If on a miss, shot will only scatter 2d10 meters.
An earthshaker cannon has a blast of 20.
Therefore, an earthshaker will almost always hit on target, no matter the distance, no matter the penalty (barring direct GM intervention).
Also, it seems odd that the shot will actually hit the ground in the same round it is fired, even if the thing is 12km away. I'll admit, I'm no ballistics specialist, but this would appear to indicate a muzzle velocity of about 2400 m/s, which, according to wikipedia, is almost 4-5 times that of a modern howitzer. I know I tend to be one to say it is silly to try to compare things to real life, but this feels "uncinematic" to me.
Also, strictly RAW, the wording on what the GM should actually modify is that the GM assigns the penalties to hit the target. At the end of the day, the penalties don't matter, as even a miss will still roughly blow away a target.
I'm fine with it being a deadly weapon, but it seems more likely to TPK a party if they ever come under fire, not severely maim them. It would be nice if this system had a Shadowrun style blast system, where they deal less damage as a function of distance (with the nasty side effect of shockwaves while in tight corridors). If in AP12 cover, a basilisk will still dish out 4d10-1 damage to a party (assuming they're tightly knit). This is accounting for AP4 and TB3. On average, this is about 21 damage to PCs. If using the human average of 12 wounds, this puts them at 9 crit damage, almost guaranteeably dead in the system.