Abstract
Recently, determinations of focal parameters have been widely investigated by comparing observed seismograms with synthesized ones. As a source model, the dislocation model has been accepted as a realistic one, since it was introduced in the field of seismology. For the medium, however, in almost all the studies a very simple model such as an infinite or a semiinfinite medium is assumed. In this case, one should recognize limitations in the appropriateness of the assumption.
In the present study, theoretical seismograms due to a moving fault in a layered medium are compared with those in an infinite and a semi-infinite medium. The results are summarized as follows:
Roughly speaking, if waves are incident on each interface of the layered medium with a small angle of incidence, then the seismograms in the simple medium differ little from those in the layered medium. But even for this case, if the infinite medium is assumed, one should take account of the development of the surface SP waves and Rayleigh waves, contributed from the upper part of the fault. If the angle of incidence is so large that almost total reflection takes place at the lower boundary beneath the source, the difference is significant. For this case, computations on only major phases are recommended to save computation time, referring to the reflection and transmission coefficients at each boundary due to the incidence of the plane wave. Waves which undergo P-S or S-P conversion at the boundary are in general not major phases.
It seems quite difficult and not practical to obtain synthesized seismograms of particle accelerations because of inadequate accuracy and long computation time. It is better to estimate the maximum acceleration using the rough relation (maximum acceleration)=(maximum velocity/rise time).
