The favorite model for earthquake rupture now is the "slip pulse". As you say, the rupture starts at the hypocenter, and spreads, usually mainly unilaterally (just in one direction), but sometimes fairly symmetrically out in both directions along the fault.

The waves from the entire duration of the rupture interfere, sometimes constructively, sometimes destructively. Part of the interference comes from the fact that there are several types of waves travelling at different velocities (P, S, and surface waves). Directivity, as you mention, is strongest in the direction of the rupture.

Another factor in the pattern of the strength of shaking is the radiation pattern. These are the butterfly-shaped patterns - strike-slip, thrust, and normal faults each have a distinct pattern of P and S waves they send out, related to their displacements on the fault.

If you check the link, you can see how the rupture developed in the Sumatran earthquake.

John

• our model of the Sumatran rupture