Having such a minuscule mass in freefall orbit would not create much in the way of seismic disturbances. I mean, the pedantic and accurate Vulcans would probably notice them all right, but they wouldn't declare national emergency over the issue.
However, "low geosynch" appears to call for non-freefall, essentially dictating that the Narada be using some sort of a drive to hover over Vulcan. That drive, especially if antigravitic in nature, might register as a seismic disturbance of such an exceptional type that Vulcans would be within their rights to get alarmed.
However, later on, Pike's starship enters a similar hover right next to the Romulan vessel. It doesn't appear to be an exceptional event at all.
Basically, three things specific to the movie could create noticeable, exceptional and alarming seismic disturbances: drilling with the beam drill, creating a subterranean black hole with red matter, or creating a timehole with red matter. All of these could be plausible explanations for the event.
1) We know Nero started drilling after the disturbances were recorded. But that might have been the second time he started drilling; the first hole might have failed for some reason, perhaps hitting too thick a stretch of bedrock, and forcing Nero to start anew next to the Sarek mansion.
2) We know Nero inserted red matter into Vulcan through the hole he drilled. But perhaps he first tried it without drilling a hole, and this black hole just fizzled out because it didn't have enough surrounding matter feeding into it. The pressure of Vulcan's mantle would provide the hole with this crucial mass feed.
3) We know Nero and Spock were delivered to the 23rd century thanks to the deliberate release of red matter into the 24th century supernova by Spock; this created the timehole into which both parties then accidentally fell. But Nero, knowing this and now possessing red matter in abundance, might have deliberately released more red matter to create a second timehole that allowed him to take Vulcan by surprise (perhaps bypassing some of the heaviest defenses ever devised by the Federation, namely those between Vulcan and Romulus). A bit unlikely for a mentally unstable bluecollar, I guess - unless he blackmailed Spock into doing it for him.
A variant of this would have Nero defuse the supernova in the 23rd century, using the same technique Spock used too late in the 24th. He did claim he had "prevented genocide"; dropping some of Spock's red matter into the would-be supernova would fit that bill. And if the red matter were dropped into a star dangerously close to Romulus, it might also be dangerously close to Vulcan...
Timo Saloniemi