The 30-year, fixed-rate mortgage hit lows not seen in five decades this week as the Federal Reserve committed to keeping the federal funds rate low through 2013, according to the Freddie Mac Primary Mortgage Market Survey.

European debt concerns also riled the market, staving off a hike in mortgage rates, according to Frank Nothaft, vice president and chief economist for Freddie Mac.

The 30-year, fixed-rate mortgage dropped to 4.15% from 4.32% a week earlier and 4.42% last year. Nothaft said 30-year, fixed mortgage rates are at their lowest levels in five decades.

Meanwhile, the 15-year, FRM hit 3.36%, down from 3.50% a week earlier and 3.90% last year. In addition, the 5-year Treasury-indexed hybrid adjustable-rate mortgage averaged 3.08% this week, down from 3.13% a week earlier and 3.56% a year ago, while the one-year Treasury-indexed ARM averaged 2.86%, down from 2.89% last week and 3.53% last year.

The low rates kept refinancing activity high. In the first half of 2011, refinancing applications represented nearly 70% of all mortgage activity.

Bankrate attributes the record low interest rates to "weakness in the U.S. economy and the accompanying demand for Treasury securities."

According to Bankrate's survey, the 30-year, FRM hit 4.45%, down from 4.46% last week, while the 15-year, FRM hit 3.58%, down from 3.61% a week earlier.

The 5/1 ARM fell to 3.15% from 3.24% a week earlier.