Civil Engineering Reference
In-Depth Information
0.04
g.
However, at the Lake Zone, the peak ground accelerations
a
max
were up to 5 times
greater than at the rock site (Kramer 1996). In addition, the characteristic site periods were
estimated to be 1.9 to 2.8 s (Stone et al. 1987). This longer period of vibration of the ground
tended to coincide with the natural period of vibration of the taller buildings in the 5- to 20-
story range. The increased peak ground acceleration and the effect of resonance caused
either collapse or severe damage of these taller buildings, such as shown in Figs. 4.23 to
4.25. To explain this condition of an increased peak ground acceleration and a longer period
of surface vibration, an analogy is often made between the shaking of these soft clays and
the shaking of a bowl of jelly.
Loma Prieta Earthquake in San Francisco Bay Area on October 17, 1989.
A second
example of soft ground effects is the Loma Prieta earthquake on October 17, 1989. Figure
4.26 presents the ground accelerations (east-west direction) at Yerba Buena Island and at
Treasure Island (R. B. Seed et al. 1990). Both sites are about the same distance from the
epicenter of the Loma Prieta earthquake. However, the Yerba Buena Island seismograph is
located directly on a rock outcrop, while the Treasure Island seismograph is underlain by
FIGURE 4.25
Building damage and tilting in Mexico City caused
by the Michoacan earthquake in Mexico on September 19, 1985.
(
Photograph from the Steinbrugge Collection, EERC, University of
California, Berkeley.
)
