Equity markets in the United States suffered their second consecutive month of volume dropping more than 20% compared to a year ago, according to the Equity Research Desk in Greenwich, Conn.

Volumes in cash equities were down 25% in September, overall, the provider of statistics and fundamental analysis of global capital markets said. Tape A, which tracks New York Stock Exchange listings, was down 29%. Tape B, which tracks NYSE Arca and NYSE Amex listings, was down 20%, and Tape C, which tracks Nasdaq listings, was down 16%.

In August, U.S. cash equity volumes were down 24% overall, the Equity Research Desk previously reported. Tape A was down 31%, year over year; Tape B was down 17%; and Tape C was down 10%.

In shares, average daily volume on the consolidated tape was 7.2 billion shares in September, the same as in August.

But volume has shown a steady decline -- 40% all told -- since its peak in May, the month of the volatility largely instigated by the Flash Crash of May 6. That was when liquidity temporarily disappeared, as the Dow Jones Industrial Average fell hundreds of points in a matter of minutes.

In May, average daily volume was 12.0 billion shares, according to Equity Research Desk. June’s average was 9.3 million and July 8.5 million.

The trigger for the Flash Crash was found by the Securities and Exchange Commission and the Commodity Futures Trading Commission to be a large series of trades in E-Mini futures contracts instigated by a large mutual fund company.