Modify Member Directory Search Query to “start with” keyword

I was wondering if you could point me in the direction on the best place to change the search ability so that in a member directory, when you search for a first name or last name, it looks for the keyword you type in and finds records that have that keyword at the start of the field, not just anywhere in the field. In MySql terms, it would be a like query with a wildcard only at the end.

So if I type “th” into the last name search, it returns “thornell” but not “smith”

How would I accomplish this? Thanks for any help you can provide.