By PAMELA SAMPSONBANGKOK (AP) â¿¿ Japan's benchmark stock index surged above 13,000 for the first time in more than four years Friday, a day after the country's central bank announced aggressive action to lift the economy out of an extended slump. The Bank of Japan unveiled plans Thursday to pump huge amounts of money into the financial system in order to spur spending and borrowing in an economy that has suffered from growth-crippling deflation for years. The central bank said it wanted to double the money supply and achieve a 2 percent inflation target within about two years. "The Nikkei is off to the races, continuing its strong momentum from yesterday's trade and breaking to levels not seen since 2008," said Stan Shamu of IG Markets in Melbourne. The BOJ's plans include buying $530 billion a year in government bonds. BOJ governor Haruhiko Kuroda described the scale of monetary stimulus as "large beyond reason," but said the inflation target would remain out of reach if the central bank stuck to incremental steps. The sweeping shift in monetary policy came as a surprise to analysts, even though Kuroda has vowed to do whatever necessary to break Japan's economy out of its deflationary slump. Falling prices have crippled growth in the world's No. 3 economy for the past two decades. "The size of monetary easing announced yesterday far exceeded expectations," said analysts at DBS Bank Ltd. in Singapore in a commentary. The Nikkei 225 in Tokyo soared 3.8 percent to 13,109.58, its highest level since August 2008. The benchmark has soared 25 percent this year, rejuvenated by Prime Minister Shinzo Abe's election in December and enthusiasm for his economic and monetary program that favors a massive expansion of the money supply to create inflation and jolt Japan out of its slump. The central bank's announcement dragged down the yen, giving a boost to shares of Japan's powerhouse manufacturers. A cheaper currency makes Japanese goods less costly for Americans and other foreigners and raises the value of repatriated profits. Nissan Motor Co. jumped 7.2 percent. Ricoh Co. advanced 6.1 percent. Komatsu soared 7.6 percent. Japanese banks also benefited. Mitsubishi UFJ Financial Group gained 6.4 percent.