In seismology an earthquake source is described in terms of a fault with
a particular rupture size. The faulting process of large earthquakes has
been investigated in the last two decades through analyses of
long-period seismo- grams produced by advanced digital seismometry. By
long-period far-field approximation, the earthquake source has been
represented by physical parameters such as s ismic moment, fault
dimension and earthquake mag- nitude. Meanwhile, destruction often
results from strong ground motion due to large earthquakes at short
distances. Since periods of strong ground motion are far shorter than
those of seismic waves at teleseismic distances, the theory of
long-period source process of earthquakes cannot be applied directly to
strong ground motion at short distances. The excitation and propagation
of high-frequency seismic waves are of special interest in recent
earthquake seismology. In particular, the descrip- tion and simulation
of strong ground motion are very important not only for problems
directly relevant to earthquake engineering, but also to the frac- ture
mechanics of earthquake faulting. Understanding of earthquake sources
has been developed by investigating the complexity of faulting processes
for the case of large earthquakes. Laboratory results on rock failures
have also advanced the understanding of faulting mechanisms. Various
attempts have been made to simulate, theoretically and empirically, the
propagation of short-period seismic waves in the heterogeneous real
earth.
