how to avoid repetition in comparing two things in a line graph

I have a problem with repetition for describing this graph. This graph represents the strong ground motion acceleration (vertical axes) over the period (the horizontal axes) for different return periods with and without incorporating an effect called the directivity effect. Dashed lines are the ones including the directivity effect and solid lines do not contain this effect.
The upper two lines (i.e. the red line with its dashed line) represent the acceleration for the longer return period (2475 years), while the lower lines (the green line with its dashed line) show the acceleration for a shorter return period (475 years).
I want to say this: As the period (horizontal axes) increases, the dashed line of the red line increases more than the dashed line of the green one. It means the effect of directivity is higher on the 2475 years return period and is smaller on the 475 years return period. I wrote this:

"For longer return periods, the effect of directivity on strong ground motion acceleration is more than the effect of directivity on strong ground motion acceleration for shorter return periods."

I tried to avoid repetition using "than that of" but I thought it could not be appropriate. I was wondering if you could let me know how I can avoid the repetition of "the effect of directivity on strong ground motion acceleration".