Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 260

Figure 7.37 Influence of pulse period on the acceleration and velocity
(after Somerville, 2003)
Number of significant pulses. The number of significant pulses is defined as the
number of half-cycle velocity pulses crossing the axis, whose amplitudes are at
least 50% of the peak velocity of ground motion (Fig. 7.38). Examining the same
recorded earthquakes (see Figs. 7.27 to 7.29 and 7.33), one can observe that the
main characteristic of near-source earthquakes is the reduced number of significant
velocity pulses satisfying these criteria and that there are cases with one, two, three
or maximum four pulses. The number of significant pulses in fault normal
component of 48 near-fault records (including Californian, Japanese and Turkey
earthquakes) is presented in Figure 7.39 (Rodriguez-Marek, 2000, Stewart et al,
2001).