very simple ROT:
500lb'ers for frag
2000lb'ers for blast - blast is what you would need in this case. They would have been slightly delay fused (milli-seconds) to allow detonation on the inside of the hull, possibly the reason it sank to the left (i.e underwater holes on the port side).
From the IR footage, it looks like different trajectories for the two bombs, which suggest 1 bomb each from 2 x F111 dropping near simultaneously from a wide formation. (mind you, have only had one quick look at it so could be wrong).
Disclaimer: having said all above, 500lb'ers work great on tanks due to better manoeuvreability, smaller target and can carry more.
Also as an aside, the F18 will finally be getting 1000lb'ers which is basically what the aeroplane was designed around - only taken 20 years!!