SCEC Award Number 18093 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Mechanisms of unsteady shallow creep on major crustal faults
Investigator(s)
Name Organization
Yuri Fialko University of California, San Diego
Other Participants Junle Jiang - postdoc
SCEC Priorities 1d, 3g, 5a SCEC Groups FARM, SDOT
Report Due Date 03/15/2019 Date Report Submitted 05/14/2019
Project Abstract
 A number of active continental strike-slip faults are associated with geodetically detectable shallow creep, while other faults appear to be locked all the way to the surface over the interseismic period. Faults that exhibit shallow creep also often host episodic accelerated creep events. Spatially dense InSAR observations and temporally continuous creepmeter records, taken together, indicate that shallow creep events can involve large fault areas (up to tens of km long and several km down-dip) and persist throughout much of interseismic periods.
The physical mechanisms and fault zone properties giving rise to shallow transient aseismic slip remain poorly understood. A traditional interpretation of shallow creep in terms of a velocity-strengthening (VS) layer atop the velocity-weakening (VW) seismogenic zone (SZ) fails to explain spontaneous, episodic creep events. Special subsurface structure and lithological conditions, e.g., a thin VW layer within the VS shallow crust, may promote transient slip but seems unlikely to be always present to explain the large-scale and widespread occurrence of shallow unsteady creep events on many major crustal faults. In this project we have started to explore the rheologic controls on aseismic episodic slip events and their implications for seismic faulting in models of faults governed by laboratory-based constitutive laws, motivated by observations from the San Andreas and Superstition Hills faults in Southern Californa.
Intellectual Merit Our main findings are:
• We reproduce spontaneous slow slip events, with displacements of millimeters and periods of years that are consistent with interseismic observations from SHF and NAF, in a fault zone model having monotonic depth variations in rate-and-state friction parameters at shallow depths—a transition zone with mildly VW properties between the VS surface layer and the VW deeper seismogenic zone. Simulated slow slip events occur in the transitional layer due to recurring aseismic frictional instability in a region with a large nucleation size, with their characteristics controlled by frictional parameters, a, b, L, the nucleation zone size h∗ , and layer thickness W .
• Simulationsoflong-termfaultsliphistoryoverhundredsofyearssuggestsignificantoverlapandinteractions between seismic and aseismic slip within different layers and over different periods. For example, shallow VS layer modulates the behavior of slow slip events, as well as the fault failure potential during earthquake ruptures and the ensued afterslip. Additional fault strengthening mechanisms, such as dilatancy, may be needed to reconcile model results with postseismic observations from SHF and NAF.
• Ourmodelsdemonstratetwophysicallyplausiblescenariosforthelong-termbehaviorandseismicpotential of shallow crustal faults. In the first scenario, slow-slip areas are purely friction-controlled, susceptible to earthquake penetration and capable of limited afterslip. In the second scenario, slow-slip regions are also subject to additional fluid-related strain-strengthening processes, thus acting as a more effective barrier to earthquake rupture and producing larger afterslip.
Broader Impacts Overall, the modeling and parameter estimation schemes developed in this project provide new ways to probe depth-dependent fault friction properties and assess the extent of surface rupture of potential earthquakes. The framework of connecting observations of different seismic-cycle phases to fault zone properties for the two selected strike-slip faults also provide insights relevant to studying shallow sections of subduction zones. The developed numerical models will ultimately improve our understanding of seismic hazard to populated areas in Southern California. This project has provided training and support for one postdoctoral scholar (Junle Jiang).
Exemplary Figure Figure 3.
Shallow unsteady creep in a rate-and-state fault model. (a--b) The evolution of fault slip rates and slip at different depths following a large earthquake rupture. (c) The evolution of fault slip rates along depth with time. The fault slip rates, color-coded on a logarithmic scale, are averaged over one day and hence smaller than actual peak slip rates during transient events. Horizontal dashed lines indicate the interfaces between different layers. (d) Moment magnitudes of simulated large earthquakes at corresponding time.