SCEC Award Number 15180 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, Geodesy, Seismology
Report Due Date 03/15/2016 Date Report Submitted 03/20/2016
Project Abstract
 Dynamic rupture simulations suggest that fault heterogeneity can strongly influence dynamic rupture and earth- quake 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. We find that large earthquake events can penetrate into deeper creeping regions, if enhanced co-seismic weakening is activated. Such deeper penetration results in lack of seismicity concentration 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 observa- tions for several major SAF fault segments with large historical events, suggesting that those segments can host large events with deeper slip. In simulations, fault segments hosting such deeper-penetrating earthquakes are char- acterized by deeper coseismic slip, larger spatial extent of ground motion, a broad time-dependent depth transition from fully locked to near-plate-rate creeping rates, and the associated time-dependent locking depth. Temporal and spatial variability in earthquake slip, event patterns, and depth extent of slip naturally arise due to interaction be- tween dynamic processes and depth-dependent fault properties, even in models with uniform properties along strike. Despite the increasing dynamic weakening in larger events, the events of different sizes have comparable static stress drops.
Intellectual Merit Dynamic rupture simulations suggest that fault heterogeneity can strongly influence dynamic rupture and earth- quake patterns. Its effects are typically studied in simulations of isolated dynamic events. To study the long-term effects of heterogeneity, we simulate earthquake sequences, slow slip, and their interaction in fault models with laboratory-derived friction laws, including enhanced co-seismic weakening due to shear heating. Our simulations resolve all the stages of every earthquake in detail, including nucleation, dynamic rupture propagation and arrest, as well as reproduce post-seismic slippage and interseismic creep. We find that large earthquake events can pene- trate into deeper creeping regions, if enhanced co-seismic weakening is activated there. Such deeper penetration results in lack of seismicity concentration 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 observa- tions for several major SAF fault segments with large historical events, suggesting that those segments can host large events with deeper slip. In simulations, fault segments hosting such deeper-penetrating earthquakes are char- acterized by deeper coseismic slip, larger spatial extent of ground motion, a broad time-dependent depth transition from fully locked to near-plate creeping rates, and the associated time-dependent locking depth. One important implication is that using interseismic geodetic observations alone may underestimate the rupture extent of such earthquakes. Temporal and spatial variability in earthquake slip, event patterns, and depth extent of slip naturally arise due to interaction between dynamic processes and depth-dependent fault properties, even in models with uni- form properties along strike. Despite the increasing dynamic weakening in larger events, the events have compara- ble stress drops.
Broader Impacts Large-scale dynamic rupture 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 as- sessment 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 real- istic scaling laws for large events. Graduate students have gained valuable research experience by par- ticipating in the project and interacting with the SCEC community.
Exemplary Figure Figure 1. Possibility of large earthquake slip extending into the deeper creeping fault extensions and its effect on microseismicity and fault coupling (locking). (Left) Model with dynamic weakening (DW) re- stricted to the velocity-weakening (VW) region. (Right) Model with DW extending deeper into velocity- strengthening (VS) regions, which leads to deeper rupture. (Top panel) Coseismic slip distribution during a large event in color and microseismicity before the event (blue circles) and after the event (red circles). Clearly, the microseismicity is suppressed for the model with deeper penetration. (Middle and bottom panels) Early and late interseismic fault coupling (ISC, defined as 1 – V/Vpl, where V is the current fault slip rate and Vpl is the plate rate) shown in color with contours of 0.2. The transition between creeping and locked portions of the fault is time-dependent and occurs over a broad depth range. The locked region (black) narrows, making the top of the transition shallower, and the full creeping rates are achieved deeper, due to decrease of creeping rates that compensates for postseismic slip.