Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 269

1 4

2 5

3 6

Figure 7.46 Evolution of rupture along the rupture surface (after Aagaard, 2003)

The attenuation of these velocities is very strong at site D, being only a quarter
of the maximum value. Contrary, the horizontal velocity components remain very
high, showing a reduced attenuation with the distance from the source.
The second examined problem is related to the differences between normal and
parallel ground motions (Aagaard, 2000). For the same Earth block the velocities
at two sites S1 and S2 are examined (Fig. 7.47). Site S1 lies 10 km North of the
North end of the fault surface and site S2 lies 10 km East of the fault center. One
can see that only the velocities of site S1, situated on forward directivity, have
important pulse velocity values. The site S2 is practically unaffected by the ground
motion. The maximum horizontal velocity obtained at 5 km from the top of the
fault is about 140 cm/sec.
The third analyzed problem refers to the influence of dip angles (Aagaard et al,
2001c). The results show that the maximum horizontal value is obtained for a dip
angle of around 60 degrees and the horizontal velocities are higher than the vertical
ones in the field of 90 to 60 degrees. Contrary, the vertical velocities have the
highest values for 45-50 degrees, being higher than the horizontal ones in this field
(Fig. 7.48). These results confirm the great values of the vertical components
recorded in some seismic stations during the Northridge and Taiwan earthquakes.