Thursday's move by major central banks to prevent a worsening of the European sovereign debt crisis unleashed a flurry of protective put selling across a broad spectrum of U.S. exchange-traded funds.

The put sales signaled a sigh of relief among wary investors, who apparently took comfort from the move by the U.S. Federal Reserve, the European Central Bank and other major central banks to offer U.S.-dollar loans to prevent money markets from freezing.

The put sales included ETFs that track retail, consumer, mining, financial and energy sectors, options analysts said.

I think what you are seeing is that investors are taking the good news out of Europe this week and deciding that they do not need the same amount of protection in several ETFS they had over the last few weeks, said TD Ameritrade chief derivatives strategist J.J. Kinahan.

The announcement from the ECB, the Fed, the Bank of England, the Bank of Japan and the Swiss National Bank bolstered shares of European banks, some of which had found it hard to obtain dollar funding amid nervousness over the prospect of a Greek debt default.

Wall Street stocks also rose as default fears eased.

The selling of large blocks of ETF puts could be interpreted as growing optimism among investors or as abandonment of bearish positions that have gone stale, said Andrew Wilkinson, senior market analyst at Interactive Brokers Group in Greenwich, Connecticut.

In either case, investors are less fearful today of collateral damage from the euro zone debt problems, he said.

A lot of the put positions were initiated in late August and early September. They are being closed out now as the market reacts favorably to the coordinated efforts of the global central banks, said WhatsTrading.com options strategist Frederic Ruffy. It could be that one large investor is taking off a hedge across multiple exchange-traded funds.

The common themes are that all the puts expired in October and were out-of-the-money, so they would have profited from a market collapse in the near-term, OptionMonster analyst David Russell said on the Chicago-based company's website. However, that also makes them vulnerable to losses if sentiment improves, because a market rally would depress volatility.

One big trade was in the Financial Select Sector SPDR fund as an investor collected 18 cents for the sale of a 73,000 contract block of October $11 puts, Ruffy said.

Another trade was in the Consumer Discretionary Select Sector SPDR fund, with more than 58,000 October $30 puts sold for 16 to 18 cents apiece, according to optionMonster.

OptionMonster's data shows the SPDR S&P Oil & Gas Exploration & Production Fund attracted the sale of more than 27,000 October $42 puts for 67 cents, while 37,000 October $40 puts were unloaded for 34 to 61 cents in the SPDR S&P Retail fund. Similar activity appeared in the SPDR S&P 500 Metals & Mining Fund and the Select Sector Industrial SPDR Fund, Russell said.

(Editing by Padraic Cassidy)