This is not a decision you, as a supervisor in charge, will make. Inform your immediate supervisor, your Chief, or the Deputy Director of Public Services of the situation.
Decisions to close the library are generally made at the executive level. The Deputy Director of Public Services will likely be the person to make the decision about closing. Your immediate supervisor will likely be the person to inform you. This is true whether the emergency affects all branches or only your branch.
Things that may cause a branch to close include, but are not limited to:
inclement weather (flooding, storms, ice)
power outages
water main breaks
other serious plumbing problems
natural gas leak
If your emergency involves imminent life threatening issues, you can close the branch and notify the immediate supervisor immediately. Examples of these may include fire, bomb threats, or gunfire.