The Tankentai SBS can manage variables and switches into the action sequence of an attack. For example: at the beginning you can set a random number between 1 and 10 into a variable, then check that variable if it's =1 (1 of 10 is 10%) link to your quake, doing the normal attack followed by quake. I'm not sure if it's possible to set a random number from T-SBS actions, if not, I'm sure it still can handle scripts to run a simple one that adds a random number into a variable... also can be done as a common event in the skill I think.You'll need to set a switch or variable to be checked as "actor has the sword with the chance to proc quake" on the normal attack sequence, to limit to the correct actor with the correct weapon, the chance to launch that skill. Or use a state... I mean, when equip that weapon you can add an invisible state to that actor, that can be used to do the "actor has the sword with the chance to proc quake" check in the action sequence.
So, play around with variables, conditionals and link-skills action sequences into SBS configuration to match that kind of effects you want. Must be confusing for beginners, but nothing that can't be done on trial and error.
Hope that helps... If you can understand my english