They seem to have vacillated on the topic. Of course for decades they claimed earthquakes were more intense or prevalent than before 1914, then they backed off in the 90's because the information disproving this was more easily available. Since then they seem to just include earthquakes in the mix of "The Sign" without explicitly claiming that there are more or that they're worse, it's left to be assumed by the reader. It just makes for exciting opportunity in preaching when there is one in the news.
In fact every part of "The Sign" (as they now insist it be called rather then "signs") can be debunked in a similar fashion. War, hunger and disease impact a lower percentage of the population now then ever before. A critical examination reveals the writer of the 'mini apocalypse' of Mark was either simply using basic stock apocalyptic language from the OT in a new 'prophetic' way or referencing actual then past events of either 66-70CE or 135CE or both. What is certain, the writers did not say that this stuff was evidence of something invisible in the heavens so sit and wait 100 years. They said it was the preamble to Jesus's violent return.
Note: The wording of Mark itself was modified slightly throught the years and the revisions of Mark known as Matthew and Luke each added a bit more.