Upon completing the upgrade from one to six degrees of freedom of the Outdoor Shake Table at UCSD in 2019, forced vibration tests were carried out to identify the dynamic characteristics of the reaction mass and soil system. This report describes the motivation, execution, and results from such tests, which independently excited the reaction mass in four degrees of freedom: longitudinal, transverse, yaw, and vertical. The report discusses the frequency response curves and deformation patterns from which the natural frequencies, damping ratio, mode shapes, and rigid body motion were determined. The first objective of the study was to investigate if the dynamic properties of the system had dramatically changed after the upgrade by comparing the results to those from forced vibration tests performed 20 years ago, during the construction of the facility. In addition, most recent tests also contributed with results from the vertical degree of freedom, which had never been tested. The second objective was to obtain high-quality response data of the system that will be used to develop a high-fidelity computational model of the reaction mass in future research. A comparison of results showed a slight difference of 0.5Hz in the natural frequency of 2 degrees of freedom. Moreover, maximum displacements in the recent tests were overall larger than the previous ones with few exceptions. The report thoroughly discusses the several sources of discrepancy between the past and most recent results. Finally, test results allowed us to estimate the system's response if the shake table actuators were to be used at their maximum nominal capacity. Small displacement and high damping results were consistent with those of previous tests and further validated the design of the reaction mass.