Vertical excitation of partially filled horizontal cylindrical tanks can induce large-amplitude sloshing, particularly when the forcing frequency approaches twice the fundamental sloshing frequency, leading to parametric resonance. Under these conditions, parametric resonance can trigger exponential growth of free-surface perturbations, leading to large-amplitude waves, interface breakup, and enhanced mixing. In this work, we experimentally investigate the sloshing regimes associated with this primary parametric instability. Experiments are conducted in a transparent horizontal cylindrical tank ($D=134.5\mathrm{mm}, L=336.3\mathrm{mm}$) over a range of fill ratios and excitation conditions. A data-driven, image-based methodology is developed to identify and classify sloshing regimes directly from high-speed visualizations. The approach combines multiscale proper orthogonal decomposition for feature extraction with prototype-based clustering and support vector machine classification. The results are summarized in a dimensionless regime map across three fill ratios ($H_l/D\in [0.40;0.67]$), distinguishing stable free-surface conditions, dominant longitudinal modes, and mixed or mode-interaction regimes. The map provides a structured characterization of the flow responses in the vicinity of the primary parametric resonance.

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• Instability of free-surface flows
• Interfacial flows
• Waves and free surface flows