NEW YORK, Oct. 15 -- South Korea's benchmark stock index fell 7.57 percent on Thursday and at one point triggered a trading halt, and the shock was still travelling west before Wall Street opened.

Nasdaq futures were down about 1 percent, extending Wednesday's losses, and the S&P 500 had already shed 1.11 percent. Japan's Nikkei 225 lost 4.9 percent. The 10-year Treasury yield peaked at 5.32 percent, then dropped back to 5.08 percent by morning, a swing that left traders unable to say whether the bond was a refuge or a risk.

What makes the rout hard to read is that the sector's own news was good. TSMC reported third-quarter earnings that beat forecasts and said it would spend more on capacity over the next two years. Apple closed at a r...