The number of mortgage applications filed in the United States increased 7.1% last week as mortgage rates fell across the board, an industry trade group said Wednesday. The Mortgage Bankers Association's market composite index – a measure of the nation's mortgage loan application volume – rose 7.1% on a seasonally adjusted basis and 7% on an unadjusted basis over the previous week. Meanwhile, the refinance index grew 7.8%, while the seasonally adjusted purchase index rose 5.1% from a week ago. "Treasury rates plummeted more than 20 basis points last week as all eyes were focused on the debt ceiling negotiations in Washington, and economic data depicted much slower than anticipated economic growth," said Michael Fratantoni, MBA's vice president of research and economics. "Mortgage rates fell, with the rate on 15-year mortgages reaching a new low in our survey. Refinance application volume increased, but even though 30-year mortgage rates are back below 4.5 percent, the refinance index is still almost 30 percent below last year's level." Borrowers in the most recent week remained constrained by negative equity and a weak job market, leading to a purchase index that the MBA defines as "weak by historical standards." The four-week moving averages for the seasonally adjusted market index and the seasonally adjusted purchase index are up 2.8% and 4.2%, respectively, while the four-week moving average on the purchase index declined a slight 0.4%. The share of application activity tied to refinancing grew to 70.1%, up from 69.6% a week earlier.  The adjustable-rate mortgage's share of filing activity increased to 6.6% from 6.1%. In addition, the 30-year, FRM remained well under 5% at 4.45%, down from 4.47% a week earlier, while the 15-year, FRM decreased to 3.52%, down from 3.67%, making it the lowest 15-year mortgage rate on record at the MBA. Write to Kerri Panchuk.