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We report laser probing results on a set of piezo electrically actuated square plate MEMS resonators. The plate size of the resonators is systematically varied in order to study the frequency scaling of the main, square-extensional resonance mode in the range between 13-30 MHz. In electrical impedance measurements, an unexpected splitting of the main mode branch into two separate mode branches is observed at intermediate plate size dimensions. To get further insight of the origin of the splitting phenomenon, the two observed mode branches are directly measured by laser probing. The laser probe measurements confirmed that the vibration modes on both branches had a lateral vibration field characteristic for a square-extensional mode. Furthermore, the results indicate that the square-extensional mode may couple to other resonance modes occurring around 20-22 MHz, hence causing the splitting of the mode branch.