Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 268

Figure 7.45 Evolution of vertical velocity at surface (after Aagaard, 2000)
The maximum horizontal and vertical ground motions along the directivity line
across the top of the fault provide a good indication of the severity of these
motions near to the surface rupture. There are some important problems which
must be underlining .
The first examined problem is the directivity effect . The magnitude of the
velocity vector showing the forward directivity is given in Figs 7.44 and 7.45, at
different surface sites. The velocity variation corresponds to pulse one with a half-
semi cycle. Site A lies directly above the top of the fault and site D lies at the block
boundary, at 25 km from the top of the fault One can observe that the vertical
components of velocity are higher than the horizontal ones; the maximum values
resulting in the site directly located above the top of the fault.