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The method of hybrid loading is meant to tie faults to target long-term slip rates but then load them gently in a way that does not overforce them to slip in ways they would rather not, as they are ultimately self-organizing systems (24). Traditional backslip methods that load at a constant slip rate along a fault and to the base of the seismogenic depth create stressing rate singularities at the base and ends and then generate many small events at those edges that try to fill in the imposed long-term slip-rate profile. Here, we aim to recreate the long-term slip rates along the bulk of the fault but add gentler stress-rate loading to accomplish this. The procedure is as follows:

(1) Begin with a target slip rate. Typically, this is taken to be a constant along strike and with depth, but if further information is available to modify this, other slip profiles can be used.

(2) Calculate what the stressing rates would be for the fault system loaded in backslip mode with this slip-rate profile.

(3) Smooth and modify the backslip-estimated stressing rates. This is done with a series of filters.

(i) Add upper and lower unstressed layers to represent the non-seismogenic layers. On the top Z1 km and bottom Z2 km in depth, the shear stressing rate is zeroed out. This is done to get things loading and nucleating in the seismogenic middle zone. A physical justification is the lower-modulus upper layer and the higher-temperature creep relaxation processes in the lower layer. The effect in the simulator is to improve the depth dependence of the hypocenters.

(ii) Correct the stressing rate by 1/(HZ1Z2) to maintain the overall slip rate on the fault, where H is the fault depth. An additional multiplicative factor may be needed in the case of initial slip profiles, such as constant slip with depth, which have nonuniform depth dependence. An overall multiplicative factor correcting stressing rates can be added at this stage. For the case (H = 18 km, Z1 = 2 km, and Z2 = 4 km) we will be using in our model, a factor of 1.2 was seen to give a good match to slip rates.

(iii) On Z3-km-wide edges, take the median stressing rate for that fault depth and extend it to the sides. This gets rid of fault edge singularities. (In our case, we use Z3 = 5 km.) It also allows the fault slip to taper at the edges in a self-organizing way, consistent with an applied constant stressing rate.

(4) Using this new filtered modified stressing rate, run for a long time (hundreds of thousands of years).

(5) Measure the accumulated slip over this long run and divide by time to get slip rates on faults.

(6) Use this new empirical slip rate function as input to backslip loading. This is the new backslip-from-stress loading mode. This slip-rate function can be used for rupture parameters different from those than it was generated under, since it only depends on the cumulative slip, not the individual slip events.

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