Kerosene heater is common heating method than heat pump in Japan cold area. But nowadays heat pump increasing share because it gets efficient and kerosene heating is bad for air. Especially it's prohibited to use on better insulated house, and heat pump's power is enough for such house.

I believe why natural gas heating is not popular is because they are a bit expensive and gas pipes aren't common outside big city. I'm getting curious why gas heating isn't popular even in big city (Sapporo). Your assumption (earthquake) looks reasonable for a part of reason.

Maybe gas heater is hard to install/uninstall and never use except winter. So we don't want to put all year on small house is reasonable.