Mortgage rates fell this week, with the average rate on 30-year fixed-rate mortgages retreating deeper below 5% and several others reaching lows, according to Freddie Mac's weekly survey.

The 30-year fixed-rate mortgage averaged 4.87% for the week ended Thursday, the lowest since May. It compares with last week's 4.94% average and 5.94% a year ago.

Rates on 15-year fixed-rate mortgages were 4.33%, down from 4.36% last week and 5.63% a year earlier. The latest figure is the lowest since Freddie began tracking such loans in 1991.

Five-year Treasury-indexed hybrid adjustable-rate mortgages averaged 4.35%, down from last week's 4.42% and 5.9% a year earlier. Those loans haven't had such low average rates in the four years Freddie has kept such data. The average rate on one-year Treasury-indexed ARMs rose to 4.53% from 4.49%. The prior-year average was 5.15%.

The falling rates also are spurring increased refinancing activity, which reached a 19-week high last week.