We model the San Fernando earthquake as a propagating rupture in a half-space, using for the slip-time-history on the fault plane analytical expressions which approximate the slip functions of dynamic crack models obtained by Das and Aki (1977a, b). We synthesize the strong ground motions and accelerations at the Pacoima Dam site and compute the teleseismic signals for different models of cracks. Three major featuras of the data–the strong pulse associated with the beginning of the rupture, the high acceleration phase on the Pacoima Dam records, and the presence of ripples on the teleseismic seismograms–which are not compatible with a smooth rupture process, are well explained by a crack with barriers model where the rupture encounters, along the fault plane, barriers or obstacles of high strength materials which may remain unbroken after the passage of the rupture front. A high-stress drop (400 to 500 bars) is required in the hypocentral area to explain the high-amplitude short-duration first pulse of the teleseismic records. This indicates a high level of tectonic stress in the area. A study of the earthquake series following the main shock shows that the aftershocks which took place in the region where major slipping occurred during the earthquake may represent the release of some of the barriers.