Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 225

7.2.4 Forward Directivity
After a slipping of rocks in the earthquake zone starts, due to breaking of the most
exposed asperity, the rupture propagates out from this zone. The space-time history
of rupture depends on many properties of the fault. For instance, the presence of
asperities, invaded barriers and barriers, elastic properties of fault, etc., will hinder
or encourage the propagation in a certain direction (Warren and Sheare, 2006). In
addition, a branching, a curved or stepping fault will influence the rupture
propagation. Therefore, the observations regarding large earthquakes show that
they have primarily unilateral ruptures. One of the most representative cases for the
forward directivity is the 1995 Kobe earthquake (Fig. 7.5), when the rupture
directivity was exactly under the most densely populated part of the city, producing
the above-mentioned great damage. Fortunately, the opposite was the situation of
the Northridge earthquake, where the directivity was in the North direction, a less
populated zone of the city.
Figure 7.5 Effect of the forward directivity during the 1995 Kobe earthquake
(Gioncu and Mazzolani, 2002)