This is evidence of a major disaster, an earthquake that hit the city in 62 A. D., just 17 years before the final eruption. What we find is that, in this wall for example, that we have the brick in the corner which surrounds this nice diamond shaped wall. In fact, it's broken off at this point, and is instead replaced by a much more poorly built wall. And so the walls have been rebuilt with that rubble into this much more basic design. 

 

Even though the people on Pompeii didn't know, it is clear to volcanologist  today that this earthquake of 62 B.C.(A.D.?) was a precursor of the eruption. It indicates that the magma is about to reach the surface. And a new eruption is about to occur.

 

Pompeiians definitely didn't understand the dangerous position they were in. They even built the city on a offshoot of the previous lava flat.