HONG KONG — A series of earthquakes in Gansu Province in northwestern China set off landslides and building collapses in an impoverished mountainous region Monday, killing at least 56 people and injuring more than 400, according to figures from the Gansu Provincial Seismological Bureau that were being revised upward by the hour. The seismological bureau estimated the magnitude of the main earthquake at 6.6. The United States Geological Survey listed three fairly strong earthquakes in Gansu Province on Monday morning, with magnitudes of 5.9, 5.6 and 4.7. Most of the deaths and injuries appeared to have occurred in Minxian County, although the quakes took place along the border between Minxian County and Zhangxian County, both in the south of Gansu Province.