Multiple high altitude EMP bursts to take out the power grid, GPS, Internet and phone system would only require an aggregate of 20-50kT.
Or Putin could set off one of his Sakarov Tsunami-producing, high rad, deep ocean cobalt mega-weapons they developed during the Cold War and produce a 1000 ft. tidal wave that would wipe out the Benelux countries, UK, and most of France, while devastating the east coast of the US from Boston to Baltimore up to 100 miles inland.
We've known about Soviet nuclear torpedoes for years, Putin intentionally leaked this again as a form of sabre rattling.
100-megaton warhead in Atlantic Trench could devastate US east coast and much of Europe coast with massive tsunami and intense radiation.
http://www.bbc.com/news/world-europe-34797252
On the diagram the giant torpedo's range is given as "up to 10,000km" (6,200 miles) and depth of trajectory is "up to 1,000m" (3,300ft). It was developed by Rubin, a submarine design bureau in St Petersburg.
It would, apparently, be launched by nuclear-powered submarines of the 09852 "Belgorod" and 09851 "Khabarovsk" series. Rossiiskaya Gazeta called the torpedo a "robotic mini-submarine", travelling at 100 knots (185km/h; 115mph), which would "avoid all acoustic tracking devices and other traps".
According to state-run Rossiiskaya Gazeta, the destructive power attributed to the new torpedo's warhead would fit the description of a cobalt bomb. That would be a type of thermonuclear warhead with a layer of cobalt-59, which on detonation would be transmuted into highly radioactive cobalt-60 with a half-life longer than five years.
Such a weapon would guarantee "that everything living will be killed", the paper said - there would not even be any survivors in bunkers. A cobalt bomb has never been tested because of the devastating radiation it would unleash.
"But it can be considered as a means of deterrence - like the Perimetr system, which is on combat readiness, which guarantees retaliation with all of Russia's nuclear forces even if command posts and the country's leadership have been annihilated".
Russian military experts told BBC Russian Service:
A warhead of up to 100 megatons could produce a tsunami up to 500m (1,650ft) high, wiping out all living things 1,500km (930 miles) deep inside US territory - Konstantin Sivkov, Russian Geopolitical Academy
Robotic torpedo shown could have other purposes, such as delivering deep-sea equipment or installing surveillance devices. The Russian defence ministry has a special division for deep-sea research - Konstantin Bogdanov, Lenta.ru website
This is no secret for the US, whose military is also working in the area of robotic submersibles for hunting and destroying submarines - Viktor Murakhovsky, reserve colonel, editor of Arsenal of the Fatherland magazine.
The submarine, the newspaper said, had been called “Sakharov’s Torpedo” after the Soviet physicist Andrei Sakharov, but had never been built. Meduza, a Lithuania-based news portal covering Russian affairs, cited blogs of Russian military history enthusiasts asserting that the design resurfaced in the 1980s, under the code name “Skif,” but that it was again shelved.