Most of the analytic, theoretical work and the attempts to explain the observed seismograms in the vicinity of propagating faults have used kinematic models of a moving dislocation in an infinite elastic medium. It is usually assumed that the existence of a free surface has the effect of approximately doubling the amplitude of the infinite medium solution.
The result from an exact solution for the seismic displacements caused by a finite two-dimensional dislocation in an elastic half-space which is moving with uniform velocity and constant slip along a straight line are obtained for the stepfunction time-dependence by using Cagniard's method. They are compared with the observed Pacoima Dam record of the San Fernando earthquake of 1971. The computed displacements compare significantly better with the observed record than the corresponding infinite medium solution. They show that while the free surface approximately doubles the near-field long-period motion of the infinite medium solution, the effect is more dramatic for the high-frequency motion. The solution indicates that the conversion of S to P and P to S at the surface is responsible for the highest acceleration (1.25 g) recorded on the Pacoima Dam record which is the highest acceleration ever recorded during an earthquake until now.