Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 226

7.2.5 Rupture Duration

The duration of the rupture process depends on the amount of radiated energy,
magnitude, geometry of the rupture area and speed of the rupture process (Trifunac
and Novikova, 1995a,b, Lee, 2002).The duration of rupture can be described by an
exponential function of the magnitude (Gioncu and Mazzolani, 2002). For low
magnitude earthquakes the duration is under 0.5 seconds, for moderate earthquakes
the duration is about 2 seconds and for large magnitude earthquakes the duration
can reach more than 10 seconds. For great earthquakes the rupture duration is more
than 15-20 seconds.

7.2.6 Pulse Types

The number of pulses depends on the number of active asperities. In many cases
the propagation of the fault rupture toward a site causes most of the seismic energy
to move from the rupture with a single large pulse of motion (time required for a
complete cycle) (Somerville, 2000a,b). This pulse of motion represents the
cumulative effect of almost all the seismic radiation from the fault.
Figure 7.6a presents the case of the 1994 Northridge earthquake, where only
one asperity was active on the rupture surface. One can see that the recorded
velocity has a single pulse. Contrary, in some cases where there are more asperities
on the rupture surface, the velocity record has multiple pulses. Figure 7.6b shows
the recorded velocity of the Kobe earthquake, where it is supposed that the
earthquake was produced by three asperities.
These periods increase as far as the magnitude increases (Fig. 7.7). For low
earthquakes, the pulse periods are under 0.5 seconds, while for great earthquakes,
these periods can reach over 10 seconds. The pulse period depends of fault type: is
larger for strike-slip earthquakes in comparison with the reverse (subduction) ones.

(a) (b)

Figure 7.6 Pulses of rupture: (a) One pulse (Northridge); (b) Three pulses (Kobe)
(Gioncu and Mazzolani, 2002)