The feat in question.[]
So let's cut to the cheese. In Chapter 100.1 of Mob Psycho 100 we see how Mob's alter ego (???%), by walking, creates an earthquake so powerful that it not only instantly flattened buildings around him but could be felt throughout Spice City and other neighbouring cities as well. In Chapter 76 of the manga, Mob at 100% Tenacity expands his psychic antennae and detects enemy espers around 20 km away from his house. Seeing as they are well within city limits when he arrives, we can safely estimate the radius of the city to be at least 20 km. Somewhat supported by the time they drive for around an hour to meet some spirirts in the woods but are still within the city when they arrive.
However, that's not all. In the 100th chapter it's revealed that the earthquake in question could also be felt in neighbouring cities, most notably the city where he exorcised the Kuchisake-Onna. In Chapter 53, Reigen and Mob need to take a train to this city and are at least in it long enough to hold a decent conversation. The combination of the distance between the city and town and the radius of Spice City can be used since Mob couldn't have made it too far from his home by walking when he started wrecking Japan.
According to Wikipedia, conventional line trains in Japan operate at a speed of at most 160 km/h and usually under 130 km/h. For the calc, I think 100 km/h seems most reasonable.
Distance from Spice City to Urban legend Town[]
I'll use 20, 30 and 40 minutes as the possible time-frames between them leaving Spice City and them entering the town. It seems rather unlikely that it could have taken any longer than this anyway.
Low-end: 100 km/h*0.33 hrs = 33.3 km.
Mid-end: 100 km/h*0.5 hrs = 50 km.
High-end: 100 km/h*0.66 hrs = 66.6 km.
Distance from Epicenter[]
Low-end: 20+33.3= 53.3 km
Mid-end: 20+50= 70 km
High-end: 20+66.6= 86.6 km
Total Seismic Energy[]
Magnitude 9 seems appropriate as buildings were immediately brought to the ground around him and the ground suffered massive upheaval as well.
Low-end: Magnitude at distance: 9+0.0238*53.3= 10.2685
Energy= 10^[(1.5*10.2685)+4.8]= 1.59496074971E20 Joules
Mid-end: Magnitude at distance: 9+1.1644+0.0048*70= 10.5004
Energy= 10^[(1.5*10.5004)+4.8]= 3.55303920815E20 Joules
High-end: Magnitude at distance: 9+1.1644+0.0048*86.6= 10.5801
Energy= 10^[(1.5*10.5801)+4.8]= 4.67896719183E20 Joules
Final Tally[]
Low-end: 38.12 Gigatons (Island level)
Mid-end: 84.92 Gigatons (Island level+)
High-end: 111.83 Gigatons (Large Island level)
I'm new at calcing so any criticism would be greatly appreciated.