Investors’ growing appetite for the safety of U.S. Treasury bonds in the wake of European debt troubles and domestic policy decisions aimed at jump-starting a stagnant economic recovery have driven mortgage interest rates to their lowest level in over 50 years.

Freddie Mac says both fixed- and adjustable-rate mortgages have reached all-time record lows, providing further incentive for homeowners looking to refinance.

Data released by Freddie Mac Thursday puts the average 30-year fixed-mortgage rate at 4.15 percent (0.7 point) for the week ending August 18th, a 17 basis-point drop from 4.32 percent in one week’s time.

The 15-year fixed-rate fell 14 basis points, from 3.50 percent last week to 3.36 percent (0.6 point) this week.

“Not surprising, many homeowners took advantage of this low mortgage rate environment and have already refinanced their loans,” said Frank Nothaft, Freddie Mac’s chief economist.

“The refinance share of applications averaged nearly 70 percent of all mortgage activity in the first half of this year,” Nothaft contined. “In addition, an increasing share of refinancing borrowers chose to shorten their loan terms during the second quarter.”

Adjustable-rate mortgages (ARMs) also headed lower in Freddie’s study, with the 5-year ARM falling from 3.13 percent to 3.08 percent (0.5 point), and the 1-year ARM slipping from 2.89 percent to 2.86 percent (0.6 point).

Freddie Mac’s weekly mortgage rate survey averages quotes gathered from about 125 lenders across the country.

A separate study released Thursday by Bankrate also put rates at or near record lows. The tracking firm’s results are based on data provided by the top 10 banks and thrifts in the top 10 U.S. markets.

In Bankrate’s survey, the benchmark conforming 30-year fixed-mortgage rate slipped to 4.45 percent (0.45 point), down from 4.46 percent last week.

The average 15-year fixed mortgage reset a record at 3.58 percent (0.42 point). That’s down from the 3.61 percent average reported last week.

Bankrate says the larger jumbo 30-year fixed rate fell below the 5 percent mark for the first time ever, landing at 4.89 percent. It was 5.02 percent last week.

Adjustable rate mortgages saw the biggest decreases in Bankrate’s survey, with the average 5-year ARM dropping from 3.24 percent to 3.15 percent and the 10-year ARM falling from 3.93 percent to 3.83 percent.