I'm in a bit of a morbid streak so sure, I'll bite.
Honestly the first thing we shouldn't be doing should be narrowing it to one disaster, because the thing about Mob going apeshit is that he had several effects on his surroundings. I think, for the sake of simplicity, we should compare Mob's outburst to and earthquake, a tornado and a hurricane.
The strongest earthquake I have like, more knowledge about is the earthquake that happened here in México in 1985. It had a magnitude of 8.1 on the Richter scale and it killed 3,192 people according with official documents, with other organizations rounding the numbers on 20,000. So a lot of people. And it killed them in about a minute and a half.
For the tornado, this is an easy one because the fastest and biggest tornado on modern history actually happened fairly recently. It was de tornado of El Reno in 2013. That shit was BIG. It was 2.6 miles wide (4.2 kilometers for those who do not live in the US), and it had ground wind speeds of 296 miles per hour (or 476 kilometers per hour) which according to experts are some of the fastest speeds ever recorded. However, and despite how terrifying that thing was, it doesn't have a high death count, with only 9 people dying that day and 3 of them being storm chasers (which is important because these people, as the name suggest, go arround following storm cells and record tornadoes). But the reason it didn't kill a lot of people, or destroy a lot of property was because the region it developed was fairly rural, so not a lot of stuff around to destroy.
Which is why I also mentioned hurricaines, because they're more or less the same concept as tornadoes (lots of destructive and speedy winds) but they can do things tornadoes don't do, and that is that they linger around. Naturally, the best choice for this would be hurricaine Katrina, which killed 1,392 with winds going as fast as 170 miles per hour (280 kilometers per hour) and lasting 7 days, from the 23 of august to the 30th of that same month. It cost the US 2005 dolars on damages.
What I'm going with this is that we need to consider all the damages we can see in these pictures, especially the one you put as an example, as well as consider the facts we have about Mob's outburst. Firstly, we know he caused a tornado, Serizawa mentions this and the news channels do as well. Second, neither tornadoes nor hurricaines cause those kinds of cracks on the ground (water pipes bursting don't count) so naturally the thing we need to compare it to is an earthquake. With all that taken into account...well, it's not looking very good. As I said, the earthquake here in México killed a shit ton of people in just half a minute, and as far as I know it didn't cause that much destruction, so we can assume that, if Mob was causing something similar to an earthquake, it had a magnitude above 8.1. Then we need to take into account the hurricaine/tornado aspects. Although Mob's tornado definitely wouldn't last as long as Katrina or other hurricaines (because usually hurricaines, depending on where they were formed, can last a lot of days on land) it would last longer than a tornado, simply because Mob, despite how destructive he was being, was walking at a normal speed. Like, that boy, Shiego/Mob, was taking his time, and it kind of depends on how far he was from Tsubomi when it all started. Aside from the lingering, strength and drestruction wise I think it's fair to compare it both to tornadoes and hurricaines, just because Mob is that strong (I have no idea how the fuck Reigen didn't die).
All that being said, It all also depends on the poblational density of Seasoning City. I don't think it's as dense as Tokyo, but, judging from its appearance I think Seasoning City may be closer to Saitama (which, yeah, funny, and I wouldn't put it past ONE because, you know, One Punch Man and he loves throwing easter eggs left and right), which had a poblational density of 1 218 431 million people in 2010, two years before MP100 started publishing and probably around the time ONE started planning and doing his research for the manga.
So If we like, sum every single death that these natural disasters caused we would get (for the earthquake, I'm using only the official number) 4,593 deaths. Which doesn't seem like a lot in a city with millions of people, but we can all agree it is very much a big deal.
And the reason I'm not taking into account the evacuation and Teru saving people is because I'm fairly sure the numbers I gave you do take into accounts any deaths caused by any wounds produced by the disasters. I might be wrong tho so anyone who wants to factcheck me is welcome to do so.
Anyway, I hope this somewhat quells the need you had about this :)
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