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The star is near the southwestern end of the main fault slip patch, so a much longer part of the finite-source rupture propagates northeastward. For comparable distances, ground conditions and buildings, the northeastern site receives the stronger, shorter direct pulse; the southwestern site receives the weaker, longer direct shaking. This is rupture directivity.^[ref]

For a segment of length $\ell$, rupture velocity $v_r$ and seismic wave speed $c>v_r$, successive arrival times span approximatelyForward arrivals overlap and concentrate energy into a shorter pulse. Backward arrivals spread out. The supplied finite-source rupture is not perfectly unilateral, so the prediction concerns the dominant contribution. Seismic radiation patterns, seismic attenuation, sediment amplification and building response can alter the ranking of actual damage or total coda duration.

The shorter, stronger northern pulses and broader southern pulses suggest propagation from south to north, towards the northern station. The useful evidence is the difference in pulse widths, not just the earlier northern arrivals: the latter chiefly reflect station distance.

As rupture reaches successively more northerly patches, each patch radiates a new P wave and S wave pulse. Towards the north, the later emission time is partly cancelled by a shorter propagation path; those contributions arrive closer together. Towards the south, later emission and a longer path add, stretching the signal. This is rupture directivity. It refers to the advance of the slipping region along the fault, not necessarily the direction of the local fault slip vector. A thrust can slip up-dip while its rupture front advances in a direction with a northward component.

More generally, emission from a patch at distance $s$ along the rupture arrives atThe forward direction has a smaller arrival-time spread. This is the same travel-time compression captured by isochrone-based rupture directivity calculations.^[ref] Larger northern amplitude supports the interpretation but is not decisive on its own because geometrical spreading, seismic attenuation, seismic radiation patterns and site response also affect amplitude. With only two traces, propagation, filtering and site effects remain alternatives to test; the idealized exercise is most naturally explained by northward directivity.