Underwater volcano riding a sinking tectonic plate may have unleashed major earthquakes in Japan 

 A seamount sitting on a subducting tectonic plate off the coast of Japan and plowing its way into Earth's mantle may be at the root of several magnitude 7 earthquakes in the past 40 years. 

https://cdn.mos.cms.futurecdn.net/hrLoVv9sGdkgdMqs3mjbbj.jpg

https://www.space.com/underwater-volcano-sinking-tectonic-plate-japan-earthquakes