An index measuring the San Diego economy fell by 1 percent in August, the largest decline since March 2009 and the second decrease in the past three months, causing concern about the sustainability of the tepid recovery.

The decline last month in the Index of Leading Economic Indicators for San Diego County compiled by the University of San Diego Burnham-Moores Center for Real Estate was led by sharp decreases in local stock prices, consumer confidence, and building permits. Three components showed gains: help-wanted advertising, the outlook for the national economy, and reduced claims for unemployment insurance, but these were modest in comparison to the declines, according to the Sept. 27 report.

Alan Gin, the USD economics professor who compiles the data for the index, said while economists usually look for three consecutive months of changes before signaling a turning point in the economy, the magnitude of the August decline was troubling.

Two measures — local stock prices and consumer confidence — collapsed in August, indicating other investors and consumers have serious concerns about the economy, Gin said.

“For now, the outlook remains for positive but slow growth in the local economy through the first part of 2012,” he said. “What happens after that is up in the air, and more data will be needed in the coming months to clarify the situation.”

— Mike Allen