Creep Events on Continental Faults

In the uppermost ∼3–5 km of continental strike-slip faults, a significant fraction of the total slip is typically accommodated by stable sliding, or fault creep. Creep can be continuous or episodic, lasting only a few hours, and varies throughout the earthquake cycle and from one fault to another. The most commonly used mechanical model attributes episodic creep events to the transition from unconsolidated sediments to lithified rocks at depth. However, this model cannot explain the wide variability in observed shallow creep characteristics on strike-slip faults in California.

In an article published in Nature Geoscience, my colleagues and I use numerical simulations to examine a range of alternative mechanical models that can reproduce the variability of shallow creep behaviour in both postseismic and interseismic periods. We find that geodetic observations of creep behaviour on a number of significant fault segments in California are matched when an additional unstable layer is embedded within the shallow, stable zone. This layer may result from fine-scale lithological heterogeneities within the stable zone—frictional behaviour varies with lithology, generating the instability. Our model suggests that the displacement of and interval between creep events are dependent on the thickness, stress and frictional properties of the shallow, unstable layer. We also suggest that such frictional heterogeneity may be the mechanism responsible for slow slip events in many subduction zones.

In another paper in Earth and Planetary Science Letters, we document the history of fault creep events on the Superstition Hills Fault based on data from creepmeters, InSAR, and field surveys since 1988. We focus on a subset of these creep events that were triggered by significant nearby earthquakes. We model these events by adding realistic static and dynamic perturbations to a theoretical fault model based on rate- and state-dependent friction. We find that the static stress changes from the causal earthquakes are less than 0.1 MPa and too small to instantaneously trigger creep events. In contrast, we can reproduce the characteristics of triggered slip with dynamic perturbations alone. The instantaneous triggering of creep events depends on the peak and the time-integrated amplitudes of the dynamic Coulomb stress change. Based on observations and simulations, the stress change amplitude required to trigger a creep event of a 0.01-mm surface slip is about 0.6 MPa. This threshold is at least an order of magnitude larger than the reported triggering threshold of non-volcanic tremors (2–60 kPa) and earthquakes in geothermal fields (5 kPa) and near shale gas production sites (0.2–0.4 kPa), which may result from differences in effective normal stress, fault friction, the density of nucleation sites in these systems, or triggering mechanisms. We conclude that shallow frictional heterogeneity can explain both the spontaneous and dynamically triggered creep events on the Superstition Hills Fault.

 

Figure 1. Variability of surface fault creep in California. a, Map of major faults in California. Coloured solid lines show types of shallow creep recorded by creepmeters and theodolite measurements30. b,c, Creepmeter data from Roger Bilham (University of Colorado) and USGS. Episodic creep events are defined by the step-like features in the data of type B creep. SAF, San Andreas fault; SHF, Superstition Hills fault; HF, Hayward fault; SJB, San Juan Bautista. The starting dates for the shown data are: SJB, 1 January 1991; HF, 1 January 1997; SHF, 24 November, 1987; SAF near Parkfield, 28 September 2004.
Figure 2. Two schematic models that may explain the variability of surface fault creep. Model A is a traditional model. Model B is proposed by myself and my collaborators to explain both rapid afterslip and frequent creep events observed on the SHF. In Model A, continuous creep arises from the stable zone and episodic creep originates from the conditionally stable zone. In Model B, continuous creep arises from Layers 1 and 3, whereas episodic creep originates from Layer 2.
Figure 3. Numerical simulations and observed surface slip history on the SHF between 1988 and 2015. The black lines are data. The red solid line is the reference model with no dynamic perturbations from nearby earthquakes added. The blue line shows a simulation that included dynamic perturbations from three nearby earthquakes.