What matrix would give a radial blur in circular motion?

I've been looking on the internet and can't find a matrix filter example that would yield a blur like in the following image:

Maybe I'm not searching for the correct keywords. I've found Gaussian blur matrixes and linear blur matrixes, but no circular. Therefore my question: What matrix would give a circular motion blur? Anything can do (5x5, 9x9, etc.), I just need to see the pattern of weights around the central pixel and adapt it to my matrix's size.