Environmental Engineering Reference
In-Depth Information
Many cities, particularly in California, have prepared maps of the city giving ground
amplification factors based on soil and rock conditions. They are available on the Internet.
11.2.7 Duration
The duration of strong ground motion plays a direct role in the destruction caused by an
earthquake. It is a function of the size of fault rupture and fault type, path from the source
to the site, and site geology.
Some examples illustrate the variability of duration:
The San Francisco event (1906) started with a relatively small ground motion,
which increased to a maximum amplitude at the end of about 40 sec, stopped for
10 sec, then began again more violently for another 25 sec.
●
Agadir, Morocco (1960) was essentially destroyed in 15 sec and 12,000 of a pop-
ulation of 33,000 were killed.
●
Guatemala City (1976) was first struck by an event of magnitude 6.5 which lasted
for 20 sec. The first quake was followed by a number of smaller shocks (after-
shocks). Two days later, two more shocks occurred, one with a magnitude of 7.5,
and in the following week more than 500 shocks were registered.
●
Anchorage, Alaska (1964) experienced a duration of the order of 3 min, which
resulted in widespread slope failures where they had not occurred before, even
though the area had been subjected to strong ground motion on a number of
occasions (see
Section 9.2.6
and
Section 11.3.4)
.
●
Strong ground-motion duration is currently generally defined by (1) bracketed duration
and (2) significant duration. An example is given in
Figure 11.22.
The
bracketed duration
is
August 17, 1999 Kocaeli, Turkey earthquake,
M
w
= 7.4
400
Sakarya record, EW
Bracketed duration
19.4 sec for 0.05
g
200
0
−
200
−
400
0
10
20
30
40
50
60
Time (sec)
100
95%
50
Significant duration
Husid plot
15 sec
5%
0
0
10
20
30
40
50
60
Time (sec)
FIGURE 11.22
Bracketed duration and significant duration illustrated for the Sakarya record of August 17, 1999 Kocaeli,
Turkey (
M
w
7.4) earthquake. (From Erdik, M. and Durukal, E.,
Earthquake Engineering Handbook,
Chan, W. and
Scawthorn, C., Eds., CRC Press, Boca Raton, Florida, 2003. With permission.)
