WASHINGTON Securities regulators approved new so-called circuit breakers on Thursday in an attempt to prevent a repeat of the still-unexplained May 6 stock market drop.
The Securities and Exchange Commission said the exchanges could begin implementing the new rules as soon as Friday.
The rules will require the exchanges to pause trading in certain stocks across U.S. equities markets if the price moves 10 percent or more in a five-minute period.
Regulators are still looking for answers to the May 6 crash, which exposed deep flaws in the electronic U.S. marketplace and rattled investors globally. The Dow Jones industrial average fell some 700 points in minutes that afternoon before sharply rebounding.
Too much fragmentation and too little liquidity have emerged as factors in the free-fall, but regulators have warned they may not pinpoint a single cause.
The circuit breakers will be in effect on a "pilot basis" through December 10, the SEC said.
Regulators are looking to expand beyond these trading curbs, which initially will apply only to stocks in the S&P 500 index.
"It is my hope to rapidly expand the program to thousands of additional publicly traded companies," SEC Chairman Mary Schapiro said in a statement.
The SEC said agency staff is also studying ways to address the risks of market orders, steps to deter or prohibit the use by marketmakers of so-called "stub" quotes, and the impact of other trading protocols at the exchanges, including the use of trading pauses and self-help rules.
(Reporting by Karey Wutkowski; editing by John Wallace)