What condition(s) need to be met to be able to determine the hypocenter of a microseism?

Today, this XKCD popped up in a chatroom:

comic CC BY-NC 2.5 linked to the source

User Rob mentioned that (as usual with XKCD) this is based on the findings in a research paper.

In Source location of the 26 sec microseism from cross‐correlations of ambient seismic noise by N. M. Shapiro, M. H. Ritzwoller and G. D. Bensen the method to determine the location of microseism with a period of 26 seconds is described. While the hypocenter is indicated in latitude and longitude I couldn’t find in that paper at what depth the microseism takes place.

Living in a country known for its seismic activity I’m used to pinpoint accuracy:

map from KNMI, image links to live map

To take two examples from the above map, Weert and Farmsum I learn the hypocenter of one is tectonic and one is induced. Both have a precise location, different magnitudes (1.1 vs 0.2) and a depth (12 vs 3 km).

What condition(s) need to be met to be able to determine the depth of a hypocenter of a microseism?

I’m not an Earth scientist and although I’m not scared of scientific details I appreciate if an answer comes with subtitles in layman terms.