Effect of various factors on spectral values
Near Fault Motions and Fault Rupture Directivity

For near-fault motions ag increases, but vg increases more dramatically due to effect of a long period pulse. This pulse is generally most severe in the fault normal direction (as it can cause fling), but significant displacement also occurs in the fault parallel direction. The fault parallel direction usually has much lower spectral acceleration and velocity values than the fault normal direction. Sample waveforms are located in a previous section of the notes, Factors Influencing Motion at a Site. No matter the directivity, however, the motions very close to the fault rupture tend to be more severe than those located at moderate distances.

Somerville et al. have developed a relationship which converts mean spectral values generated from attenuation relationships to either the fault parallel or fault normal component of ground motion. See the Interactive Example for a demonstration of the shift of the spectrum in the long period range.