My money is on the CHICOMS. The message being, you are operating off our coast, we are off yours, (or Up Yours).
If we are just exploring the "foreign power" option then yes, that's more likely.
Russia doesn't currently have solid fuel SLBMs; they had old solid-fuel R-39 which were decommissioned around 2004, and they have new solid-fuel Bulava (RSM-56) which doesn't even work yet. The workhorse of the Russian Navy is the liquid-fueled Sineva (R-29RM,) it launches with no smoke because the liquid fuel burns cleanly. But the missile in the photo was using solid fuel, with unburnt particles creating the contrail.
China, on the other hand, has *only* solid fuel SLBMs (JL-1 and JL-2.) There are launch photos but they were taken at low light, so I can't say what the contrail looks like under the sunlight.
But on the higher level of the discussion one would doubt that China really would need such a demonstration of military power. First, China is weaker than the USA. Second, the Chairman of the Central Bank of China has more power over the USA than the entire Chinese military. Third, the fallout of such a prank would be unpredictable and not under Chinese control.
At this moment in time, when nobody knows anything, my bets are on some sort of a launch from a US submarine. Existing photos of Trident-II launch look similar to the occurrence near the LA. Also the launch happened in the known training and test area with heavy military presence - hardly the safest place for a foreign sub.
And North Korea ~ they have submarines ~ they have missiles ~ and they don’t care what we think.
