Dear Irene,
If the staff are taking leave under emergency, then you may do the analysis of the number of leave applications received per month and out of that how many were planned ones and how many were availed of under emergent situation. Unless you calculate the percentage, you cannot take any decision.
It appears that you have written this post out of suspiciousness on the genuineness of the "emergency" reasons. However, whether under an emergent situation or otherwise, the leave application is always approved by the HOD. If HOD finds it ok, then why meddle in the internal matters of the department?
Furthermore, each employee earns a leave after working for a certain number of days. This is a statutory benefit. As long as they are availing of leave within their quota, why the company's administration should have a problem?
Worse still, you have asked whether legal action can be taken against these staff. However, availing of leave is not misconduct per see. Therefore, the provisions which act or law would you like to take an action?
The malaise of availing sudden leave could be lying somewhere else. It shows their low morale. They could be lacking the attachment towards their work or the company itself. If the employees are mentally disconnected, then no amount of the threat of legal action will work.
As stated in the first paragraph, do the analysis of the "emergency leave". Share the details with the HODs. Find out whether they come up with any solution. Involve them in finding a solution. However, it should be within the framework of law.
Thanks,
Dinesh Divekar