One way to do it is to make a rectangle shape that is a little longer then you want the smaller lines to move. Use the repeater and and then transform the repeater rotation till you get a full circle of lines
At this point you should see a bunch of lines rotated around a center point. You don’t want to do anything else to shape layer 1
Then make a circle shape and turn off the fill and add some stroke. The bigger the stroke the longer the lines will be as they move from the center to the outside. Animate the scale from 0-100 and use it as a matte so just stop the scaling wherever you want it to end . To get the lines to get smaller at the end you’ll have to animate the stroke width about halfway through the scale animation. I also added a Fast blur and animated that half way through as well as fading out the entire circle shape layer 2 frames before it stops moving.
I am not great at explaining what I did so this may make no sense whatsoever.
Lastly if you have a few bucks you can use AESweets to do something very similar with there explosions pack.
Chris Holland
Editor/Animator