def METHOD_NAME(C, ctx):
    fsite = np.ones_like(ctx.vs30)
    below = ctx.vs30 <= CONST_DURATION['Vs30']
    above = ctx.vs30 > CONST_DURATION['Vs30']
    fsite[below] = C['d5'] * np.log(ctx.vs30[below])
    fsite[above] = C['d5'] * np.log(CONST_DURATION['Vs30'])
    return fsite