Asian stock markets tumbled Thursday as more signs of a sharp downturn in the US economy spurred investors to dump shares of exporters like Sony and resource companies like BHP Billiton. Investors also reacted nervously to US Treasury Secretary Henry Paulson's announcement that the government's $700 billion financial rescue package won't purchase troubled assets from banks as originally planned. The Treasury will instead rely on buying stakes in banks and encouraging them to resume more normal lending. Japan's benchmark Nikkei 225 stock average fell 456.87 points, or 5.3 percent, to 8,238.64 and Hong Kong's Hang Seng index dived 5.2 percent to 13,221.35. Australia's benchmark index slid 5.9 percent to a four-year closing low of 3,697.3 as banks tumbled and lower commodity prices hit miners. BHP Billiton Ltd., the world's biggest miner, sank almost 12 percent and Rio Tinto was down more than 8 percent.