Investors are buying up Treasury bonds in anticipation of a move by the Federal Reserve designed to lower mortgage rates and yields on corporate debt.

As a result, the average rate for 30-year fixed loans dropped to the lowest level on records dating back to 1971, mortgage buyer Freddie Mac said Thursday. It’s down from 4.27 percent the previous week.

The average rate on 15-year fixed loans fell to 3.62 percent.

Rates have mostly fallen since spring as investors shifted money into the safety of Treasury bonds. That demand lowers their yields, which mortgage rates tend to track. The 30-year rate was 5.08 percent at the beginning of April.

The 15-year rate was 4.39 percent.

Low rates haven’t helped the struggling US housing market, which recorded its worst summer in more than a decade.