* Yen falls vs dollar, investors wary after intervention
* Euro at 1-month high vs dollar, yen after Spain auction
* Swiss franc weakens broadly after SNB meeting (Adds details, updates prices)
NEW YORK, Sept 16 (Reuters) - The yen fell against the U.S. dollar on Thursday with the Bank of Japan silent after massive yen-selling in the prior session and investors reluctant to place bets in case the central bank begins new sales to weaken the currency.
The euro rose to its highest in more than a month against the dollar and the yen after strong demand at a Spanish bond auction reinforced confidence in Europe's economic recovery.
Japan sold an estimated 1.8 trillion yen ($21.14 billion) for dollars on Wednesday, a record for a single day, to help its exporters and increase money supply to counter deflation. The dollar value represents 3.7 percent of the daily average market turnover in dollar/yen using the last available data from the Bank of International Settlements.
Japan's first currency intervention in six years knocked the yen from a 15-year high versus the dollar, but the yen's six-month uptrend does not look broken, analysts said.
"There is talk of 82 being the line in the sand but surprise is the key element for the Bank of Japan," said Samarjit Shankar, managing director of global FX strategy at BNY Mellon in Boston. "A low 84 or high 83 may be sufficient for the Bank of Japan to come back in and show the market there is two-way risk."
Prime Minister Naoto Kan reiterated on Thursday Japan would take decisive steps on yen strength, Jiji news agency reported, while Bank of Japan Governor Masaaki Shirakawa said he expected intervention would stabilize the forex market. For more see [ID:nTKF107009].
In late afternoon New York trading, the dollar was up 0.1 percent JPY= at 85.83 yen, well up on the low of 82.87 hit on electronic trading platform EBS on Wednesday.
The dollar was just off the session high of 85.84 yen on EBS and on Reuters touched hours earlier. At the peak it broke above the 50-day exponential and simple moving averages.
The dollar/yen 12- and 26-day moving average convergence divergence was at -0.33 with the nine-day signal line at -0.57 on Thursday which may indicate more gains lie ahead. The MACD was at -0.68 on Tuesday with the signal line at -0.67. The dollar/yen December future was trading at 85.75 yen JYZ0.
BOJ ON HOLD
Traders said there were no signs of yen-selling by Japanese authorities though there were indications some investors were reversing bets on the yen.
"We are beginning to see some funds moving in the same direction (as the Bank of Japan), unwinding long yen positions," said Greg Salvaggio, vice president of trading at Tempus Consulting in Washington. He added investors should look out for stops at 86.10 yen.
Analysts said the dollar's upside may be capped around 85.83 yen, where a trendline from the dollar's highs in May and June falls.
Others said Japanese authorities may have a hard time fighting the yen's strength, given narrow yield spreads between Japanese and U.S. government bonds.
"It is almost inevitable that the Japanese authorities will have to continue intervention heading into year end with dollar/yen likely to remain under downward pressure as the U.S. economy slows and the Fed moves towards renewed monetary easing," said Lee Hardman, currency economist at The Bank of Tokyo-Mitsubishi UFJ in London.
Traders said Japanese exporters want to sell the dollar above 85 yen before half-year book-closings at month end.
SPANISH BOND SALE
The euro EUR= rallied 0.6 percent to $1.3078 after rising to $1.3117 according to Reuters data, its highest since Aug. 11, after auctions of 10- and 30-year Spanish government bonds produced lower yields than the sale in June. [ID:nLDE68F0QX]
Traders in London said the euro's initial rise after the results took out stop-loss orders placed around $1.3047, the 61.8 percent retracement of its sell-off in August.
It rose as high as 112.40 yen EURJPY=, also its highest in about a month and adding to gains after dollar/yen intervention drove it up roughly 3 percent on Wednesday.
The Swiss franc weakened broadly after the Swiss National Bank kept interest rates unchanged as expected and said that it expected a slowdown in economic growth due to strength in the currency. [ID:nWEA8411]
The euro jumped 1.7 percent to 1.3273 francs EURCHF=, the biggest daily rise since May 19. The U.S. dollar gained 1.1 percent to 1.0146 francs CHF=. <^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ For dollar/yen correlations link.reuters.com/wyn43p For PDF on the yen's rise r.reuters.com/zuz33p For graphic on intervention link.reuters.com/wym42p For Insider on yen link.reuters.com/rak63p ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ (Reporting by Nick Olivari and Wanfeng Zhou; Additional reporting by Vivianne Rodrigues and Gertrude Chavez-Dreyfuss in New York and Neal Armstrong in London; Editing by James Dalgleish)