Abstract: The study on ground motion characteristics of sand liquefaction sites holds significant importance for seismic fortification of major engineering projects. Based on ground motion records from the 2010-2011 Canterbury earthquake sequence in New Zealand, this research conducted a comparative analysis of ground motion characteristics at strong motion observation station sites during liquefaction and non-liquefaction events. Through time-frequency analysis using Hilbert-Huang Transform (HHT), the impact features of liquefaction on seismic wave spectra were revealed. The results demonstrate that acceleration time histories of liquefied sites exhibit distinctive "spiky" harmonic oscillations. By analyzing specific frequency component variations in time-frequency diagrams - particularly the significant reduction of mid-high frequency components and dominance of low-frequency components - the frequency drop feature can effectively determine the timing of site liquefaction initiation. Validation using data from the Wildlife Liquefaction Array (WLA) in the United States, combined with corresponding pore pressure monitoring data, confirms the feasibility of using frequency component changes in ground motion time-frequency diagrams to characterize liquefaction triggering time. This finding provides valuable diagnostic indicators for determining sand liquefaction status based on surface strong motion records.