During an earthquake, seismic waves are emitted from the slipping part of the fault behind the rupture front. Since the rupture velocity is usually close to the shear-wave velocity, the amplitude of the seismic waves ahead of the rupture front grows progressively as more energy is added by the propagating fault. This energy buildup results in a large pulse of motion at the arrival time of each kind of seismic wave in the seismogram that contains the cumulative effect of rupture on the fault (197). The radiation pattern of the shear dislocation causes the motions of the large pulse to be oriented perpendicular to the fault. Forward rupture directivity effects require two conditions: the rupture front propagates toward the site, and the slip direction is aligned with the site. These conditions are readily met at locations away from the epicenter in strike-slip faulting and are also met during dip-slip faulting in the region located updip of the hypocenter. The enormous destructive potential of near-fault ground motions was manifested in the 1994 Northridge and 1995 Hyogoken Nanbu earthquakes. In each of these earthquakes, peak ground velocities as high as 175 centimeters per second were recorded (Figure 5.13). The periods of the near-fault pulses recorded in both of these earthquakes were in the range of 1 to 2 seconds, comparable to the natural periods of structures such as bridges and midrise buildings, many of which were severely damaged. These near-fault recordings have led to revisions of building codes in the United States.
The effects of absorption are described by the quality factor Q, which is inversely proportional to the fractional loss of energy per wave cycle. In the Earth, the Q value depends on the frequency of the seismic wave and the properties of the rocks. Values of Q in the crust and lithosphere are much lower than those in the underlying mantle, and they vary significantly with the tectonic environment. In general, attenuation is lower in tectonically stable regions, so earthquakes cause damage at much greater distances in stable regions than in tectonically active regions. Also the frequency dependence of Q is greater in areas of active tectonics (198). Proposed absorption mechanisms in the crust include frictional sliding on cracks, thermoelastic effects, grain boundary deformation, and dissipation by fluid movement within cracks and pores. For depths less than 1 kilometer, there can be strong attenuation from open cracks in near-surface rocks and losses in unconsolidated soils, causing the intensity of ground motions to diminish with increasing frequency beyond about 5