The average rate for the conventional 30-year fixed mortgage has dropped below the 4 percent mark for the first time in history, according to numbers released Thursday by Freddie Mac.

The GSE’s market analysis also shows that the 15-year fixed rate – which has become a popular refinancing option among existing homeowners – fell to its lowest level on record for the sixth consecutive week.

Freddie Mac’s regular weekly survey of mortgage rates is based on data collected from about 125 lenders across the country.

The GSE puts the average rate for a 30-year fixed mortgage at 3.94 percent (0.8 point) for the week ending October 6, 2011. That’s down 7 basis points from its average of 4.01 percent last week. As a point of comparison, last year at this time, the 30-year rate was 4.27 percent.

The 15-year fixed-rate mortgage came in at 3.26 percent (0.8 point) this week, dropping 2 basis points from 3.28 percent last week. A year ago at this time, the 15-year rate was averaging 3.72 percent.

Frank Nothaft, Freddie Mac’s chief economist, attributed the decline in fixed mortgage rates to a sharp drop in 10-year Treasuries earlier in the week as concerns over a global recession grew.

Adjustable-rate mortgages (ARMs) were mixed this week in Freddie’s study. The 5-year ARM dropped from 3.02 percent to 2.96 percent (0.6 point), while the 1-year ARM rose from 2.83 percent to 2.95 percent (0.5 point).

At this time last year, the 5-year ARM was averaging 3.47 percent, and the 1-year ARM was 3.40 percent.

Nothaft tied the rise for 1-year ARMs to shorter-term Treasuries, noting that the Federal Reserve began replacing $400 billion in short-term Treasury securities with longer-term bonds this week.