Precariously balanced rocks in seismically active regions may be used to provide constraints on probabilistic estimates of strong ground motion. A computer program is developed to study the rocking response of models of precariously balanced rocks when subjected to various levels of ground accelerations. The computer code, validated by comparing the numerical and experimental results for simple-shaped rigid blocks, is used to study the rocking response of rectangular blocks with various aspect ratios to the El Centro strong ground motion recording of the 18 May 1940 Imperial Valley earthquake. Results show that during earthquakes, the horizontal components of ground motion play a dominant role in overturning rocking objects. The rederived and corrected Housner's equation (1963) could be used to estimate the minimum overturning acceleration of precarious rocks at different frequencies by measuring the quasi-static toppling acceleration in the field.