Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 254

Vertical components. Another interesting phenomenon observed from the near-
source earthquakes is the increasing of vertical accelerations. It is generally
acknowledged that vertical motions are smaller than the horizontal ones. Recent
observations during the last important earthquakes have demonstrated that at the
near-source areas the vertical accelerations are sometimes greater than the
horizontal ones, due to the effect of last ball (Fig. 7.31a) and direct propagation of
P-waves (Fig. 7.31b) Many strong-motion recordings obtained in the near-source
areas have exceeded unity and the widely used 2/3 rule-of-thumb (Fig. 7.31c)
(Gioncu and Mazzolani, 2002, Elnashai et al, 1998). The ratio of the vertical to
horizontal spectra is shown in Figure 7.32a as a function of distance and periods
(Gioncu and Mazzolani, 2002, Bozorgnia et al, 1996) and distance and magnitude
in Figure 7.32b (Gioncu and Mazzolani, 2002, Elnashai and Papazoglu, 1997). The
increase in ratio at small distance from the fault in near-source areas can be
dramatic (Gioncu and Mazzolani, 2002, Carydis, 2005).

(a) (b)

Figure 7.31 (continues)