Real estate analytics firm CoreLogic Inc. said in a report that 14,928 mortgages were in negative equity during the fourth quarter of 2012 in Greater Des Moines. The number, representing 13.3 percent of all residential properties with a mortgage, increased slightly from the third quarter. Negative equity occurs when a property is valued at less than the amount due on the loan, there has been an increase in mortgage debt or a combination of the two. According to CoreLogic, 10.4 million properties, or 21.5 percent of all residential properties with a mortgage in the country, were in negative equity at the end of the fourth quarter of 2012, down from 10.6 million properties, or 22 percent, at the end of the third quarter.