The phrase "are mostly concentrated" is correct and usable in written English.
It can be used to describe the distribution or focus of something in a particular area or among a group.
Example: "The resources are mostly concentrated in urban areas, leading to disparities in rural regions."
Alternatives: "are primarily located" or "are largely focused".
Flash-flood warnings are mostly concentrated in south-central states.
Counties with one carrier are mostly concentrated in the South.
They are mostly concentrated in Brooklyn, and many of them work in construction and caring for the elderly.
In Miami, condo-hotels are mostly concentrated in South Beach, Miami Beach, downtown Miami and Sunny Isles.
In Tehran, the homeless are mostly concentrated in the Imamzade Abdollah neighbourhood in west Tehran, Shoush and Harandi, and parts of Azadegan Expressway in south Tehran.
Cones are mostly concentrated within the central retina (macula), which contains the fovea (depression in the retina), where no rods are present.
Kurds in Iraq are mostly concentrated in a semiautonomous region whose legal status has been in limbo despite an Iraqi constitutional directive in 2005 to negotiate the borders.
The applications of natural fibre/polymer composites in civil engineering are mostly concentrated on non-load bearing indoor components due to its vulnerability to environmental attack.
The meridional membrane and bending signal components are mostly concentrated on the high-curvature areas, while the longitudinal bending component is mostly concentrated on the relatively flat areas.
The pores are mostly concentrated along the boundaries between deposited layers and the shape of the boundaries is strongly influenced by spraying strategy.
Dams in Australia are mostly concentrated on the coastal regions of the continent and on Tasmania, while dams in New Zealand are relatively evenly distributed between North and South Island.
