I recently bought the Garmin Forerunner 70, for a general run I use the Alerts settings to set a fast and slow alert. I want to be able to set these 15 seconds apart. e.g. 5:00 per km and 5:15 per km but whenever I do this they auto adjust to be 30 seconds apart. I used to be able to do this on my old Garmin vivoactive 3 can I really not do it on the newer model?
I can reproduce this issue on my Enduro 3 (I know, very different watch, but it is using the same codebase nowadays).
The gap always readjusts to be at least 30 seconds for me as well. I guess this is to avoid constant buzzing, it's very hard to run more accurate pace than within 30 second, however if you want to annoy yourself then that should be your right. 
An alternative solution is to create a workout in Garmin Connect: https://connect.garmin.com/app/workout/create/running
There you can make a workout with a very long "Run" step, and as far as I see in the "Intensity target" you can make the from-to range arbitrarily small. I managed to set it to 5:14-5:16. Then sync the workout to your watch and start the workout when you run.
