Both, actually, though not, perhaps related.
First, towns and villages out in the middle of nowhere Siberia used to disappear on a regular basis under the Soviet regime, for various reasons, ranging from plague to famine to KGB relocation/liqudation to getting overrun and destroyed by the Chinese. Sometimes the KGB didn't look into the matter too closely (if they weren't responsible). There's still a whole lot of questions of just what the heck happened out there from Stalin on; and I really doubt that anyone will find a statistically significant percentage of the answers.
Next, yes - that's what a tactical nuke is, and these detonations seemed to appear in that class on the seismograph. There is another possible explanation, though.
Starting 1969, the Soviets were known to have deployed a good portion of their nuclear forces to the Sino-Soviet border zone, sometimes right up to the border itself. It's entirely possible that in at least one of the events, a Soviet missile position was about to be overrun by Chinese troops and faced with a "use it or lose it" scenario, the commander used the "salvage-fuse" option and set one off.
However, that's speculation as, well, nobody's talking about just *what* happened on that border.
would small nukes leave visible terrain features visible today? thinking if a person knew what to look for, google earth might provide a reference.
The other possibility is that the nukes were used strategically instead of tactically. The Soviets set off a nuke close to the field of conflict as a warning. The Chinese did the same. And both sides realized they needed to step back from the brink at that point.
I would not be surprised if, over the course of the next year, Israel sets off a nuke 400 miles south of the Iranian coastline as a warning.