This letter demonstrates the application of scanning acoustic force microscopy for the characterization of film bulk acoustic resonators beyond detection of the gigahertz vibration amplitude and acoustic wave field imaging. We present a method to measure thermal expansion effects on these resonators by modulating the driving signal amplitude and then varying the modulation frequency from a few hertz to several tens of kilohertz. For the particular device considered here, we show the influence of thermal expansion effects on its electromechanical response and the acoustic wave field images. The thermal relaxation time constant of the resonator is measured and compared with a theoretical estimation based on the heat transfer analysis of the system.

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