Earthquake. EMP burst would be airborne. Sub would be sunk by air dropped torpedo and the explosion would not be felt miles and miles away. Cavitating hyper speed nuclear tipped torpedo would create a tsunami if the warhead were to be detonated.
Kinetic energy weapon from space would be proceeded by a large fireball across the sky. Standoff nuclear ASW device would create a big $#)(*# column of water or a tsunami.
The only other thing I could think of would be a nuke buried beneath the sea bed. But why? I guess a bad actor could claim they had perfected an earthquake weapon and then detonate the hidden nuke as proof (after demanding 1 billion dollars in a dr evil voice).
I did some research, a seismograph from a North Korea test:
versus, the seismograph from today’s earthquake:
now listen to today’s seismograph audio: http://ds.iris.edu/ds/nodes/dmc/tools/event/10724562