I am modeling a fault surface (which I consider to be a plane rectangle). I got the area of the surface, but the orientation is unknown, which can be found out if the aspect ratio of the rectangle is known. Is there a way to find this using the average slip rate?
Ok, so this is a somewhat complicated question, and while this isn't a full answer I think I may be able to help you find the right direction.
It sounds like you are doing a completely theoretical study, so in this case I would advise you to look at Fault Maximum Displacement vs Fault Length Ratios (commonly referred to as D/L in te tectonics community). Here is an older but good study on D/L ratios: ftp://ftp.gps.caltech.edu/pub/avouac/OwensValley/Dawers-Geology-1993.pdf
You will have to make some assumptions based on the type of faulting that occurs in your model.
I found a observational study on D/L ratios and slip rate: http://www.sciencedirect.com/science/article/pii/S0191814101000360
Which seems to be a well cited article on the subject. It seems there is some correlation, but perhaps it is different depending on the area. I know that Mars, Earth, and Icy Moons around Jupiter/Saturn all have different average D/L ratio which is directly correlated to the strain rates and material properties. Slip rate should have a similar, if not identical correlation (though slip is usually calculated on the surface where strain is often subsurface from my understanding).
What software are you using to model this fault surface? Is it a homebrew or is it a community code? I realize you might need to be a little secretive about what you are researching when posting on the internet, but a little more information might or might not go a long way in helping people provide a better answer.
Hopefully this helped.