From the Raspivid documentation page:
--intra, -g Specify the intra refresh period (key frame rate/GoP)
Sets the intra refresh period (GoP) rate for the recorded video. H264
video uses a complete frame (I-frame) every intra refresh period from
which subsequent frames are based. This options specifies the numbers
of frames between each I-frame. Larger numbers here will reduce the
size of the resulting video, smaller numbers make the stream more
robust to error.
A sample command line to set I-frames at each 15 second interval (when recording at 25 fps) might look like this:
raspivid -g 375 -w 1280 -h 720 -fps 25 -t 10000 -b 3000000 -o test.h264
-g 375: Set 375 frames between each I-frame (15 seconds @ 25 fps)
-w 1280: width=1280 pixels
-h 720: height=720 pixels
-fps 25: 25 frames per second
-t 10000: record for 10000 milliseconds/10 seconds
-b 3000000: bitrate of 3000000 bits per second
-o test.h264: output to file 'test.h264'
While, from the looks of things, your videos are going to be very short, I think I'd be inclined towards a higher I-frame rate if I were ever planning on seeking through the clips. Media players tend to 'snap' to I-frame intervals, meaning that your sub-divided clips will effectively be un-seekable. You could perhaps aim for 3 or 5 second intervals, which would still allow for accurate 15 second divisions.