Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 250

Figure 7.27 Characteristic records for near-source earthquakes: 1994 Northridge
earthquakes (Gioncu and Mazzolani, 2002)
front, while the fling step is the permanent displacement of the ground near the
rupture fault. The estimation of ground motions close to an active fault should
account for these characteristics of near-source ground motions.
An earthquake is produced by a shear dislocation which begins at a given point
on a fault (asperity) and spreads with a velocity being almost as large as the shear
wave velocity. This phenomenon is known as rupture directivity pulse. The fault
rupture tends to propagate slightly slower than the shear wave generated by slip
and the rupture reinforces the shear waves as far as it propagates. If the rupture
propagates towards a site (forward direction), then the energy arrives over a much
shorter time interval than the duration of the entire rupture, whereas if the rupture
propagates away from the site (backward direction), the energy arrives over a time
interval which roughly matches the duration of the entire rupture. This feature
follows the same principle as the Doppler Effect. Not all near-source earthquakes
present this effect, because two conditions must be satisfied: the rupture front
propagates towards the site and the direction of slip on the fault is aligned with the
site. The propagation of fault rupture toward a site at very high velocity causes
most of the seismic energy from the rupture to arrive in a single large long period
pulse of motion (Somerville, 1997). This pulse of motion represents the cumulative