Detailed analyses were conducted on the accelerograms and their associated Husid plots, IA(t)/IA(tf) versus t. It was found that a main parameter controlling the duration of strong ground motion was the total duration of rupture at the fault, d. Rock records are dominated by a strong part, having a duration, Δ, which is essentially a straight line in the Husid plot, and which is Δ ≲ D. This strong part coincides approximately with the arrivals of S waves which followed a direct path between source and station (direct S waves). The values of both Δ and D are mainly controlled by the duration of rupture at the fault, d, for M ≲ 7. Many soil records have, in addition to the strong part, another part which is moderately strong, has longer periods and appears after the direct S wave arrives. This additional part is responsible for the increased values of D observed for soil, and also for the curved shape of the Husid plot observed in many soil records. This part also contributes significantly to the observed richer content of long periods in soil records, reported by several authors. Some evidence suggests this part of the record may be mostly associated with the amplification by the soil of indirect, multipath body-wave arrivals, and with surface wave effects.