NEW YORK (MarketWatch) -- The national average interest rate on the benchmark 30-year, fixed-rate loan averaged 4.85% in the week ending Thursday, setting a new record low dating back to 1971, according to Freddie Mac's weekly survey. That rate is down from last week's 4.98% and the year-ago 5.85%. The 15-year fixed-rate loan averaged 4.58%, a record low dating back to 1991, down from the week-ago 4.61% and the year-ago 5.34%. The five-year Treasury-indexed hybrid adjustable-rate mortgage averaged 4.96%, compared with 4.98% a week ago and 5.67% a year ago. One-year Treasury-indexed ARMs averaged 4.85% this week, down from last week's 4.91% and the year-ago 5.24%. "The Federal Reserve's announcement that it intends to purchase Treasury securities over the next six months caused bond yields to drop and mortgage rates followed," said Frank Nothaft, Freddie Mac vice president and chief economist.