Deployment of cabled stations on the ocean bottom enhances the effectiveness of early warning systems for suboceanic earthquakes in terms of early signal detection and improved accuracy of hypocentre analyses. We investigated strong-motion data at cabled ocean-bottom stations deployed in the source area of the 2011 Tohoku earthquake (M_w 9.0) in northeastern Japan. We found a significantly large change in the direct current component in accelerations due to the roll and pitch motions of the cabled stations that are proportional to strong motions. We further found that the roll angle is affected more by the strong motions than the pitch angle owing to the cylindrical shape of the station housing in a long cable. The observed large roll motions caused anomalous displacement waveforms with significantly large amplitudes, which have contributed to the overestimation of magnitude in the two components perpendicular to the cable direction. Applying the scheme to three-component data to estimate real-time magnitude for early warnings in Japan resulted in high-magnitude estimates of M9.10–9.76 for the 2011 Tohoku earthquake, which are higher than the magnitude saturation and moment magnitude M_w 9.0 of the earthquake. Such an overestimation was suppressed when the single-component data along the cable axis direction were used instead, with the resulting magnitude of M8.47–8.63 almost coinciding with the catalogue magnitude of M_JMA 8.4. For other event data that are not affected by the roll and pitch motions, no large systematic difference was found between the magnitudes estimated using the single- and three-component data. We therefore suggest that the use of single-component data could be an effective approach for stable magnitude estimation at cabled ocean-bottom stations for early warnings.