TOKYO — Shares tumbled in Asia on Tuesday after a wild day for U.S. markets that resulted in the biggest drop in the Dow Jones industrial average in six and a half years. Japan's Nikkei 225 index skidded 7.1 percent by early Tuesday afternoon but was trading down 6.6 percent a bit later. Hong Kong's Hang Seng index lost 5.0 percent and Australia's benchmark S&P ASX 200 slid 3.4 percent. South Korea's Kospi declined 2.4 percent and the Shanghai Composite index was off 2.2 percent.