On 24 May 1940, a 1940 Lima earthquake destroyed most of the city, which at that time was mostly built of adobe and ''quincha''.  In the 1940s, Lima started a period of rapid growth spurred by migration from the Andean region, as rural people sought opportunities for work and education. The population, estimated at 600,000 in 1940, reached 1.9 million by 1960 and 4.8 million by 1980. At the start of this period, the urban area was confined to a triangular area bounded by the city's historic center, Callao and Chorrillos; in the following decades settlements spread to the north, beyond the Rímac River, to the east, along the Central Highway and to the south. The new migrants, at first confined to slums in downtown Lima, led this expansion through large-scale land invasions, which evolved into shanty towns, known as ''pueblos jóvenes''.