Figure 1. A schematic diagram of three-dimensional fault geometry. On the fault plane, (ab ) for Scenario 3 is shown as an example (L = 40 km; d = 0 km; Figure 2 and Table 1). The dip angle of the fault plane is 15˚. The gap between the colored fault patch and the bounding dashed line is set to be velocity strengthening.
2.3 Parameter choices
Model parameters are explicitly stated in this section. Wherever possible, values of parameters are chosen to match values from appropriate laboratory experiments or field observations. One exception is the critical slip distance Dc : we constrain the parameter mainly based on considerations of computational tractability. We refer to the temperature dependence of the RSF constitutive parameters (ab ) for phyllosilicate/quartz-rich fault gouge under hydrothermal conditions reported by den Hartog and Spiers (2013) and a classic characterization of the megathrust frictional environment proposed by Lay (2015).
The transition at the updip from velocity strengthening (ab > 0) to velocity weakening (ab< 0) takes place approximately at 250˚C, corresponding to 10-16 km depth (40-60 km along dip in our models) assuming a geothermal gradient of 16-25˚C/km. We choose a lower bound of (ab ) of -0.0030 in the unstable sliding regime (Figure 6a in den Hartog & Spiers, 2013) for our rupture scenarios. We thus construct the depth profile of (ab ) as shown in Figure 2a. In addition, we employ an apparent along-strike (or dip) thickness of 30 km (true depth thickness of ~8 km) of a velocity-strengthening layer on left, right, and bottom boundaries of the fault plane to gradually arrest the rupture in our models (white area on the fault plane in Figure 1). The critical slip distance,Dc , is 0.015 m and is homogeneously distributed at all depth. As we will introduce in Section 2.5, we design different scenarios with different apparent thicknesses of updip transition (L ) and conditionally stable layer (d ) to examine effects of depth-varying friction properties on rupture characteristics (Figures 2a and 3). The (ab ) value of a conditionally stable layer is set to be -0.0015 so that it is closer to velocity neutral behavior.