Three-dimensional wedge analyses were performed for each of
the 137 recorded Northridge earthquake motions. All results are
presented with the recorded horizontal components set to azimuthal
directions of 0° and 90°. Calculations were performed for
normalized yield accelerations (ay /amax values! of 0.9, 0.8, and
0.5. The reference acceleration amax by which the yield acceleration
was normalized, was taken as the Pythagorean sum of the
maximum positive accelerations ~those causing downslope movement!
for the two recorded horizontal components (amax
5Aa1max,1
2 1a1max,2
2 where a1max,i is the maximum positive acceleration
for the ith horizontal component!; depending on the
individual motion, this reference acceleration may be greater or
less than the highest peak acceleration in any particular azimuthal
direction. Detailed results will be illustrated for two of the 137
motions—the SYH ~Sylmar Hospital parking lot! and LAS ~Los
Angeles 116th St. School! motions. Station SYH was located approximately
2 km northeast of the rupture area, in an area likely
to experience directivity and other near-field effects. Station LAS
was located approximately 37 km southeast of the rupture area,
i.e., in an area unlikely to experience significant near-field effects.
Time histories of the SYH and LAS motions are shown in Fig. 9.
The azimuthal variations of peak horizontal acceleration and
single-component Arias intensity ~computed from time histories
obtained by resolving the two horizontal components of each motion
in 360 azimuthal directions! are shown in Fig. 10. Both peak horizontal acceleration and Arias intensity can be seen to exhibit
considerably more azimuthal variation at the SYH ~near-field! site
than the LAS ~far-field! site.