We get 6-8's all the time, we do however only have two skyscrapers in the whole country lol.
The sky scrapers here aren't the problem. I'm pretty confident most if not all can withstand a big earthquake. It's the few hundred thousand 4 to 8 floor buildings... quite often having been built before the latest Earthquake standards (which I believe was the late 80s).

Uggh... this is worse than I thought. Here's some lovely excerpts from here which is a reflection of what would happen in current day Tokyo if another 1923-sized Earthquake hit. Note that more peopled died in the ensuing fires after the quake (including 14,000 people in one single location) than the quake itself.

Much of Tokyo's old reclaimed land remains vulnerable to liquefaction. This is a process where shaking ground causes the soil to sink and the water content to rise to the top (as happens when a person pats sand on the beach). Some of Tokyo's major industrial plants containing hazardous materials are constructed on such land.
Two million of Tokyo's 2.6 million structures, including mid-rise concrete apartments, were erected before the earthquake standards for new construction were instituted in 1974 and strengthened in 1981.
City officials in Tokyo themselves expect a magnitude 7.2 earthquake in Tokyo to kill more than 7,000 people, injure 160,000, leave at least 2.3 million people homeless and destroy more than 500,000 buildings.
There's a Wikipedia entry somewhere that tries to calculate implications on the world economy if an Earth-shattering quake hit Tokyo. The truth of the matter is, not only does the majority of the population aggregate to this one spot on weekdays (w/ commuters included, you've got some 30-40 million people in Tokyo on any given day, which is 20-25% of the population), but the entire government body resides here. Most companies have their HQ in or near the area. It is redefinition of all eggs in one basket.