Increased urban populations and the use of new construction systems whose resistance to earthquakes has not been sufficiently studied have led to an increase of the earthquake hazard. Also, in spite of the considerable achievements of theoretical and experimental research into the origin and the nature of earthquakes the dynamics of soils, the response of structures and soil-construction interaction, the results are not yet generally reflected in existing building codes and regulations.
The 19 September 1985 Mexico City earthquake left thousands dead and tens of thousands homeless, and a pattern of devastation in its wake. Many buildings made of structural steel designed to sway flexibly during an earthquake remained intact, while immediately next door brick or concrete buildings came crashing down. The city's tallest buildings (the Pemex Tower with 46 stories and the Latin American Tower with 43 stories) suffered little damage, while less than 2 miles away the 5 story Televisa building collapsed, killing 50-60 employees.
The earthquake of June 1990 in Iran caused estimated human losses of 40,000 dead. 500,000 people were rendered homeless. A large part of the existing housing stock is still highly vulnerable to earthquakes and other natural hazards.