Explain where African-Americans were concentrated in the United States before Interregional migration occurred and explain the historical reason behind the concentration in that region.

In the 50 years following the end of Reconstruction, African Americans transformed American life once more: They moved. Driven in part by economic concerns, and in part by frustration with the straitened social conditions of the South, in the 1870s African Americans began moving North and West in great numbers. In the 1890s, the number of African Americans moving to the Northeast and the Midwest was double that of the previous decade. In 1910, it doubled again, then again in 1920. In the 1920s, more than 750,000 African Americans left the South--a greater movement of people than had occurred in the Irish potato famine of the 1840s.

The large-scale relocation to the Northeast and West brought many other changes with it, as many largely rural people moved into cities for the first time. Housing was difficult to come by, and in many cities the non-African American residents demanded strict segregation, relegating the new arrivals to self-contained neighborhoods in undesirable parts of town. In addition, most of the available work in the cities was industrial, and many migrating African Americans faced the prospect of learning new trades, generally at lower rates of pay than European Americans received. Tensions between longtime residents and new migrants frequently flared, and during the first decades of the century race riots struck many of the nation's cities and towns, from Springfield, Illinois, and Rosewood, Florida, to New York, Los Angeles, Detroit, and Tulsa.,

Hope this helped out somehow, this is not my own answer, it comes from (https://www.loc.gov/classroom-materials/immigration/african/moving-north-heading-west/) you can read the full passage and explanation there yah, good luck M8!