depending on how deep coseismic slip reaches, the microseismicity at depth could be eliminated in most or all of the interseismic period (Figure 1); hence microseismicity patterns at the bottom of the seismogenic zone can provide clues to the depth extent of prior large events. Furthermore, parts of the deeper creeping region that experienced coseismic slip and hence stress drop are locked right after a seismic event but start creeping relatively soon, resulting in the time-dependent decrease of locking depth with time (Figure 2). Therefore, using current interseismic coupling may underestimate the coseismic rupture extent in this case.