Spirit/Demon/Sacred energy aura can kill seemingly when exerted at full power (or close to it). As
@ATARi.HMB stated, Koenma even says that Sensui could have killed Team Urameshi just by flexing his Sacred energy at full (or close to full) power. This is why I have doubts about scaling Team Urameshi (Upper A-class keys only) to 6-A.
Great Kanto Quake was natural, and would be in the High 6-C to 6-B ballpark, correct? King Enma is also stated to be responsible for some of the most severe natural disasters in human history. And those were just him throwing hissy fits.
As Yusuke was reviving, it was announced to Spirit World Special Forces that "destruction of a country is imminent" (at least in the viz translation; don't know what the original Japanese says). The Spirit World Special Forces believe themselves to be capable of handling any Demon World threat, including seemingly the strongest A-class beings (which would also put them on Upper A-class). But they don't know about the S-class beings' true power and shit themselves just on sensing the power of a relatively weak Low S-class; and Kurama later on states that S-class applies to any demon/threat that Spirit World's people can't handle. All of this points to Upper A-class being at least country level threats.