They do of course get a small fraction from geothermal - heat rising from the core - something like 2W per m2 of surface - especially in winter, when nett solar replenishment has stopped or gone negative at the surface, and the GSHP is drawing entirely on u/ground stored or sourced heat, AKA freezing the subsoil. That effectively locally accelerates the constant, non-seasonal dissipation of core heat to atmosphere, creating a local temperature depression or 'vacuum' that sucks more of same in from surrounding ground. Under those circumstances, a GSHP may be getting as much as 5W/m2, 24/7, from geothermal.

My book on renewable energy claims that at boundaries between plates there is an average of 300 mW/m^2 (0.3 x 10^-1 W.m^-2) , the Earth average is 60 mW/m^2 (0.06 x 10^-1 W.m^-2).
Radioactive decay is is estimated at 5 x 10^20 J.a^-1
So not sure where you have that thermometer.

A lot depends on local geology. Around here here there is a lot of radioactive decay near the surface, but still tiny compared to the solar absorbed by the rocks. Though a lot of heat pump energy comes via water that has run off the surface and soaked into the ground. This increases the capture area immensely. Rainwater is very definitely solar energy. Though yesterday there was lots of rain and little sun!

Posted By: SteamyTeaEarth average is 60 mW/m^2

As little as that? somewhere I have a map showing enormous differences even leaving aside hotspots like Iceland. Old rock masses like most of Africa v low; ocean beds v much higher; mixed geology like western Europe in between.

Posted By: SteamyTeaa lot of heat pump energy comes via water that has run off the surface and soaked into the ground

That only applies during the summer solar-replenishment phase; will actually be a ground-cooling mechanism in winter. In any case, does not widen the catchment area of solar collection.

What may (or may not - it's a lottery) contribute greatly as prime source of heat collected by GSHP, is groundwater flow, either horizontal by mass flow, or vertical by water table rise and fall.

Posted By: fostertomThat only applies during the summer solar-replenishment phase; will actually be a ground-cooling mechanism in winter. In any case, does not widen the catchment area of solar collection.

There is solar gain in other times/ways/places as well.
Think where the rain predominately comes from, (in the UK, Canada's will be a USA import ), after a few phase changes it falls as either rain, hail or snow. All ways it is energy transfer from the tropics to the higher latitudes, we would be, at 50-60N a lot colder without it. Convert from the Celsius to the Kelvin scale to get a true measure of it.

Posted By: fostertomprecipitation is cooling the ground

But not all of it. If you think of the SHC of water, and then how much falls as rain onto given a m^2 of ground, you can work out the energy transfer. This has to be against the backdrop of how a heat pump extracts energy from the ground. It uses a fluid that is cooled to around 0C (or 273K in real money), that fluid is then warmed slightly before it cools in the HP. Now as water hold more energy per unit mass than soil, as long as it is above 0C when it soaks in, more energy can be extracted at a higher rate from wet ground than dry ground even if the overall temperature is lower. Sounds odd, but it works.

Posted By: bot de paille
presumably badgers dig their burrows below the frost line?

Heavier than air gas needed then