If August wasn't bad enough, historical trading patterns suggest that investors shouldn't expect a quick rebound this month.

September is statistically the worst month of the year for the stock market, with the S&P 500 index logging an average decline of 1.03 percent in the month over the last 87 years, according to data from S&P Dow Jones Indices.

While there is no specific reason for the stock market's so-called seasonality, traders and investors do closely follow the historical trends. That could make them a little more nervous this September and, as a result, stocks could remain volatile for a few weeks yet.

"If you're a watcher of seasonality you would not be optimistic for the next six weeks or so," said Scott Wren, senior global equity strategist at Wells Fargo Investment Institute. "It's just a pattern that has played out often enough for people to pay attention to."

As well as worries about the health of the Chinese economy, investors are also concerned about the outlook for U.S. interest rates. There is speculation that Federal Reserve policymakers could lift rates for the first time in close to a decade as soon as this month.

Here's a look back at some recent September slumps.