Foreclosure activity in the greater Phoenix area jumped 26.5% between December and January, real estate analytics firm DataQuick said Friday. The increase comes as robo-signing issues, which had slowed the foreclosure activity, wanes. As the area’s foreclosure inventory grew last month, prices declined to 13-year lows, with the median Phoenix-area home price hovering at $119,175, down roughly 1% from December and 9.4% from last year. In January, the Phoenix market reported 6,821 new and resale home sales, down 21.7% from December and 9.5% from last year. Of the sales that did occur in January, 54.5% were classified as foreclosure resales as investors and cash buyers flooded the market looking for affordable deals. Investors put even more pressure on home values by focusing their efforts in the under-$100,000 market segment — a trend that is consistent with what was happening in the market during the month of  December. Homes valued below $100,000 made up 40.3% of all January transactions, up from 32.9% a year ago. Write to Kerri Panchuk.