Complex sentence: Since then it started growing decade by decade, mostly because of the migrants from the rural areas and the surrounding smaller towns, reaching its peak in the period 1987-1991 when the population exceeded 90,000.
Split into simpler sentences: Since then it started growing decade by decade, mostly because of the migrants from the rural areas and the surrounding smaller towns. It reached its peak in the period 1987-1991, when the population exceeded 90,000.