Mortgage rates fell to a new record low for the fourth time in five weeks. But low rates haven't been enough to lift a struggling housing market.

The average rate for 30-year fixed loans this week was 4.56 percent, down from 4.57 last week, mortgage company Freddie Mac said Thursday. That's the lowest since Freddie Mac began tracking rates in 1971.

The last time home loan rates were lower was during the 1950s, when most mortgages lasted just 20 or 25 years.

The rate on the 15-year fixed loan dropped to 4.03 percent, down from 4.06 percent last week and the lowest on records dating back to 1991.

Rates have fallen since the spring. Investors worried about the European debt crisis have shifted money into the safety of Treasury bonds. That has forced those yields down. Mortgage rates tend to track yields on Treasury debt.

However, low rates have yet to spark home sales and refinancing activity remains moderate.

Sales of previously occupied homes fell in June and are expected to keep sinking. The National Association of Realtors said Thursday that last month's sales fell 5.1 percent to a seasonally adjusted annual rate of 5.37 million.