Mortgage rates inched up almost imperceptibly this week from historic lows that helped boost demand for purchase loans last week by more than 10 percent from a year ago.

Rates on 30-year fixed-rate mortgage averaged 3.39 percent with an average 0.7 point for the week ending Oct. 11, up from 3.36 percent last week but down from 4.12 percent a year ago, Freddie Mac said in releasing the results of its latest Primary Mortgage Market Survey. Last week’s rates were an all-time low in Freddie Mac records dating to 1971.

For 15-year fixed-rate loans, popular with homeowners refinancing, rates averaged 2.7 percent with an average 0.6 point, up from 2.69 percent last week but down from 3.37 percent from a year ago. Rates on 15-year fixed-rate mortgages were at an all-time low last week in records dating to 1991.

Rates on five-year Treasury-indexed hybrid adjustable-rate mortgage (ARM) loans averaged 2.73 percent with an average 0.6 point, up from 2.72 percent last week but down from 3.06 percent a year ago. Rates on five-year ARM loans hit a low in records dating to 2005 of 2.69 percent during the week ending July 19.

For one-year Treasury-indexed ARMs, rates averaged 2.59 percent with an average 0.4 point, up from 2.57 percent last week but down from 2.9 percent a year ago. Rates on one-year ARM loans were at an all-time low last week in records dating to 1984.

An $40-billion-per-month increase in government purchases of mortgage-backed securities guaranteed by Fannie Mae and Freddie Mac announced by the Federal Reserve on Sept. 13 is expected to bolster MBS prices and keep yields down for an indefinite period.

Looking back a week, a separate survey by the Mortgage Bankers Association showed demand for purchase mortgages was up a seasonally adjusted 2 percent during the week ending Oct. 5 compared to a week earlier. Demand for purchase loans was up 12 percent from a year ago, the MBA said, but applications to refinance still accounted for 83 percent of all loan requests.