Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 251

effect of almost all of the seismic radiation from the fault. Figure 7.28 shows the
directivity effect of ground velocity in strike-slip earthquake from two near-source
recordings of the 1992 Landers earthquake (M 7.3). This recorded earthquake is
the most representative one among the large number of records for the near-source
effects, where one can see the differences in records in the forward and backward
rupture directivity.
Due to the forward directivity, the Lucerne record shows a large brief velocity
pulse of motion, while the Joshua Tree record, situated in the backward directivity
region, consists of a long duration without significant velocity pulses and low
amplitude record. Although this example refers to a strike-slip earthquake, the
conditions required for forward directivity are also met in thrust earthquakes
(resulting from subduction or collision faults). The alignment of both the rupture
direction and the slip direction up the fault plane produces rupture directivity
effects at sites located around the surface exposure of the fault. Consequently, it is
a general case when all sites located near the surface exposure of crustal faults
experience forward rupture directivity for an earthquake occurring on these faults.
Velocity pulse
Figure 7.28 Directivity effect during 1992 Landers (USA) earthquake
(after Somerville, 1997)