The phrase "is mainly concentrated" is correct and usable in written English.
You can use it to indicate that the focus of a particular thing is primarily on one particular area. For example, you might say "The majority of the population is mainly concentrated in urban areas."
Dunkin' Donuts is mainly concentrated in the eastern United States.
Opium cultivation is mainly concentrated in four southern and southwestern provinces: Helmand, Kandahar, Farah and Uruzgan.
In Pakistan, quake activity is mainly concentrated in the northern and western sections of the country, along the boundary of the Indian, Iranian and Eurasian plates.
Industry is mainly concentrated around the larger centres, such as Roermond, Sittard, Venlo, and Weert, and includes the manufacture of light metals, clothing, hosiery, and chemicals.
So far, little seems to have discouraged the stubborn insurgency, which is mainly concentrated in a swath of territory west and north of Baghdad called the Sunni Triangle, encompassing Falluja.
"In practice this problem is mainly concentrated in poor countries, but we can be sure the true number of under-fives left alone is going to be much higher".
Many MPs expressed fears about rejoining military action in the region, while others raised doubts over why the Commons was not giving its backing to similar strikes on targets in Syria, where Isis is mainly concentrated.
Attention is mainly concentrated on tackling the parameter uncertainty, unknown transition probabilities and output quantization.
Heat transfer enhancement of twisted oval tube is mainly concentrated in the high Reynolds number region.
However, concrete crushing and rebar fracture is mainly concentrated in the flanges.
In particular, the size of the Pd nanoparticles is mainly concentrated at 6 nm.
