Average rate on 15-year mortgage dips below 4 pct.

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NEW YORK (AP) — Fixed mortgage rates tumbled this week and the 15-year loan dipped below 4 percent for the first time in three months.

Freddie Mac says the average rate on the 15-year fixed mortgage dropped to 3.97 percent from 4.15 percent. The last time the rate was below 4 percent was in mid-December. It reached 3.57 percent in November, the lowest level on records dating back to 1991.

The average rate on the 30-year fixed mortgage fell to 4.76 percent from 4.88 percent the previous week. It hit a 40-year low of 4.17 percent in November.

Mortgage rates tend to track the yield on the 10-year Treasury note. Those yields have tumbled as investors sought safer investments on fears that Japan's earthquake and ensuing nuclear crisis could slow economic growth.