Why are there so many people from Henan in Xi 'an and so many people from Shaanxi in Henan?

In the eighteenth year of the Republic of China, a catastrophe occurred in Shaanxi with a population of more than 10 million. More than 3 million people starved to death, and more than 6 million people were displaced and begged for food. There were 9 rooms in 10, which was ten times more miserable than 1942 100 (see Absolute Qin Shu for details). Since then, the populations of Henan, Shanxi, Gansu, Sichuan and other provinces around Shaanxi have all come to Shaanxi, so there are more migrants in the west of China.