First of all you absolutely need to stop physically touching employees. Even if you think its in a friendly, joking way its easy to misconstrue, folks may not like it and be afraid to say anything, and of course you now have the repercussions of the allegations you described. Its simply not appropriate under any circumstances. Even the "funny statements" are not a good idea. While you didn't describe them, its again something that easily can be misconstrued. As a manager you need to be professional and set a good example.
Otherwise, its hard to undo what's occurred and her workers compensation claim will have to play itself out. Be very careful in your future dealings with this employee, and as appropriate carefully document any future performance issues that she may have... If the threats you described are work related and disruptive, my guess is that at some point you will need to address them with her... Best of luck to you...
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Your employer will have its workers comp. insurance carrier defend the employee's claim. It will probably have its attorney contact you to discuss what happened. Be cooperative and truthful. Your employer's civil or workers comp counsel will provide better advice than anything you will get on a free website. The one clear thing you must do from this point, stop with the taps on the head or anywhere else. You may think it is funny but not everyone may agree and you are subjecting yourself and your employer to a possible lawsuit.
They say you get what you pay for, and this response is free, so take it for what it is worth. This is my opinion based on very limited information. My opinion should not be taken as legal advice. For true advice, we would require a confidential consultation where I would ask you questions and get your complete story. This is a public forum, so remember, nothing here is confidential. Nor am I your attorney. I do not know who you are and you have not hired me to provide any legal service. To do so would require us to meet and sign written retainer agreement. My responses are intended for general information only.
Sadly, all it takes is one complaint to "ruin" a fun environment. Keep in mind, however, that it probably more fun for those who were doing the tapping than it was for those being tapped.
Have you apologized to her? That may help lessen any resentment she may be carrying toward you and her coworkers. A little show of compassion can go a long way. More importantly, you must stop joking around on a physical level with your employees.
You should let the workers' comp carrier or TPA deal with the claim and refrain from any involvement in that other than what is necessary for your job in terms of reporting the injury and whatnot. Do NOT under any circumstances tease her or gossip about her work injury. Further, you should refrain from taking any adverse action against her for filing that claim, as that may amount to discrimination under Labor Code section 132a.
If you feel that you have information that is relevant to the compensability of the workers' compensation claim, you should give that information to the adjuster, then wash your hands of it. Let them do the investigating.
I agree with what my colleagues have said as well. I just want to end with one last note: you say that no harm was intended... just realize that the intention isn't as important as the fact that harm may have been brought about. As they say, it's all fun and games until someone gets hurt.
Best of luck to you.
The answers you have already received are worth their weight in gold; please take them to heart. I have one more suggestion: your misadventure would not likely have occurred if you had been properly trained as a supervisor regarding hostile work environment and sexual harassment. No such training would fail to point out that supervisors must not physically touch subordinates, however benign or playful their intentions. Such training is required by law for supervisors if the employer has more than 50 employees. If your employer does not have that many employees, fine, the training is not required by law - but it is still a good idea and, please don't take this the wrong way, but you obviously need it. Do some research on line, find training course for supervisors, ask your employer to sign you up for the training (or, if they won't, just do it yourself). Education is the best way to protect yourself from unfounded claims.
This response creates no attorney client relationship; consult a local lawyer for help if you proceed.