Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of insurer Flagstone Reinsurance (NYSE: FSR) were quaking today, losing as much as 11% after an 8.9 magnitude earthquake and a huge tsunami hit Japan.

So what: There are plenty of reasons to be upset about the disaster in Japan, but investors in insurers that cover catastrophes will almost certainly have to brace themselves for losses. Flagstone's annual report notes that it does "a significant amount of catastrophe business in Japan," so there is good reason for investors to prepare themselves for a hit. Platinum Underwriters (NYSE: PTP) and Montpelier Re (NYSE: MRH) are among the other catastrophe insurers with exposure to Japan. Warren Buffett's Berkshire Hathaway (NYSE: BRK-A)(NYSE: BRK-B) may also have exposure through General Re and Berkshire Hathaway Reinsurance, as well as its investment in Munich Re.

Now what: The extent to which any particular insurer will get whacked is still conjecture at this point. However, the best insurers should be able to handle the fallout and keep both customers and shareholders happy. I'd keep an eye out for market overreactions here that could make some insurers' stocks attractive.

Want to keep up to date on these stocks?

Berkshire Hathaway is a Motley Fool Inside Value pick. Berkshire Hathaway and Montpelier Re Holdings are Motley Fool Stock Advisor choices. Montpelier Re Holdings is a Motley Fool Hidden Gems recommendation. The Fool owns shares of Berkshire Hathaway. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.

Fool contributor Matt Koppenheffer owns shares of Berkshire Hathaway but does not own shares of any of the other companies mentioned. You can check out what Matt is keeping an eye on by visiting his CAPS portfolio, or you can follow Matt on Twitter @KoppTheFool or on his RSS feed. The Fool's disclosure policy prefers dividends over a sharp stick in the eye.