Path Effects in Strong Motion Seismology.

Records of earthquake- and explosion- induced motions are studied by seismologists 1 to learn the details of earthquake and explosion sources 2 to determine earth structure and 3 from the knowledge of sources and structures, to predict the shaking at the surface of the earth from earthquakes and explosions. This paper explores methods of computing the motions that result from elastic waves propagating through complex structures. We apply these methods to understand the effect of laterally varying near-surface geologic structure on ground motions. Strong ground motion waveforms recorded by seismic stations on soft rock sites are generally more complicated than those recorded on hard rock sites. Unfortunately, from a modeling point of view, most earthquakes occur in complicated geologic settings. Since cities, and therefore strong-motion instruments, tend to be in basins interpretations of the strongest motion records require consideration of propagation through complex structures. Some progress has been made in modeling longer period waveforms, particularly from nuclear blasts. Fewer parameters suffice to describe explosions than earthquakes and they are generally better known. Some aspects of explosion modeling are reviewed. A source description is developed for use in 2-dimensional finite difference or finite element methods that can produce approximate point source seismograms that include the effects of complex structures.