Civil Engineering Reference
In-Depth Information
Figure 1.17
Directivity effects on sites towards and away from direction of fault rupture (
adapted from
Singh,
1985 )
as ' directivity ' . Benioff ( 1955 ) and Ben -Menachem (1961) demonstrated that such directivity can lead
to azimuthal differences in ground motions. Directivity occurs because fault ruptures are moving wave
sources, which travel at a fi nite velocity along the fault. The engineering implication of such directivity
effects is that sites that are equidistant from the source will be subjected to varying degrees of shaking
from the same earthquake, thus casting doubt over the concept of distance-based attenuation relation-
ships discussed in Section 3.3. In Figure 1.17, a pictorial representation of directivity effects on ground
motions at sites in the direction of, and away from, fault rupture is given. As the fault rupture (or
earthquake source) moves away from the epicentre, it generates ground motion from each segment of
the breaking fault. The ground motion radiates outward in all directions and the seismic energy propa-
gates through expanding wavefronts.
The over -riding of stress waves or 'constructive interference' results in larger ground- motion mag-
nifi cation with shorter total duration in the direction of rupture propagation. Lower amplitude motions
and longer total duration are exhibited in the opposite direction. This effect increases as the velocity
of the fault rupture reaches the speed of seismic waves and as the angle between the point of observa-
tion (e.g. the recording station and construction site) and the direction of rupture propagation is reduced.
Constructive interference, which is in essence a Doppler effect, generates strong pulses of large dis-
placement or ' fl ing' at nearby sites towards which the fault rupture is progressing (Singh, 1985 ; Somer-
ville
et al
., 1997), e.g. towards the left in Figure 1.17. Rupture directivity also causes the polarization
of ground motion, i.e. differences between the fault-normal and fault-parallel components of horizontal
ground - motion amplitudes (Stewart
et al
., 2001). This polarization causes more intense shaking in the
fault-normal direction than in the fault-parallel direction. Where suffi cient information exists, directivity
effects should be taken into account in estimating earthquake design parameters. Directivity or focusing
of seismic energy caused severe damage to residential buildings and transportation systems in urban
areas during the 1994 Northridge and 1995 Kobe earthquakes (Broderick
et al
., 1994 ; AIJ, 1995 ).
Damage to structures during past earthquakes is illustrated in detail in Appendix B.
1.3.2 Site Effects
The characteristics of the site affect the frequency and duration of earthquake ground motions. Struc-
tures founded on rock will, in general, be subjected to short-period (high frequency) motion, while soft
sites result in longer period (low frequency) excitation. The ratio between the period of the site and
that of the building is important in estimating the amplifi cation effects; this is known as ' site resonance
