Basically, Orochi survives a hit to the head that is so strong that it generates a seismic energy that makes one of Doppo's portraits on his home to shake and fall from a shelf.
First, on chapter 55, we get the information that for Doppo to get from his house to the arena it was needed a car and that he left his home afternoon and when he got to said arena it was night. So i'll assume a 2h timeframe. Using 25km/h for the average speed of the car should be alright considering there's transit and all that, for a total 50km of distance between the arena and his home.
So, we have a seismic energy that is capable to make the equivalent of a Magnitude 3 earthquake since all the effect we really see is the portrait falling from the shelf.
" A magnitude 3.0 earthquake is defined as the size event that generates a maximum ground motion of 1 millimeter (mm) at 100 km distance. "
"At 100km distance" must be refering to the diameter, the radius being 50km. So all conditions meet for we to assume the energy of that hit is the same as the energy of a radiated wave from a magnitude 3 earthquake. That is listed here as 0.47 tons, 8-C.
If we were to assume total seismic energy of the earthquake, it'd be 9.3 kilotons and it would upgrade prime Doppo to 7-C.