We validated the source inversions of the intensity data set of the Whittier Narrows ML 5.9 1987 earthquake by comparing the source characteristics retrieved by our inversion technique with those already known from instrumental measurements. In our grid-search technique the squared residuals (calculated minus observed intensity at the sparse sites) are weighted by the Voronoi polygons tessellation, and then the sum of the weighted squared residuals is minimized. For comparison, we also minimize the sum of the unweighted squared residuals. The intensities are calculated by our kinematic KF function. Twelve source parameters are involved in the inversions, the most sensitive of which are the epicentral coordinates and the fault-plane solution. To find the minimum variance model, we explore the whole space of the angular values that control the fault-plane solution and place some reasonable constraints on the other parameters. Two minimum variance models were determined by the inversions: one source that is in agreement with that already known from instrumental measurements and one almost coinciding with its auxiliary plane in the fault-plane solution. This finding confirms that, notwithstanding the introduction of more source parameters, the nature of the problem of the inversion of an almost pure dip-slip earthquake, is still close to bimodality. It is demonstrated that it is possible to get an approximate idea of the source of the studied earthquake by inverting the U.S. Geological Survey intensities. This result encourages us to validate our inversion technique with more well-documented earthquakes and to treat intensities of preinstrumental earthquakes, which are the principal target of our work.