The Basel earthquake has been estimated with a intensity of IX, which can probably be correlated to a Richter scale (or the more accurate Seismic Moment). I found a hazard analysis on Basel here wherein they write:

Earthquake Scenarios for the City of Basel

The calculated scenarios are an intensity VII - VIII earthquake and one of intensity IX. These represent the expected value with return period of 475 years generally used in building codes, and the intensity of the earthquake in 1356.

Meaning, the building codes do no provide for earthquakes the magnitude of the 14th century Basel quake, and the estimated damage is considerable (Fig 1). How likely is it that we'll have another one? Don't know, but the very fact that one happened should bear caution - it's a known unknown. If stress can build up in that region to such levels, it can happen again.

The simplification of the plates in the picture works as a first approximation concept - there is no nicely defined 8 pixel line, especially when it concerns converging plates which is a very messy process. The idea one would get from the picture above when focussed on the African-EuroAsian plate margins is completely wrong. One would have to zoom in on the Mediterranean, start here.

Yes, most of the tectonic action is concentrated at a particular zone and you're correct that the majority of the major earthquakes will be centred around the Mediterranean countries. The uncertainty is: what's the extent of major seismic event towards the north? For now, we know at least Basel, and the north of Italy, can be affected. Although, for Basel, it's not clear to me if it's because of plate convergence to the south or by normal faulting as a response to crustal loading.

The most powerful recent quakes were

• in 1756 at Düren (6.2, two dead)
  
• in 1951 at Euskirchen (5.7-5.8, no dead)
  
• in 1978 at Onstmettingen (in Baden-Württenberg, 5.7)
  
• in 1992 at Roermond/Netherlands (near both Belgium and Germany, 5.9-6.1, no causalties)

However, I recall reading an article by some geologists critical of the ruling assumption that the central part of the rift zone shifts in a 'lubricated' way -- that assumption is unproven, and that would imply the possibility of strong earthquakes with a low frequency. I.e., what the USA had at New Madrid in 1812 (incidentally the strongest for the contiguous states in recorded history), and China at Tangshan in 1976. Such low-frequency events are definitely missing from risk calculations, especially for high-rises, which haven't been around for long.

_{*Lunatic*, n.
One whose delusions are out of fashion.}

We know that large magnitude events have occurred prior to the 20th century. Has modern architecture (eg. flats and office blocks) in these countries withstood a direct hit of major earthquakes? No - because they're untested. There were no major events in this time period. And Messina was bad enough.

Developments and architecture work on hindsight, retrospection and, worse, amnesia - not foresight. I've used the housing planning in England's river stormbeds during last year's floodings as example of that. I don't believe it's really that different for earthquakes.