Possible Clues:

• In the main, it’s a source of upheaval
• Big ocean motion
• In the main it constitutes a seismic convulsion
• Cause of a tidal wave
• Underwater rocks, light blue and black, finally in sight
• One might shake the bed
• Source of big waves
• Look for foamy water, as you might say? It'll come with this!
• Source of some big waves
• Seismic disturbance under the ocean