The crisis in Japan pushed mortgage rates down this week, according to Freddie Mac's Primary Mortgage Market Survey, which was released on Thursday. The average rate on a 30-year fixed-rate mortgage fell to 4.76 percent, down from last week's average of 4.88 percent.

Rates on 15-year fixed mortgages fell under 4 percent, averaging 3.97 percent. That's down from last week when the average was 4.15 percent.

"With the crisis in Japan, investors rushed to buy the security of U.S. Treasury bonds, which lowered its yields and other interest rates as well," said Frank Nothaft, Freddie Mac chief economist. "This allowed fixed mortgage rates to drift lower this week."

Adjustable rates also fell this week. The five-year adjustable-rate mortgage averaged 3.57 percent, down from 3.73 percent. The one-year ARM averaged 3.17 percent, which was lower than last week's average of 3.21 percent.

Interest rates continue to be highly affordable for home buyers. Last year at this time the 30-year fixed-rate mortgage averaged 4.96 percent.