30 June 1908: The asteroid fragment that OTL exploded above Siberia in the Tunguska Event instead, due to its travelling slightly faster than OTL, explodes directly above St. Petersburg with a force equivalent to 15 Megatons of TNT, completely destroying the city, wrecking the Russian Baltic Fleet, instantly killing the Tsar and his entire family, and decapitating the Russian government and military command structure.
What happens next?