What everyone believes isn't as relevent as what the Bible actually says. And, like I said, I'm no scholar on the subject, but I'm pretty sure the Bible covers the notion of God's wrath coming in the form of natural disasters. Anybody who believes God would be willing to flood the entire planet as a means of communicating his displeasure with mankind shouldn't have any problem believing that the Big Guy might change it up and throw down an earthquake or two.
And most Christians do believe the bit about the flooding. So why is it unreasonable that they believe this could be something along those lines?
Look, even if you want to use the biblical account of the flood as a comparison, God gave the world decades of warning. It didn't come as a bolt out of the blue after some antediluvian court decision made Him mad over the weekend.