More than half of all the people in the world now live in dense urban centres. The rapid expansion of cities, particularly in low-income nations, has enabled the economic and social development of millions of people. However, many of these cities are located near active tectonic faults that have not produced an earthquake in recent memory, raising the risk of losing the hard-earned progress through a devastating earthquake.

In this paper published last year (https://doi.org/10.5194/nhess-2019-30) we explore the possible impact that earthquakes can pose to the city of Santiago in Chile from various potential near-field and distant earthquake sources. We use high resolution stereo satellite imagery and derived digital elevation models to accurately map the trace of the San Ramón Fault, a recently recognised active fault located along the eastern margins of the city.

For the San Ramón rupture scenario in the magnitude range 7–7.5 under the current residential exposure, we would anticipate over 100,000 building collapses, 10,000+ fatalities and a replacement cost of 8-10 billion USD. For all our scenarios, the most vulnerable building class is unreinforced masonry, while reinforced concrete and wooden structures are the most resilient to earthquake shaking. Therefore, effective near-term risk reduction measures could target unreinforced masonry homes for retrofitting campaigns, particularly in Ñuñoa, Santiago and Macul, while in the mid-long term a drive towards reinforced concrete homes would significantly reduce the risks to future earthquakes both from near and far-field sources.

This work also reinforces the need to identify active faults adjacent to or beneath cities in actively deforming zones, and the need to update the exposure models as such cities encroach onto these faults. We have highlighted that local crustal earthquakes in the magnitude range 6-7.5 can have at much greater impact than distant larger earthquakes. Therefore the frequency of distal major earthquakes has to be balanced by the potential for infrequent but much more potent local smaller earthquakes on less active faults.