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It would be difficult to find out the user who deleted Site collection. However you can use Get-SPDeletedSite -ContentDatabase "WSS_CONTENT" to obtain a deleted site collection.Try to find the /_layouts/deleteweb.aspx and /_admin/delsite.aspx. Also check whether automatic deletion of site is not enabled in central admin.

For more information,

Please check the following links,

https://social.technet.microsoft.com/Forums/office/en-US/6423bc37-b320-40ff-b953-f3b8f2d845fe/get-the-user-information-who-deleted-the-site-collection?forum=sharepointgeneralprevious

Trace who and when a site collection got deleted Permanently in sharepoint 2013