S&P stock index futures briefly turned positive on Friday before returning to negative territory.

Futures were lower earlier after a massive earthquake hit Japan and accelerating inflation in China rattled investors. Also, data showed U.S. retail sales rose 1 percent in February.

After briefly turning positive, S&P 500 futures lost 2.8 points and were below fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures fell 25 points, and Nasdaq 100 futures dropped 4.5 points.

(Reporting by Ryan Vlastelica)