The downgrade of U.S. debt may have led to more market volatility than the Flash Crash.

That’s the initial result of comparing the average daily volume of stock trading in August 2011 to the volume in May 2010, when stocks fell nearly 1,000 points at one point in a single day of trading.

The comparison, tallied by agency broker and market structure analyst Rosenblatt Securities, shows average daily volume of 12.5 billion shares traded in trading sessions conducted during the first 15 days of August.

By comparison, an average of 12.1 billion shares changed hands each day of trading in May 2010.

The Flash Crash, which saw 600 points lost and regained in a matter of minutes occurred on May 6 last year. The first-ever downgrade of U.S. debt, which led to this month’s volatility, occurred on the evening of August 5. But the first day that traders could react to Standard & Poor’s decision to lower its rating of U.S. bonds to AA+ from AAA was Monday August 8.

The comparison may not hold up over the course of the rest of August. Rosenblatt, in a report issued late Wednesday, noted that “activity during the past few days has eased from the near-record sessions seen early in the month.

But the Dow Jones Industrial Average fell 470 points in early trading Thursday (August 18), after a slew of economic data created gloom about the global economy.

In the previous six months, from February to July, the average volume of trading each day had not exceeded 8 billion shares, by Rosenblatt's counts. Average daily volume was 8.1 billion in January – but 6.7 billion in December.

By contrast, average daily volume stayed at 8.2 billion shares or above in every month from January through July of 2010.

The 12.5 billion shares traded on average each day in the first half of August is 75.1 percent higher than the 7.2 billion shares traded in July.

This surge in volume on public venues is in keeping with that on so-called nondisplayed markets, known as dark pools. The Investment Technology Group Wednesday said that volume on its POSIT dark pool for institutional trading was up 74 percent in the first half of August, at 144 million shares traded a day.

Liquidnet said it set a daily volume record of 109 million shares on August 9.

Among the public exchanges, Rosenblatt said the BATS BZX exchange gained 1.12% market share in the period, Nasdaq Stock Market 0.93%, NYSE Arca 0.81% and the New York Stock Exchange 0.71%, compared to July.

Nasdaq had the largest share, at 19.39%. NYSE Arca came in at 13.89% and NYSE itself at 13.61%. BATS BZX accounted for 10.01% of trading.

Off-exchange trading, on dark pools, on the wholesale market and elsewhere, fell 4.19%.

Exchange-traded funds showed a huge leap in usage, amid the volatility.

Average daily volume soared 149%, to 2.4 billion shares traded a day, up from 1.1 billion the prior monthand as little as 721 million shares a day in December.