I'm not sure what the sensor was used, but according to news reports they are pressure sensors placed on the sea-bed, which are linked by cable to a floating buoy with GPS capabilities. Since pressure depends on the height of the column of water above it, and normal sea waves can be metres high while tsunami waves are merely inches or at most feets high in the open seas, the signal would probably be noisy. But if you have a large network of it, the signals could probably be processed to identify a wavefront.